text:
How to Fix Data Reading Errors in R
Probably the most common mistakes in R are made while reading in data from text files using read.table() or read.csv(). Many mistakes result in R throwing errors, but sometimes you only notice something went wrong when you look at the structure of your data. In the latter case you often find that some or all variables are converted to factors when they really shouldn't be.
When R gives errors or the structure of your data isn't what you think it should be, check the following:
Did you forget to specify the argument header=TRUE? If so, R will see the column names as values and, as a result, convert every variable to a factor as it always does with character data in a text file.
Did you have spaces in your column names or data? The read.table() function can interpret spaces in, for example, column names or in string data as a separator. You then get errors telling you 'line x did not have y elements'.
Did you have a different decimal separator? In some countries, decimals are separated by a comma. You have to specifically tell R that's the case by using the argument dec="," in the read.table() function.
Did you forget to specify stringsAsFactors = FALSE? By default, R changes character data to factors, so you always have to add this argument if you want your data to remain character variables.
Did you have another way of specifying missing values? R reads 'NA' in a text file as a missing value, but the file may use a different code (for example, 'missing'). R will see that as text and again convert that variable to a factor. You solve this by specifying the argument na.strings in the read.table() function.
If you always check the structure of your data immediately after you read it in, you can catch errors much earlier and avoid hours of frustration. Your best bet is to use str() for information on the types and head() to see if the values are what you expected.


text:
The two issues, which could lead to yet another SCOTUS review sometime during the 2013-2014 court session, involve a challenge to the employer mandate—requiring that employers with 50 or more full-time employees provide health insurance to its workers or pay a fine—and the contraception mandate requiring that health care benefits include a contraceptive benefit without the need for a co-pay.
The Court's ruling was in response to a request by Liberty University—the Christian educational institution founded by Reverend Jerry Falwell—to force the 4th Circuit to give the university its day in court after the 4th Circuit dismissed the action last year, refusing to hear substantive claims and arguments in support of Liberty's position. The 4th Circuit's reasoning for tossing the case was grounded in the Anti-Injunction Act—the tax law that requires that a tax actually be paid before a challenge can take place. While this was one of the issues raised in this year's challenge to Obamacare before the Supreme Court, SCOTUS quickly disposed of this as an issue that would have prevented the case from moving forward, thereby allowing the Supremes to get into the more substantive issues.
Liberty's employer mandate claim is based on the argument that Congress overstepped its constitutional boundaries with respect to this provision of Obamacare. The university additionally is arguing that requiring it to provide the contraceptive benefit violates its right to freely exercise religion.
Interestingly, the Obama Administration chose not to challenge the effort by Liberty to get the case before the 4th Circuit, noting that the issues raised by the university had not been adjudicated by the Supreme Court in its landmark decision.
However, the administration's lack of concern might have more to do with the fact that the 4th Circuit is one of the most liberal appellate courts in the nation and had sent signals during the initial action that they would be more likely than not to side with the Obama Administration on the issues raised by Liberty had they decided to actually hear the case.
Were Liberty to succeed in its lawsuit, a major part of the Affordable Care Act—the employer mandate—could be cut from the law. Further, the controversial requirement involving contraception would be placed in doubt. While a victory by Liberty is unlikely at the appellate level, a loss by Liberty—and it's co-appellants—would likely bring a request that the U.S. Supreme Court take up the matter during next year's session.
While many legal scholars expect that SCOTUS would be reluctant to jump back into the Obamacare fray—particularly given the fact that the law would be in full effect by the time the Court would likely get to the matter late in the 2013-2014 session— a decision to hear the matter would present Chief Justice John Roberts with an opportunity to square his credentials with conservatives who have yet to forgive his initial Obamacare ruling.
Related on Forbes:
contact Rick at firstname.lastname@example.com and follow me on Twitter and Facebook

subset: commoncrawl


text:
Product description
☑ Product specification:
Perfect for any occasion, such as shopping, outdoor, party ect.
Please note the size chart. (In CM), Asian size is usually smaller than EU size.
Before buying, please check the detail size of the product.
Due to the manual measurement, please allow 2-3 cm difference.
Note: Due to different monitors, the image may not always show the actual color.
We can guarantee that the actual product matches with the picture, but are NOT sure when wearing it, is beautiful as a model. Thank you for your comprehension.
Welcome to our shop Battercake, we wish you a pleasant shopping!
you may also like
£128.25 £53.58
Save: 58% off
£165.05 £78.79
Save: 52% off
£69.68 £30.09
Save: 57% off
£213.30 £76.83
Save: 64% off


text:
Coming off the enthusiastic reception of the Bennett Miller-directed drama "Foxcatcher" at the Cannes Film Festival last month, Channing Tatum looks to be working toward his own version of a McConaissance.
Like Matthew McConaughey, who made the leap from rom-com heartthrob to serious dramatic actor via movies such as "Dallas Buyers Club" and "Mud," Tatum is showing ambitions of moving beyond the confines of mainstream studio fare, and with some top auteurs to boot.
The "22 Jump Street" star is in talks to join the cast Joel and Ethan Coen's "Hail, Caesar!," about a fixer in 1950s Hollywood who works for the studios to keep their stars out of trouble, according to a Variety report.
The Universal film, which the Oscar-winning siblings are directing from their own original script, also stars George Clooney and Josh Brolin. According to the Hollywood Reporter, Tilda Swinton and Ralph Fiennes may come aboard as well. (Universal could not be reached for comment.)
Tatum has made a name for himself as a beefy action hero with a deft comedic touch. He's saved the day in shoot-'em-ups like "White House Down" and "G.I. Joe: The Rise of Cobra" and also played his jocky physique for laughs in "21 Jump Street" and its recent sequel.
The 34-year-old actor's first foray outside the studio box and his own wheelhouse came when he collaborated with Steven Soderbergh on the male-stripper-movie "Magic Mike" (based on Tatum's days as an exotic dancer) and the psychological thriller "Side Effects."
"Foxcatcher" could mark another significant step for Tatum. The film, which tells the story of the twisted relationship between chemical-fortune heir John Eleuthere du Pont (Steve Carell) and Olympic wrestling brothers Dave (Mark Ruffalo) and Mark Schultz (Tatum), has already been the subject of awards-season buzz ahead of its Nov. 14 release from Sony Pictures Classics.
Follow @ogettell for movie news


text:
BROCKTON, Mass. – Pitching was the story of the night as baseball and Keene State College each walked away with a victory in another close Little East Conference doubleheader Saturday afternoon at Campanelli Stadium. Junior lefty Kyle Szatrowski (Agawam, Mass.) led the Beacons to a 6-3 game-one victory, while in the nightcap senior righty Shawn Egan carried the Owls to a 5-1 decision.
With Saturday's split the Beacons continue to go back-and-forth with that .500 mark as they now stand at 13-14 overall and 4-5 in the Little East Conference. The Owls are now 7-15 on the season and 2-7 in the Little East following the twinbill.
Szatrowski coupled with home runs from Jamill Moquete (Boston, Mass.) and Jose Pena (Boston, Mass.) helped the Beacons to the three-run victory. Moquete hit his fourth homer of the year, a solo shot, in the bottom of the first to make it 1-0. In the sixth, Pena crushed his first home run of the year over the right field wall to make it 3-0.
During that time Szatrowski was perfect through the first four innings. An error in the fifth ended the lefty's bid at perfection, but the Agawam native carried a no-hitter into the seventh.
In the nightcap, Szatrowski would record the lone Beacons hit as Egan held UMass hitless until the bottom of the eighth. The pitcher turned first baseman ripped a line shot up the middle as he tried to spark some late inning heroics. Unfortunately that hit coupled with an unearned run for the Beacons would be the only action UMass Boston would produce as the Owls took game two.
GAME ONE – UMass Boston 6, Keene State College 3
Szatrowski's day consisted of eight strong innings in which he allowed two runs on two hits. The only pitcher to record a no-hitter in program history, he also fanned four and walked two to improve to 2-1 on the year. Moquete's and Pena's big bats helped keep the momentum in favor of the Beacons.
The Owls ended Szatrowski's no-no in the seventh on a leadoff single. They would then make it a one-run ball game on a two-run shot by Andrew Gummow. However Szatrowski was able to settle down and got out of the inning with the Beacons lead still intact.
The Beacons pushed their lead to four in the bottom half of the next inning scoring three runs on a bases loaded walk, an RBI groundout, and on an error by the Owls.
In to close out the ninth was senior righty Daniel Heefner (Tracy, Calif.). The California native had to overcome a pair of infield errors but did manage to end the game with the bases loaded while only surrendering an unearned run.
GAME TWO – UMass Boston 1, Keene State College 5
The nightcap belonged to Egan as the righty surrendered just the one Beacons hit to earn his first win of the 2014 season. He would strike out four and walk one in eight complete innings of work. Saturday's loss went to Lucas Ilges (Gloucester, Mass.) who went 6 1/3. The senior allowed four runs, three earned, on eight hits and struck out two.
The Owls would score one run in each of the first three innings to give their pitcher some breathing room. Egan would mirror Szatrowski's showing in game one as he was perfect through the first 3 2/3 before surrendering his only walk of the day to Moquete.
From there he shut down the Beacons bats, and got out of some late jams thanks in part to his defense despite their four errors on the day. The lone Beacon run came on an infield error in the bottom of the eighth which scored Charlie LaPaglia (Marshfield, Mass.).
Vinnie Mangiagli closed out the ninth retiring the side in order to give the Owls their second conference win of the season.
With Saturday's split the Beacons currently sit tied for fourth in the conference standings with Rhode Island College. UMass Boston will face RIC in the final doubleheader of the season (May 2), but still have to play a doubleheader with Plymouth State University (April 26) and a single game with the University of Southern Maine (April 22).
The Beacons return to action on Monday when they travel to Milton to play a single non-conference game with Curry College at 3 p.m.


text:
Friday, June 24, 2011
Breadcrumbs: Etched in Stone
Next time you receive a flyer to support a fundraiser for a local charity, take a second look. A park in my town was raising money for improvements. They decided to sell engraved bricks to raise funds. The donors could have whatever they wanted put on the bricks. I chose a very simple "The Pierre-Louis Family."
Wouldn't it be fun if generations from now my descendants discovered where I lived in census records or city directories and then decided to take a ride and visit the town? If they happen to stop in the park they will find our family legacy.
It's fun and feels good to know that I am helping the park where my children play right now and that I'm also getting something back in return - a little monument to the future.
The next time you get a chance to buy a brick for a fundraiser - do it! Leave a legacy behind etched in stone.
6 comments:
I love the brick idea. We have that all around in our county. The new library has bricks and my favorite is the new Confederate Memorial. Our state park used to be a Confederate Prison Camp and they built a new memorial that's beautiful and one half is bricks of the prisoners names and mixed in the other half is donors to the memorial.ReplyDelete
Brilliant! You've left a wonderful "bread crumb", and supported a local charity.ReplyDelete
Cheers to you,
Good for you! When I visit an ancestor's town the first thing I do is to stop on the common to read all the names on the war memorials, names on benches, and then peek inside libraries and churches to see if there are names on the walls (benefactor plaques, names on pews, contributions to stained glass windows, etc), and then I hit the graveyards and town hall. I never fail to find similar surnames, and often matching names somewhere! These bricks are exactly what folks will find in the future.ReplyDelete
My siblings and I just purchased 3 bricks for my father and his two brothers for a local Veteran's Memorial Park. All three served in WW II. The bricks have their names, branch and service years. Will be a great clue to some future researcher.ReplyDelete
My dad purchased bricks in the names of our Chickasaw ancestors for one of the Chickasaw cultural buildings. It was really moving to see those names in stone. Thank you for suggesting it to others!ReplyDelete
love this Marian!ReplyDelete


text:
Security Innovation, a company that specializes in application and crypto security, has announced availability of its NTRU public key cryptography system for free use in the open-source software community.
NTRU is a public key cryptosystem that uses lattice-based cryptography to encrypt and decrypt data. It consists of two algorithms: NTRUEncrypt, which is used for encryption, and NTRUSign, which is used for digital signatures.
"More diversity in choice of cryptography can only make communications on the Internet safer," the firm said, adding that NTRU is "poised to become the de facto crypto in the post-RSA world."
RSA, an algorithm for public key encryption, and ECC, for Elliptic Curve Cryptography, are the two most common public key crypto systems in use today. But at the 2013 Black Hat conference, according to SI, researchers declared that the math for cracking encryption algorithms could soon become so efficient that it would render the RSA crypto algorithm obsolete.
The resulting crypto vacuum could set up what SI called a "cryptopocalypse," with organizations scrambling to retrofit systems with new, yet trusted, public key crypto systems.
With the open-source license, NTRU can be deployed in open-source products such as Web browsers and TLS/SSL servers, the firm said. For proprietary products, a commercial license is available.
The firm cited Charles Kolodgy, research vice president for security products at IDC, who said Internet e-commerce is "being protected by a limited set of encryption algorithms. This lack of diversity can be a single point of failure."
Kolodgy said that by offering NTRU under a public license, SI is "expanding the diversity of encryption available on the Internet."
"Open sourcing NTRU ensures that the implementation is solid and without the backdoors that we have learned about in proprietary implementations," said William Whyte, chief scientist at SI and chair of the IEEE 1363 Working Group.
"We are fussy in the crypto world and want to ensure that any adopted crypto is transparent and battle-tested. NTRU has been successfully scrutinized by numerous government agencies and universities for over a decade," Whyte said.
For more information, visit the NTRU Public Key Cryptography and Reference Code on GitHub.
About the Author
Connect with the GCN staff on Twitter @GCNtech.
E-Mail this page
Printable Format
Advanced Search
Sign up for our newsletter.
I agree to this site's Privacy Policy.


text:
Red Cross calls for ceasefire as Homs faces shortages
Issued on: Modified:
Residents of the besieged city of Homs are running low on food and water as the Red Cross called for a daily humanitarian ceasefire. Two Western journalists, including a French national, were killed in the shelling, a rights group said Wednesday.
AP - Food and water are running dangerously low in the besieged Syrian city of Homs, with frantic cries for help from residents amid the latest government shelling that has pounded rebel strongholds and killed at least 30 people, activists said.
Shells reportedly rained down Tuesday on rebellious districts at a rate of 10 per minute at one point and the Red Cross called for a daily two-hour cease-fire so that it can deliver emergency aid to the wounded and sick.
"If they don't die in the shelling, they will die of hunger," activist and resident Omar Shaker told The Associated Press after hours of intense shelling concentrated on the rebel-held neighborhood of Baba Amr that the opposition has extolled as a symbol of their 11-month uprising against President Bashar Assad's regime.
Another 33 people were killed in northern Syria's mountainous Jabal al-Zawiya region when government forces raided a town in pursuit of regime opponents, raising Tuesday's overall death toll to 63, activists said. The Local Coordination Committees, an opposition group, said more than 100 were killed Tuesday, but the report could not immediately be confirmed by others.
Russia, one of Assad's remaining allies, urged the United Nations to send a special envoy to Syria to help coordinate security issues and delivery of humanitarian assistance.
Assad's forces showed no sign of easing their assault on Homs, Syria's third-largest city, whose defiance has become an embarrassing counterpoint to the regime's insistence that the opposition is mostly armed factions with limited public support.
The rebel defenses in Homs are believed to be bolstered by hundreds of military defectors, which has possibly complicated attempts by Syrian troops to stage an offensive. On Monday, reinforcements of Syrian tanks and soldiers massed outside the city in what could be a prelude to a ground attack.
"Government troops have been unable to advance because of stiff resistance from defectors inside," an activist in Homs told the AP on condition of anonymity, because of fears of government reprisal. Another activist in Homs said the shelling started after repeated attempts by troops to storm the edges of Baba Amr, which the opposition has dubbed "Syria's Misrata" after the Libyan city that refused to fall to withering government attacks last year.
One Homs resident, communicating with the AP by Internet chat, said many people are unable or too scared to go to the hospital for treatment. Some are bleeding to death at home.
"My cousin is a doctor and he said they've given up on treating serious wounds. The numbers are too many to cope with especially with so little supplies," said the resident, who has provided reliable information in the past. The resident spoke on condition of anonymity because of the fear of reprisal.
The resident, who lives just outside Baba Amr, said people in the neighborhood were surviving mostly on stocks of rice and canned corn and tuna, but those supplies also were running out fast after several weeks of attacks.
Some people go without bread for days, and when grocery stores and bakeries reopen during a lull in the shelling, long lines form quickly, the resident said, adding that shortages exist of all kinds of foodstuffs and vegetables.
The Red Cross said it has been negotiating with Syrian authorities and members of the opposition to agree a temporary cease-fire so emergency aid can reach beleaguered parts of the country.
"The current situation requires an immediate decision to implement a humanitarian pause in the fighting," said Jakob Kellenberger, the president of the Geneva-based International Committee of the Red Cross. "In Homs and in other affected areas, entire families have been stuck for days in their homes, unable to step outside to get bread, other food or water, or to obtain medical care."
Kellenberger said the cease-fire should last at least two hours daily, so that Red Cross staff and Syrian Arab Red Crescent volunteers have enough time to deliver aid and evacuate the wounded.
Beatrice Megevand-Roggo, the ICRC's head of operations for the Middle East, described Homs as "sort of a ghost city," adding that other parts of Syria also were badly affected by the fighting.
White House spokesman Jay Carney backed a Red Cross call for a daily cease-fire in Syria in order to deliver humanitarian aid.
"The reprehensible actions perpetrated by the Syrian regime, the brutal violence perpetrated by the Syrian leader against his own people, has led us to this situation where basic supplies, humanitarian supplies are very scarce and therefore action needs to be taken," Carney said.
U.S. State Department spokeswoman Victoria Nuland said Washington was focused on "increasing the international isolation and the international pressure on the Assad regime to stop the violence altogether, so that we can move on to a democratic transition."
In the northern province of Aleppo, the government said a Syrian businessman, Mahmoud Ramadan, was shot to death in front of his home in what appeared to be the latest in a series of targeted killings. The attacks, which include the slaying of an Aleppo city council member Saturday, suggest that rebel factions are increasing turning to arms to strike back at members of Assad's ruling system.
Residents and activists say a monthslong siege and stepped up attacks on Baba Amr recently have left the district without enough food, water, medicine and electricity.
"They bombed all the water tanks on the roofs of buildings. There's no water. Some people have gone without bread for days," said Shaker, who estimated the shells fell at a rate of about 10 per minute at some points in the attack. More than 200 people were wounded, he said, adding that two children were among the dead.
Phone lines with Homs have been cut, making it difficult to get firsthand accounts from residents.
One amateur video posted on the Internet showed thick smoke and shells slamming behind a building in Baba Amr. Another showed a shop on the ground floor of a building on fire as a narrator cries: "We are dying. Where are the Arabs?"
The Arab League has tried to pressure Assad into a peace process with the opposition, but he has refused.
In another possible shift away from Assad, about 500 Palestinians gathered in Gaza at a Hamas-authorized demonstration in solidarity with Syrian protesters.
Assad has long hosted and supported leaders of the Islamic Hamas movement, which rules Gaza. But as the body count in Syria continues to rise, Hamas has been trying to distance itself from Damascus. Hamas has forged closer ties with rich Gulf states that oppose the Syrian regime and seeks to undercut Iran's influence.
A planned international meeting later this week in Tunisia will seek ways to help the Syrian people.
"People don't care if it's the devil intervening to save us from Bashar.
We need the world's help," Shaker said.
In Moscow, Russian Foreign Ministry spokesman Alexander Lukashevich said Tuesday it will not attend the planned "Friends of Syria" meeting because organizers did not invite Syrian government representatives.
Russia and China have vetoed two U.N. Security Council resolutions backing Arab League plans aimed at ending the conflict and condemning Assad's crackdown on protests that killed 5,400 people in 2011 alone, according to the U.N. Hundreds more have been killed since, activist groups say. One of the groups puts the toll at more than 7,300.
Lukashevich said the meeting wouldn't help a dialogue, saying that the global community should act as friends of all the Syrian people, not just one part.
"It looks like an attempt to forge some kind of international coalition like it was with the setting up of a 'Contact Group' for Libya," he said.
Russia has said it will block any U.N. resolution that could pave the way for a replay of what happened in Libya. In that case, Russia abstained from a vote, which cleared the way for months of NATO airstrikes that helped Libyans end Moammar Gadhafi's regime.
In Jerusalem, Sen. John McCain condemned Russia and China for vetoing sanctions against Syria, saying their action was "not the behavior of mature nations." He suggested that weapons should be sent to those fighting the regime.
Iran -- Syria's other strong ally -- sent two warships through the Suez Canal on Tuesday on their way back from the Syrian port of Tartus. The ships had reportedly docked in Syria over the weekend on a mission to provide training for Syria's naval forces, according to Iranian media reports.
The Pentagon disputed those reports, saying there was no indication the ships had docked or delivered any cargo. U.S. Defense Department spokesman George Little said the Iranian ships now appear to be going back through the Suez Canal again.
Assad has announced a Feb. 26 referendum on a new constitution. The charter would allow a bigger role for political opposition to challenge Assad's Baath Party, which has controlled Syria since a 1963 coup. But leaders of the uprising have dismissed the referendum as an attempt at superficial reforms that do nothing to break the regime's hold on power.
In Jordan, Bernardino Leon, the EU's representative for the Southern Mediterranean, said Assad's regime missed the opportunity for reforms. "Syria is definitely not in a transition despite announcements of changes, despite plans for a referendum," Leon told reporters.


text:
Be the first to review this product
Availability: In stock
Same day dispatch available if ordered before 2pm.
Call us now for more info about our products.
Straightforward return and warranty policy
Free mainland UK shipping for orders over £10
Only registered users can write reviews. Please, log in or register
Use spaces to separate tags. Use single quotes (') for phrases.
Satsecure are specialists in the CCTV & satellite industry with over 30 years combined experience. We have stores in Stockport and Leicester, and we aim to provide top quality products at the most competitive prices available online. You can contact us for further advice on any of the products that you've seen advertised.
Call us0161 4303849
Email us
Click here to send us an email
Find usClick here for our Store Locator


text:
Servicios Personalizados
- Citado por SciELO
- Accesos
Links relacionados
- Citado por Google
- Similares en SciELO
- Similares en Google
Revista chilena de historia natural
versión impresa ISSN 0716-078X
Rev. chil. hist. nat. v.78 n.2 Santiago 2005
Revista Chilena de Historia Natural
78: 323-336, 2005
An evolutionary frame of work to study physiological adaptation to high altitudes
Un marco conceptual para estudiar adaptaciones fisiológicas a altas altitudes
ENRICO L. REZENDE1*, FERNANDO R. GOMES1, CAMERON K. GHALAMBOR2, GREGORY A. RUSSELL1,3 & MARK A. CHAPPELLl1
1Department of Biology, University of California, Riverside, California 92521, USA
2Department of Biology, Colorado State University, Fort Collins, Colorado 80523, USA
3University of California White Mountain Research Station, Bishop, California 93514, USA
*E-mail for corresponding author: email@example.com
How complex physiological systems evolve is one of the major questions in evolutionary physiology. For example, how traits interact at the physiological and genetic level, what are the roles of development and plasticity in Darwinian evolution, and eventually how physiological traits will evolve, remains poorly understood. In this article we summarize the current frame of work evolutionary physiologists are employing to study the evolution of physiological adaptations, as well as the role of developmental and reversible phenotypic plasticity in this context. We also highlight representative examples of how the integration of evolutionary and developmental physiology, concomitantly with the mechanistic understanding of physiological systems, can provide a deeper insight on how endothermic vertebrates could cope with reduced ambient temperatures and oxygen availability characteristic of high altitude environments. In this context, high altitude offers a unique system to study the evolution of physiological traits, and we believe much can be gained by integrating theoretical and empirical knowledge from evolutionary biology, such as life-history theory or the comparative method, with the mechanistic understanding of physiological processes.
Key words: adaptation, evolutionary processes, natural selection, life-history, oxygen availability, phenotypic plasticity.
Una de las preguntas más importantes en fisiología evolutiva es como evolucionan los sistemas fisiológicos complejos. Por ejemplo, actualmente sabemos poco sobre la interacción entre varios rasgos a niveles genéticos y fisiológicos, sobre el papel de la plasticidad fenotípica durante distintas etapas del desarrollo y madurez para la evolución fisiológica dentro de un linaje. En este trabajo explicamos el marco conceptual ocupado por fisiólogos evolutivos en la actualidad para estudiar adaptaciones fisiológicas a nivel evolutivo y el papel de la plasticidad dentro de la evolución Darviniana. Citamos ejemplos de como la integración de la fisiología evolutiva y del desarrollo nos permitió un mayor entendimiento de como vertebrados endotérmicos pueden adaptarse a altas altitudes. Los organismos de alta altitud ofrecen un excelente sistema para estudiar la evolución de rasgos fisiológicos, y hay mucho por aprender en ese contexto al integrarse el conocimiento teórico y empírico de la biología evolutiva, tales como teoría de historia de vida o el método comparativo, con el conocimiento mecanicista de los procesos fisiológicos.
Palabras clave: adaptación, procesos evolutivos, selección natural, historia de vida, disponibilidad de oxígeno, plasticidad fenotípica.
Studying metabolic adaptations to high altitude (e.g., Rosenmann & Morrison 1975) provides a very interesting and useful model to understand the evolution of complex physiological systems, for many reasons. First, the selective pressures involved are known: hypoxia and cold (lower hypobaria or higher atmospheric radiation seem to be less relevant for birds or mammals; Monge & León-Velarde 1991). Second, considerable research on the physiological basis of aerobic performance has been done and the transport of O2 from lungs to tissues have been carefully described (Richardson et al. 1999, Bassett & Howley 2000). Third, we now have a better understanding of the genetic basis of aerobic metabolism and its physiological correlates through the use of quantitative genetics and selection experiments (e.g., Dohm et al. 2001, Nespolo et al. 2003). Finally, the tight association between aerobic capacity and sustained locomotor and/or thermoregulatory performance in endotherms makes the former a likely target of selection, which has been recently shown to be the case in wild deer mice in natural habitats (Hayes & OConnor 1999).
Despite considerable research on the topic, many questions about how organisms adapt to high altitudes remain. For example, the relative contributions of genetic, maternal, developmental and environmental factors determining the adult phenotype (and how these factors interact) are not fully understood (e.g., Brutsaert 2001, Rupert & Hochachka 2001). How animals respond differently to variable altitudes given their inherent nature and evolutionary history (e.g. mammals do not have feathers), and which responses are adaptive in the true Darwinian sense may depend on several factors (e.g., Garland & Adolph 1991, Garland & Carter 1994, Feder et al. 2000). In light of the current knowledge of evolutionary biology physiologists acknowledge that not all traits are adaptive, and past history, genetic structure of the population, among many other factors, can influence for physiological patterns observed in different conditions. Here we attempt to (1) summarize the current framework on how to study physiological adaptation, given the background physiologists now have from evolutionary biology; (2) demonstrate how this framework can be applied in the study of altitude adaptation and; (3) provide incentive those students interested in physiology to learn not only on how animals work, but also how physiological systems evolve.
Cardiac output and the concept of adaptation
Endotherms inhabiting high altitudes face a double challenge: they must thermoregulate and power activity in an environment where both temperature and oxygen partial pressures are low. Physiologists have implicitly assumed that maximizing O2 delivery to the tissues in hypoxic environments is adaptive. However, even if this is the case, adaptation and its various definitions must be differentiated because of their underlying causes and the levels in which they occur (see also Monge & León-Velarde 1991). Consider the following hypothetical example: a population of mammals inhabiting lowlands colonizes higher altitudes. Initially, as animals move higher, heart rate and stroke volume increase to provide more O2 to the tissues, and these changes are perceived as adaptive because without them, animals would have asphyxiated as O2 availability decreased. Phenotypic plasticity would account for this pattern (e.g., physiological changes occurred within each individuals lifetime), and therefore higher cardiac output would be a physiological adaptation (more precisely, an acclimatory response) to increased altitudes. An animals physiology can change within seconds (e.g., during sprint running), days or months (acclimatization), or during the course of development.
In the same hypothetical population, however, chronically high cardiac output is detrimental in the long-term because of hypertension and cardiac diseases, and individuals with this phenotype are selected against in the course of many generations. In an evolutionary perspective, high cardiac outputs would therefore be maladaptive (i.e., decreasing overall fitness), and selection would favor those individuals able to provide enough O2 to the tissues without higher workloads. If this phenotype has an underlying genetic component (and assuming that hypertense individuals had a lower probability of survival before they could pass their genes on), allelic combinations allowing for higher metabolic rates sustained by a lower cardiac output would increase in frequency in the population. These are true adaptations in the Darwinian sense, and we shall refer to those as genetic adaptations, which can be defined as changes in the mean phenotype of a population due to changes in gene frequencies as a result of natural selection. (Genetic drift and founder effects, also involved in phenotypic evolution through genotypic changes, are considered by evolutionary biologists as non-adaptive processes; see below).
Darwins proposition of natural selection as the evolutionary process explaining biological adaptation relied on three major observations: species do not grow exponentially through time as predicted (Malthusian growth), animals from a single population are not all alike (e.g., many populations have considerable individual variation), and this variation is in part heritable. With competition and variation, organisms with traits that would increase their survival and reproductive success (i.e., fitness) would contribute differentially to the next generation. After several generations, that trait would be more frequent in the population and could eventually become the norm. Although the concept may seem trivial, the origin of species was a huge collection of information emphasizing the complexity of evolutionary processes and patterns (even more when heredity mechanisms were a matter of speculation). Natural selection, however, was and remains (e.g., Nespolo 2003) the underlying common mechanism responsible for the origin of biological adaptations.
Today, the study of adaptation through natural selection relies on a slightly modified framework (see Nespolo 2003). First, phenotypic variation is the result of genetic (additive and non-additive) and environmental components (Fig. 1); the proportion of the phenotypic variance of a trait due to the (additive) genetic variation in the population is the heritability of that trait (Falconer 1989, Roff 1997, Rupert & Hochachka 2001). Second, the rate of evolution of a trait in response to selection will be proportional to the additive genetic variance of that trait (fundamental theorem of natural selection, Fisher 1930), and the intensity of selection. With this framework, modern evolutionary geneticists now have a robust body of knowledge on how several traits coevolve, the evolution of life histories and complex phenotypes (see Roff 1997, 2002 for reviews). In this context, several aspects may affect how animals will (or will not) adapt to their environment (i.e., high altitude) in a Darwinian sense.
Evolutionary history
Evolutionary responses to a given selective pressure will first depend on the nature of the organism/lineage under selection. Many traits may be present in a lineage not because it is adaptive to current conditions, but due to past events of selection, genetic bottlenecks, drift, etc. Nevertheless, evolutionary history has been practically ignored by many comparative physiologists (see Garland & Adolph 1994). Recent advances in the comparative method (see below), for instance, highlight how misleading studying Darwinian adaptation without considering the phylogenetic history of a lineage can be (e.g., Felsenstein 1985, Garland & Ives 2000, Rezende & Garland 2003).
Because of evolutionary history, many physiological responses may be deleterious or pathological when colonizing a different environment. For example, physiologists initially considered decreased Hb-O2 affinities and increased red blood cells in humans as beneficial adaptations to high altitudes, whereas now physiologists acknowledge that these responses may simply not be adaptive (Monge & León-Velarde 1991, see Physiology and life-history section below). Furthermore, it is now acknowledged that high Hb-O2 affinity is characteristic of hypoxia tolerant species, and such pattern has been observed across widely divergent vertebrate species (Hopkins & Powell 2001).
The opposite may also be the case, and some traits already present in a lineage may facilitate the movement and survival of that lineage in a new niche or environment. These are considered exaptations (Gould 1991); although they are adaptive (i.e., increasing overall fitness) to these new conditions, they evolved in that lineage before this new selective regime was encountered. For example, the high Hb-O2 affinity observed in South American camelids (llamas and vicuñas) was initially thought to be an adaptation to high altitudes. Studies in Old World camels and dromedaries now show that high Hb-O2 is a common trait in the entire family, being present in the lineage before the colonization of the Andes, suggesting that camelids in South America were preadapted to colonize high altitudes (Monge & León-Velarde 1991). (Many physiologists refer to exaptations as preadaptations, in spite of the misleading connotation of evolving structures in anticipation to future need).
Population structure and genetic background
Knowledge of the structure of a population and its genetic composition is now essential to understand phenotypic change on a microevolutionary scale (e.g., Grant & Grant 1995). Natural selection will act upon the variation already present in the population, and evolution will occur when that variation is to some extent hereditary. Although optimality models have been applied quite successfully to study phenotypic evolution (e.g., Alexander 1982, Roff 2002), evolution ultimately depends on population structure and the genetic background of that population. Indeed, quantitative genetic models show that, although correlated traits could eventually evolve to their optimal, it may take several to many generations to attain that optimal in response to selection (Roff 1997).
Population structure can affect phenotypic evolution in several ways. First, it ultimately determines how many alleles may be immersed in the populations gene pool, setting an upper limit to allelic variation. Second, as the effective population size decreases, genetic drift becomes increasingly important in determining evolutionary trajectories, and it may overcome the effects of natural selection as population size decreases. Third, immigration may provide enough gene flow to counteract the effects natural selection. This factor is particularly important in studies of altitude adaptation. Geographically isolated populations at high altitude may be under strong selection, and still not evolve or adapt because additional genes are flowing from low lands (see also Comparative method section below). In the hypothetical example above, for example, the scenario would be different if there was a high flow of individuals from the source population many carrying genes potentially detrimental in the long-term to high lands. According to Monge & León-Velarde (1991), natural selection does not seem to have operated in humans as much as in other high-altitude animals, probably due to their migratory habits (see also Brutsaert 2001).
Genes carry the information from one generation to another, and much physiological variation has a genetic component. For example, by crossing two inbred lines of laboratory mice, McCall & Frierson (1997) found that the inheritance of running performance in hypoxia (hypoxic exercise tolerance) is consistent with expectations from a two-locus segregation model. At the population level, how information is passed on in each generation will ultimately determine the trajectories of phenotypic evolution. According to the fundamental theorem of natural selection, populations lacking additive genetic variance (e.g., high levels of inbreeding) will have a negligible response to selection. Genetic correlations may lead to co-adaptation of several traits in response to a single selective pressure, even when these traits do not provide any increased fitness (Lande & Arnold 1983). As a case study, Rezende et al. (2004a) have shown that rodents from cold environments can attain higher maximum metabolic rates (MMR), and suggested that basal or resting metabolic rates (BMR) might be also higher in colder climates because both metabolic indexes were positively correlated. On the other hand, negative correlations (genetic trade-off) may constrain or delay the overall response to selection, and lead to adaptive valleys in the fitness landscape (i.e., certain combination of traits would have lower fitness).
Identifying selective pressures
High altitude environments impose two major constraints for endothermic organisms: cold temperatures, and hence higher thermoregulatory requirements, and low O2 availability. In addition, local factors may be influencing phenotypic evolution and must also be considered. For example, one would expect different intensities of selection (and evolutionary rates, if there is enough genetic variation) depending on the predatory regime populations encounter (e.g., Reznick & Bryga 1987). To understand how these selective pressures are acting in a population is considerably more difficult, however, and many factors should be considered.
How organisms interact with their biotic and abiotic environment (behavior and/or ecology, in a broad sense) varies among individuals, populations and species, and will ultimately determine the nature and intensity of selection, as well as its spatial and temporal patterns. For example, although lower atmospheric pressures may constrain metabolic power output in any aerobic organism, reduced air density becomes an additional challenge for hummingbirds due to their hovering flight (Altshuler & Dudley 2002). Mortality during winter would vary in intensity depending on whether individuals migrate to lower altitudes and warmer environments, hence selection could account for higher thermogenic capacities or hibernation in gregarious high-altitude populations, or increased ability to avoid cold temperatures in migrant populations. Behavior plays a crucial role in phenotypic evolution, therefore, and its importance in the evolution of physiological systems is now explicitly acknowledged (e.g., the centrality of organismal performance paradigm; Garland & Carter 1994, p. 593).
Selection may also be acting throughout ontogeny, and a single selective pressure may affect fitness functions of similar genotypes in many different ways. Many studies have reported high mortality rates in avian embryos relocated to high altitudes, for instance. Beattie & Smith (1975) described an overall increase in egg hatchability from 16 to 56 %, after raising chickens from six generations in high altitude, highlighting how strong selection may be during ontogeny and how fast populations may respond in an evolutionary scale. In this context, physiology should evolve to decrease mortality during development and still lead to viable adult phenotypes, and novel developmental trajectories may be selected as a new environment is colonized (see Developmental plasticity section below).
Physiology and life-History
Understanding how selection shapes physiological traits and leads to adaptive systems ultimately depends on the linkages between an organisms physiology and its life history. Life history traits are those traits that contribute directly to the number of offspring an organism produces over the course of its lifetime. Commonly measured life history traits include the size and age when organism begins to reproduce, how often it reproduces, the number and size of offspring, and the relative allocation of time and energy to reproduction, versus growth and maintenance (Roff 2002, Stearns 1992). Life history traits thus make up the major components of Darwinian fitness and variation in life history traits amongst individuals in a population determine which genotypes will be represented in subsequent generations.
Physiology and life history are intimately related to each other because it is largely through physiological mechanisms that selection acts to produce adaptive strategies for allocating limited resources to the competing functions of growth, survival, and reproduction (Stearns 1992). Physiological adaptations for survival can thus place constraints on, or come at a cost to, the kinds of life history strategies that evolve. From this perspective, the evolution of physiological systems and life history strategies share the common feature that adaptations are embedded in a complex phenotype that incorporates trade-offs between integrated traits (see also Ghalambor et al. 2004). Comparisons of physiology and life history strategies between low and high altitude populations or species provide a particularly good framework for investigating these linkages because both physiological and life history traits often exhibit predictable patterns of variation across altitudinal gradients. Yet, few attempts have been made to critically examine the linkages between adaptive changes in physiological and life history traits.
Do physiological adaptations to breeding at high altitudes constrain the kinds of life history strategies that evolve? While such constraints can be predicted on theoretical grounds (Hayes et al. 1992), few empirical examples exist, although various lines of evidence from endothermic vertebrates suggest linkages between physiological and life history strategies. An examination of life history strategies of birds occupying low and high altitudes shows a repeated pattern of reduced fecundity and increased parental care at high altitudes (Badyaev 1997, Badyaev & Ghalambor 2001). The increase in parental care at high altitudes is driven primarily by an increase in the amount of male contribution to the incubation, nestling, and post-fledging periods. Badyaev & Ghalambor (2001) argue that this pattern arises because at high altitudes the colder temperatures, reduced food availability, and greater climatic unpredictability negatively impact juvenile survival, thus favoring a life history strategy of producing fewer offspring of higher quality as a buffer to these environmental conditions. Alternatively, or contributing to this pattern could be that the increased energetic costs of breeding at high altitude (Weathers et al. 2002) reduces the amount of energy available for investment in offspring number. Evaluating such physiological constraints will require considerable more research on the physiological variability of birds along altitudinal gradients.
In contrast to birds, physiological adaptations to high altitude environments are much better known in mammals, whereas life history variation is less well understood. However, mammal life histories do appear to show a similar pattern of reduced fecundity at higher elevations as observed in birds (e.g., Bronson 1979, Zammuto & Millar 1985). One of the few attempts to explore the conflicts between physiological and life history strategies is work summarized by Wynne-Edwards (1998) on the closely related dwarf hamster species in the genus Phodopus of central Asia. One species, P. campbelli occurs at higher elevations, experiences colder and more arid conditions and has a lower critical maximum temperature compared to a closely related species P. sungorus (Wynne-Edwards 1998). The less extreme habitat of P. sungorus allows females to rear litters alone, whereas in P. campbelli bi-parental care is necessary to alleviate thermoregulatory and water balance stresses on the female at higher elevations (Wynne-Edwards 1998). In this case, it appears that physiological adaptations for survival to a more extreme environment, place constraints on the kind of reproductive strategies favored by selection (Wynne-Edwards 1998). Thus, as is observed in birds, male Phodopus act to alleviate the challenges imposed by high altitudes on females attempting to rear offspring on their own. While it remains unknown whether similar physiological pathways shape avian life histories at high altitude, examination of the joint evolution of physiological and life history strategies offers a useful framework for examining the linkages and constraints between these complex systems.In a more holistic perspective, physiology is always under selection: many physiological dysfunctions, at virtually any level of organization, may compromise homeostasis leading to decreased fitness or death (physiologists often take this for granted). The inherent complexity of physiological systems and their multifunctional nature is, therefore, a central component to be considered when studying adaptation. The cardiovascular system, for example, must deliver O2, hormones and energetic substrates to tissues, remove CO2 to lungs and metabolic wastes to kidneys, etc. Furthermore, regulatory systems must ensure adequate oxygenation of different tissues with variable metabolic workloads (e.g., rest or exercise), in a wide range of conditions, responding at different temporal scales (acute or chronic stimulus) with many different constraints.
Physics is the first factor constraining physiological function. A mouse cannot have the same absolute cardiac output of a whale simply because of size, and lungs will lose water during respiration through the same diffusive process by which O2 eventually reaches tissues. In a similar way, many physiological processes such as membrane potential, or blood pH - will have narrow functional ranges, potentially constraining increased performance. Constraints also occur when a single physiological system is responsible for maintain properly different functions (trade-offs). In very cold and hypoxic environments, for example, mammals attempt to maintain both oxygenation and body temperature, although conflicts can arise because of the respiratory heat loss associated with the increase in ventilation (Mortola & Frappell 2000). Finally, given the non-linear behavior of many physiological functions, a response could be adaptive when it is moderate, and detrimental or pathological as it increases. For example, although a higher hematocrit may increase blood O2 carrying capacity, abnormally high values could be pathological (e.g., polycythemia) because of increased blood viscosity.
Non-Mendelian parental effects
Phenotypic variation will depend on genetic and environmental components, and how these factors interact. In this context, the phenotype of the offspring may be dependent not only on its own genotype, but also on the phenotypes of their parents (predominantly the mother). Maternal effects may or may not have a genetic component, having different sources: (i) the mitochondria or in cytoplasmatic factors inherited in the egg (e.g., more or less yolk to nurture the embryo), (ii) genetic differences underlying variation in parental care, and (iii) environmental conditions experienced by the mother may affect its contribution to the offsprings phenotype (Roff 1997, p. 241). In mammals, for example, the capacity of providing O2 to the embryos during gestation, and the females lactation performance (e.g., phenotype of the mother) may affect drastically the offsprings body size (and related life-history traits), and probably has a genetic component. However, a starving pregnant female cannot nourish her offspring, and this source of variation is entirely environmental.
Although most of the knowledge in ecological physiology has been built on studies comparing phenotypes of different individuals, populations, or species (see Comparative Method below), an additional level of variation must be considered: phenotypic changes within the lifetime of each individual (Fig. 1). Such changes are possible because the phenotype is a product of the interaction between genes and the environment. The set of phenotypic expression of a single genotype in response to naturally occurring (or experimentally imposed) environmental variation is called phenotypic plasticity. An environmental stimulus can change a phenotype both by (i) short-term modulation of the pre-existing physiological and biochemical systems (e.g. changing the concentration of modulators of the enzyme-substrate affinity, or altering the membrane fluidity through changes in its constituents), and/or by (ii) changing gene expression. Genetic adjustments usually take a longer time to be fully expressed and include (i) altering the concentration of the same enzyme isoforms or (ii) expressing different isoforms with different catalytic properties (Hochachka & Somero 2002).
Fig. 1: Schematic diagram showing the potential effects of plasticity – either through different developmental trajectories, or because of short-term acclimation or acclimatization – on phenotypic differences between species or populations (see also Garland & Adolph 1994). (A) Two populations genetically different (i.e., population 2 is in average smaller than 1) develop in environments with different partial pressures of O2 (PO2). In both hypothetical populations, hypoxia leads to smaller body sizes, either by affecting growth rates during development (often irreversible effects, large arrows), or through modulations in food ingestion and activity levels during adulthood (reversible effects, small arrows with asterisks). (B) Expected distribution of size in each species when phenotypic measurements are performed completely 'at random' (e.g., different ages, sexes, developmental conditions, acclimated to either hypoxia or normoxia, etc). (C) Expected variation when both populations are studied under 'common garden' conditions (i.e., at similar PO2; all variance in this case is genetic or within-individual variation, because environmental variance is close to 0). (D) Hypothetical pattern if subsets of the populations developed at different controlled PO2 (N = normoxia, H = hypoxia). Note that specific developmental trajectories may constrain irreversibly the adult phenotype (dotted arrows), and groups developing in N and H within a population do not overlap despite of their genetic similitude (polyphenisms, Huey & Berrigan 1996). (E) Hypothetical diagram of reversible plasticity (solid arrows), when both populations developed under controlled conditions and are submitted to different PO2 when adults (short-term acclimation or acclimatization experiments).
Several studies have described how environmental stimuli can alter developmental trajectories. This developmental plasticity may be crucial, since it tends to produce long-term and often permanent phenotypic changes (Spicer & Gaston 1999, Wilson & Franklin 2002, Spicer & Burggren 2003). In humans, for example, several studies support an important developmental component explaining the bigger thoracic dimensions observed in populations at high altitudes, as well as in people growing up with untreated chronic respiratory diseases such as asthma (Monge & León-Velarde 1991). Some authors claim that it is neither the genes themselves nor the discrete adult phenotypes that are main the target of natural selection, but the interaction between the genes and the environment through the ontogenetic trajectory that determine the capacity to deal with the environmental contingency (Schlichting & Pigliucci 1993, 1995, McNamara & Houston 1996).
The sensitivity of the phenotype to a particular environmental stimulus changes during development (Spicer & Burggren 2003). If specific challenges are presented during sensitive periods of the ontogeny, often called critical windows, the responses can be particularly influential on the adult phenotype. One example is the large and irreversible effects on adult respiratory patterns caused by a brief exposure to hypoxia during the first few weeks after birth in rats (Strohl & Thomas 1997). Maternal effects (above) can be very important, because the environment the mother provides (e.g., an appropriate nesting site in egg-laying species) will determine how the embryo develops. In humans, maternal residence at high altitude promotes a decrease in birth weight when compared to sea-level pregnancies, possibly due to a reduction of the uteroplacental and fetal volumetric blood flows (despite of the compensatory response of increased placental angiogenesis under such conditions). This constraint has been linked to a downregulation of fetal growth reflected in decreased materno-fetal circulating growth factors, placental nutrient transport, and fetal nutrition (Zamudio 2003).
During adulthood a short to medium-duration exposure to a different environment can also promote phenotypic changes. These events of phenotypic plasticity are usually referred as acclimatization when they occur in response to environmental changes in nature, and as acclimation when they are experimentally induced by environmental manipulation in the laboratory. Unlike developmental plasticity, such changes can be at least partially reversible. For example, reversible seasonal acclimatization can allow an individual to tolerate temperatures in winter that would be lethal during summer and vice versa. As discussed above, many factors could account for non-adaptive plastic responses to environmental changes. Furthermore, it is important to remember that the ability of the regulatory systems to respond to a particular environmental stimulus may have evolved in a context different from the one under study. For example, responses to tissue hypoxia and their underlying regulatory systems may have evolved to deal with exercise instead of high altitudes, and many of these responses might be deleterious when chronic.
Diagrama representando como la plasticidad durante el desarrollo o aclimatación y aclimatización puede afectar comparaciones entre dos especies o poblaciones. (A) Dos poblaciones genéticamente distintas (i.e., la población 2 es en promedio mayor que la 1) se desarrollan en ambientes con distintos PO2. En las dos poblaciones hipotéticas, la hipoxia tiene a disminuir el tamaño corporal, ya sea cambiando las tasas de crecimiento durante el desarrollo (efectos comúnmente irreversibles, flechas grandes), o al afectar la ingesta o actividad durante la madurez (efectos reversibles, flechas chicas con astersiscos). (B) La distribución de tamaño corporal esperada si las comparaciones entre poblaciones o especies se hace sin controlar ninguna otra variable (e.g., se agrupan distintas edades, sexos, distintos PO2, etc). (C) Variación esperada si se compara las poblaciones en un ambiente controlado similar (common garden; toda la varianza fenotípica es genética o intra-individual, ya que la varianza ambiental es cercana a 0). (D) Patrón esperado si muestras dentro de cada población se desarrollan en PO2 contrastantes (N = normoxia, H = hipoxia). Nótese que las trayectorias de desarrollo específicas pueden generar diferencias fenotípicas irreversibles (líneas punteadas). (E) Diagrama hipotético mostrando la variación fenotípica cuando la plasticidad es reversible (líneas sólidas), como en experimentos de aclimatación o aclimatización en animales adultos.
Rates of acclimation can be highly variable, not only among individuals, but also between different traits in a single individual (e.g., Rezende et al. 2004b), and many of the factors underlying physiological responses must have a genetic component. Whether phenotypic plasticity can be considered a character itself, under genetic control at least partially independent of the mean phenotypic value, has been a major point of debate. Empirical support to the view of phenotypic plasticity as an independent character has come from studies demonstrating independent evolution of trait means and plasticities (e.g., Huey & Berrigan 1996), as well as from experiments showing that phenotypic plasticity is responsive to selection (Schlichting & Pigliucci 1998, for a review).
Multiple levels of organization from genes to ecosystems must be considered when studying Darwinian evolution and adaptation. Although proving that a particular trait is an adaptation may be logistically difficult (i.e., by rejecting all other alternative hypotheses), physiologists now have tools to approach evolutionary problems more rigorously. The fundamental problem lies in demonstrating that (1) allelic changes underlying a particular trait were the result of selection, and (2) the correlation between that trait, performance and ultimately fitness. Only by combining different techniques, from organismal physiology to population genetics, can one study these two factors (Fig. 2). We summarize current methods employed to infer adaptive changes in response to selection, highlighting studies in deer mice (Peromyscus), given widespread geographic distribution of this genus, and the variety of techniques employed to study this rodent models evolutionary history and altitude adaptation.
Fig. 2: Diagram showing three of the major approaches used to study evolutionary physiology. (1) The 'gene to phenotype' approach consists in screening for candidate genes affected by natural selection, and then studying the physiological and ecological mechanisms explaining the observed differences in allelic frequencies for that gene. (2) The 'phenotype to gene' approach initially identifies which phenotypes have evolved as true adaptations (e.g., comparative method, selection experiments), approaching the problem at lower levels of organization until the identification of the genetic architecture underlying these adaptations. (3) Factorial experiments, studying the interaction between genotypes (in this example, being the same) and the environment, can be used to study plasticity during both development and adulthood. (All approaches can be applied at different levels of organization, from whole individual performance to lower levels such as Hb-O2 affinity, cardiac output, etc).
Comparative method
From Darwin's time to the present, comparing species or populations has remained the most general technique for addressing questions about long-term patterns of evolutionary change. In five species of Peromyscus reared under common garden conditions, BMR was significantly correlated with habitat primary productivity (Mueller & Diamond 2001); and negatively correlated with temperature in 31 deer mice populations (although animals were not reared in common garden, MacMillen & Garland 1989). These results suggest that BMR has evolved in response to different environmental productivity and temperature (which is correlated with altitude). After controlling for acclimatization effects by comparing species in similar thermal environments, Hayes (1989) showed that populations of Peromyscus from high altitude have higher BMR than those from low altitude, suggesting that metabolism has evolved with altitude.
The comparative method has radically changed in recent years (e.g., Garland & Adolph 1994) and with the development of analytical methods that incorporate phylogenetic information and use explicit models of character evolution to allow statistical inference (see Rezende & Garland 2003, Garland et al. in press, for reviews). Importantly, many additional questions can now only be conceived in a phylogenetic context. Randomization methods developed to estimate phylogenetic signal (the tendency of more related species to resemble each other, Blomberg et al. 2003), for instance, can provide insights on how geographical factors may have affected historical patterns of speciation and vicariance. Among 57 species of rodents, altitude did not show significant signal (whereas latitude signal was highly significant) and was positively correlated with MMR (Rezende et al. 2004a); which suggests that (i) altitude was not a major geographical barrier to migration or colonization at that temporal scale (e.g., thousands to millions of years of divergence), and (ii) there is selection for increased MMR at higher altitudes, probably due to lower environmental temperatures.
Selection experiments and quantitative genetics
Quantitative genetics studies how quantitative traits are inherited in a population, and ultimately the role of the genetic background in phenotypic evolution (Roff 1997). Evolutionary physiologists are now studying, for instance, the underlying genetics of hypoxic tolerance in laboratory strains of Mus (see above), as well as aerobic performance and subordinate traits in laboratory and wild rodents (Dohm et al. 2001, Nespolo et al. 2003, Bacigalupe et al. 2004). These studies have reported very low heritabilities and additive genetic variance for BMR and MMR (during cold or exercise), and no genetic correlations between traits (although new results suggest a heritability of about 0.6 for MMR in a population of Phyllotis darwini, Nespolo personnel communication). These studies require very large sample sizes and complex breeding designs. Another approach relies on measurements of individual consistency (repeatability) of a trait in a population (e.g., how consistent are the differences between individuals, Hayes & Jenkins 1997). Repeatability is a prerequisite for natural selection to affect trait variation, and it may set the upper limit on the narrow sense heritability of the trait (Dohm 2002).
Selection experiments allow researchers to study evolution of complex phenotypes in action, under controlled conditions. Major advantages of selection experiments over comparative studies include: (i) evolutionary inferences do not rely only on a correlational approach, and (ii) environmental factors can be either controlled or included in the design (genotype by environment interaction). Because previous authors have already reviewed how selection experiments can be employed for studying physiological evolution and adaptation to hypoxic environments (e.g., Garland 2001, 2003), we will not address the topic in more detail here.
Measuring selection in nature
Studying natural selection in action in the field, and subsequent phenotypic evolution (e.g., Grant & Grant 1995), is an ultimate goal of evolutionary physiology. Very few studies, however, have attempted to measure whether selection acts on individual variation in physiological traits and overall performance in natural populations (e.g., Jayne & Bennett 1990). The basic protocol to study selection in performance is as follows. Some estimate of performance (e.g., running speed or endurance) is measured in a cohort of individuals of known age, which is then released in the field and the survivors are recaptured some time later. Several statistical procedures allow determining whether the probability of survivorship is correlated with a particular estimate of performance. Using this approach, Hayes & OConnor (1999) reported significant directional selection for higher MMR in a high altitude population of Peromyscus, during one particularly cold winter. (Note that recapture rates are not necessarily a good index of survivorship or fitness, if emigration occurs).
An alternative approach consists of comparing allelic or allozyme variation in different population (i.e., comparative study) to infer selection from patterns of geographic variation. In this case, candidate genes to be under selection are first identified, their contribution to phenotypic variation and ultimately fitness would then be addressed (Fig. 2). For instance, Peromyscus show an array of hemoglobin (Hb) polymorphisms, which are inherited basically as two different haplotypes (i.e., similar to what is expected with Mendelian inheritance of a single locus with two alleles), and haplotype frequencies are correlated with altitude (Snyder et al. 1988). Further research showed that individuals with the high-altitude haplotype had increased Hb-O2 affinities, and higher MMR during cold-exposure or forced exercised when measured at high altitudes, whereas the opposite was observed in the low-altitude haplotype (Chappell & Snyder 1984). Again, different lines of evidence support that the correlation between allelic frequencies and altitude may be due to local adaptation.
Esquema mostrando tres metodologías para estudiar la evolución de rasgos fisiológicos. (1) Del gen al fenotipo se refiere a aquellos estudios donde primero se buscan genes potencialmente afectados por la selección natural, seguidos por estudios fisiológicos y ecológicos que explicarían mecanísticamente los cambios alélicos en esos genes. (2) Del fenotipo al gen se refiere a estudios que primero intentan determinar los fenotipos que evolucionaron como adaptaciones Darwinianas, para después elucidar la estructura genética que conlleva a esos fenotipos. (3) Experimentos factoriales, que estudian la interacción entre el genotipo (que es constante en nuestro ejemplo) y el ambiente, pueden ocuparse para estudiar plasticidad fenotípica durante el desarrollo o madurez. (Cualquiera de esos métodos puede ser aplicado para estudiar rasgos fisiológicos a distintos niveles de organización
Similarly, population geneticists can now determine candidate genes affected by selection, by comparing frequencies across several loci. As summarized by Storz & Nachman (2003), the basic idea is that, if allelic variation at most loci is simply tracking stochastic demographic processes, loci under selection should produce a detectable signal against the genome-wide backdrop of neutral variation. After a multilocus survey of allozyme variation in Peromyscus populations across a steep altitudinal gradient, Storz and colleagues have suggested that the albumin locus (or perhaps a closely linked gene) is a candidate for local adaptation. Nevertheless, albumin is associated with several physiological processes, and how variation at this locus translates in differences in physiology, performance and fitness, is not clear (Storz & Dubach 2004).
Developmental physiology and acclimation experiments
Because the period of development is such a complex series of processes that results in a fully-functioning individual, it is important to understand how environmental perturbations can affect organisms during this period (Spicer & Gaston 1999). Challenges occurring early in development may have quite different effects than those occurring at later life stages, since different organ systems have different critical windows during which environmental perturbations will exert maximal effects (Dzialowski et al. 2002). Similarly, the amount of plasticity allowed for during ontogeny can define an individuals physiological capacities early during growth and development, which may have consequences for later survival (e.g., Tracy & Walsberg 2001).
In this context, factorial designs are another useful tool to study the contributions of developmental and reversible plasticity in the adult phenotype. In this sense, many experimental designs may be employed to address different questions. Specific designs can provide insights about critical windows of development, whereas other approaches may allow partitioning of the phenotypic variance on a population in developmental and reversible components, and study their relative contributions to overall plasticity during adulthood.
We emphasize that, although we have some knowledge of general effects of environmental variables in a phenotype, much remains to be done. For example, studies of reaction norms in physiological traits across many different environments, addressing physiological responses to combined stimulus (e.g., how would mice acclimate to different temperatures as O2 availability changes?), or determining the effects of the maternal environment in the offsprings phenotype, are scarce in the literature. The costs of phenotypic plasticity, or the adaptive character of many physiological responses, must be tested rigorously (Huey & Berrigan 1996).
The fields of physiology and evolutionary biology have long functioned as separate disciplines, with each providing their own important contributions to science, but without consistent attempts to examine their natural interrelationships. Comparative and environmental physiologists have been particularly successful describing (and demonstrating the functional importance of) patterns of physiological variation associated to life in challenging environments, such as high altitudes. However, a comprehensive understanding of how organisms adapt to different environments requires robust knowledge of the genetic, developmental and evolutionary mechanisms underlying these patterns of phenotypic variation. To achieve this goal, evolutionary physiologists need to embrace conceptual and methodological advances provided by evolutionary biology. Factors such as previous evolutionary history, population genetic structure, developmental plasticity, acclimatization, performance and fitness consequences of physiological variation, and the interactions of physiological systems with aspects of behavior and life-history traits need to be properly investigated in order to avoid the circular reasoning typical of the adaptationist program (Gould & Lewontin 1979). Similarly, evolutionary biologists will benefit from the quantitative mechanistic understanding of fitness-determining performance traits provided by physiological methods. High altitude offers a unique system to study the evolution of physiological traits and fortunately, several tools such as the phylogenetically corrected comparative analyses, quantitative genetic analyses, selection and factorial experiments among others, are now available to achieve this goal.
We are particularly grateful to the staff of the White Mountains Research Station at Barcroft for their constant support throughout the years; several studies cited here and performed by some of us would not be possible without their help. GAR also acknowledges a mini-grant from WMRS. ELR and CKG thank C del Agua for insights and support. This work was supported in part by NSF IBN-0111604 (KA Hammond and MAC). Finally, thanks to Mario Rosenmann for the great tutor he was, always sharing his knowledge and enthusiasm about science and life. Profe, muchas gracias por todo, y vamos a extrañarte.
ALEXANDER RM (1982) Optima for animals. Edward Arnold, London, United Kingdom. 112 pp. [ Links ]
ALTSHULER DL & R DUDLEY (2002) The ecological and evolutionary interface of hummingbird flight physiology. Journal of Experimental Biology 205: 2325-2336. [ Links ]
BACIGALUPE LD, RF NESPOLO, DM BUSTAMANTE & F BOZINOVIC (2004) The quantitative genetics of sustained energy budget in a wild mouse. Evolution 58: 421-429. [ Links ]
BADYAEV AV (1997) Avian life history variation along altitudinal gradients: an example with cardueline finches. Oecologia 111: 365-374. [ Links ]
BADYAEV AV & CK GHALAMBOR (2001) Evolution of life histories along elevational gradients: trade-off between parental care and fecundity. Ecology 82: 2948-2960. [ Links ]
BASSETT DR & ET HOWLEY (2000) Limiting factors of maximum oxygen uptake and determinants of endurance performance. Medicine and Science in Sports and Exercise 32: 70-84. [ Links ]
BEATTIE J & AH SMITH (1975) Metabolic adaptation of the chick embryo to chronic hypoxia. American Journal of Physiology 228: 1346-1350. [ Links ]
BLOMBERG SP, T GARLAND& AR Ives (2003) Testing for phylogenetic signal in comparative data: behavioral traits are more labile. Evolution 57: 717-745. [ Links ]
BRONSON MT (1979) Altitudinal variation in the life-history of the golden-mantled ground-squirrel (Spermophilus lateralis). Ecology 60: 272-279. [ Links ]
BRUTSAERT TD (2001) Genetic and environmental adaptation in high altitude natives. In: Roach RC, PD Wagner & PH Hackett (eds) Hypoxia: from genes to the bedside: 133-151. Kluwer Academic, New York, New York, USA. [ Links ]
CHAPPELL MA & LRG SNYDER (1984) Biochemical and physiological correlates of deer mouse a-chain hemoglobin polymorphisms. Proceedings of the National Academy of Sciences USA 81: 5484-5488. [ Links ]
DZIALOWSKI EM, D VON PLETTENBERG, NA ELMONOUFY & WW BURGGREN (2002) Chronic hypoxia alters the physiological and morphological trajectories of developing chicken embryos. Comparative Biochemistry and Physiology 131A: 713-724. [ Links ]
DOHM MR (2002) Repeatability estimates do not always set an upper limit to heritability. Functional Ecology 16: 273-280. [ Links ]
DOHM MR, JP HAYES & T GARLAND (2001) The quantitative genetics of maximal and basal metabolic rates of oxygen consumption in mice. Genetics 159: 267-277. [ Links ]
FALCONER DS (1989) Introduction to quantitative genetics. Third edition. Longman, London, United Kingdom. 438 pp. [ Links ]
FEDER ME, AF BENNETT & RB HUEY (2000) Evolutionary physiology. Annual Review of Ecology and Systematics 31:315-341. [ Links ]
FELSENSTEIN J (1985) Phylogenies and the comparative method. American Naturalist 125: 1-15. [ Links ]
FISHER RA (1930) The genetical theory of natural selection. A complete variorum edition. Oxford University Press, New York, New York, USA. 318 pp. [ Links ]
GARLAND T (2001) Phylogenetic comparison and artificial selection: two approaches in evolutionary physiology. In: Roach RC, PD Wagner & PH Hackett (eds) Hypoxia: from genes to the bedside: 107-132. Kluwer Academic, New York, New York, USA. [ Links ]
GARLAND T (2003) Selection experiments: an under-utilized tool in biomechanics and organismal biology. In: Bels VL, JP Gasc & A Casinos (eds) Vertebrate biomechanics and evolution: 23-56. BIOS Scientific Publishers, Oxford, United Kingdom. [ Links ]
GARLAND T & SC ADOLPH (1991) Physiological differentiation of vertebrate populations. Annual Review of Ecology and Systematics 22:193-228. [ Links ]
GARLAND T & SC ADOLPH (1994) Why not to do two-species comparative studies: limitations on inferring adaptation. Physiological Zoology 67:797-828. [ Links ]
GARLAND T & PA CARTER (1994) Evolutionary physiology. Annual Review of Physiology 56: 579-621. [ Links ]
GARLAND T & AR IVES (2000) Using the past to predict the present: confidence intervals for regression equations in phylogenetic comparative methods. American Naturalist 155: 346-364. [ Links ]
GARLAND T, AF BENNETT & EL REZENDE (in press) Phylogenetic approaches in comparative physiology. Journal of Experimental Biology. [ Links ]
GHALAMBOR CK, DN REZNICK& JA WALKER (2004) Constraints on adaptive evolution: the functional trade-off between reproduction and fast-start swimming performance in the trinidadian guppy (Poecilia reticulata). American Naturalist 164: 38-50. [ Links ]
GOULD SJ (1991) Exaptation: a crucial tool for an evolutionary psychology. Journal of Social Issues 47: 43-65. [ Links ]
GOULD SJ & RC LEWONTIN (1979) Spandrels of San-Marco and the Panglossian paradigm - a critique of the adaptationist program. Proceedings of the Royal Society of London B 205: 581-598. [ Links ]
GRANT PR & BR GRANT (1995) Predicting microevolutionary responses to directional selection on heritable variation. Evolution 49: 241-251. [ Links ]
HAYES JP (1989) Altitudinal and seasonal effects on aerobic metabolism in deer mice. Journal of Comparative Physiology 159B: 453-459. [ Links ]
HAYES JP, T GARLAND & MR DOHM (1992) Individual variation in metabolism and reproduction of Mus: are energetics and life-history linked? Functional Ecology 6: 5-14. [ Links ]
HAYES JP & SH JENKINS (1997) Individual variation in mammals. Journal of Mammalogy 78: 274-293. [ Links ]
HAYES JP & CS O'CONNOR (1999) Natural selection on thermogenic capacity of high-altitude deer mice. Evolution 53:1280-1287. [ Links ]
HOCHACHKA PW & GN SOMERO (2002) Biochemical adaptation: mechanisms and process in physiological evolution. Oxford University Press, New York, New York, USA. 466 pp. [ Links ]
HOPKINS SR & FL POWELL (2001) Common themes of adaptation to hypoxia: insights from comparative physiology. In: Roach RC, PD Wagner & PH Hackett (eds) Hypoxia: from genes to the bedside: 153-167. Kluwer Academic, New York, New York, USA. [ Links ]
HUEY RB & D BERRIGAN (1996) Testing evolutionary hypotheses of acclimation. In: Johnston IA & AF Bennett (eds) Phenotypic and evolutionary adaptation to temperature: 205-237. Cambridge University Press, Cambridge, United Kingdom. [ Links ]
JAYNE BC & AF BENNETT (1990) Selection on locomotor performance capacity in a natural population of garter snakes. Evolution 44: 1204-1209. [ Links ]
LANDE R & SJ ARNOLD (1983) The measurement of selection on correlated characters. Evolution 37:1210-1226. [ Links ]
LYNCH GR, CB LYNCH, M DUBE & C ALLEN (1976) Early cold exposure: effects on behavioral and physiological thermoregulation in the house mouse, Mus musculus. Physiological Zoology 49:191-199. [ Links ]
MACMILLEN RE & T GARLAND (1989) Adaptive physiology. In: Kirkland GL & JN Layne (eds) Advances in the study of Peromyscus (Rodentia): 143-168. Texas Tech University Press, Lubbock, Texas, USA. [ Links ]
McCALL RD & D FRIERSON (1997) Inheritance of hypoxic exercise tolerance in mice. Behavioral Genetics 27: 181-190. [ Links ]
McNAMARA JM & AI HOUSTON (1996) State-dependent life histories. Nature 380: 215-221. [ Links ]
MONGE C & F LEÓN-VELARDE (1991) Physiological adaptation to high altitude: oxygen transport in mammals and birds. Physiological Reviews 71: 1135-1172. [ Links ]
MORTOLA JP & PB FRAPPELL (2000) Ventilatory responses to changes in temperature in mammals and other vertebrates. Annual Review of Physiology 62: 847-874. [ Links ]
MUELLER P & J DIAMOND (2001) Metabolic rate and environmental productivity: well-provisioned animals evolved to run and idle fast. Proceedings of the National Academy of Sciences USA 98: 12550-12554. [ Links ]
NESPOLO RF (2003) Evolution by natural selection: more evidence than ever before. Revista Chilena de Historia Natural 76: 699-716. [ Links ]
NESPOLO RF, LD BACIGALUPE & F BOZINOVIC (2003) Heritability of energetics in a wild mammal, the leaf-eared mouse (Phyllotis darwini). Evolution 57: 1679-1688. [ Links ]
REZENDE EL & T GARLAND (2003) Comparaciones interespecíficas y métodos estadísticos filogenéticos. In: Bozinovic F (ed) Fisiologia ecológica y evolutiva: teoría y casos de estudio en animales: 79-98. Ediciones Universidad Católica de Chile, Santiago, Chile. [ Links ]
REZENDE EL, F BOZINOVIC & T GARLAND (2004a) Climatic adaptation and the evolution of maximum and basal rates of metabolism in rodents. Evolution 58: 1361-1374. [ Links ]
REZENDE EL, MA CHAPPELL & KA HAMMOND (2004b) Cold-acclimation in Peromyscus: temporal effects and individual variation on maximum metabolism and ventilatory traits. Journal of Experimental Biology 207: 295-305. [ Links ]
REZNICK DN & H BRYGA (1987) Life-history evolution in guppies (Poecilia reticulata) .1. Phenotypic and genetic changes in an introduction experiment. Evolution 41: 1370-1385. [ Links ]
RICHARDSON RS, CA HARMS, B GRASSI & RT HEPPLE (1999) Skeletal muscle: master or slave of the cardiovascular system? Medicine and Science in Sports and Exercise 32: 89-93. [ Links ]
ROFF DA (1997) Evolutionary quantitative genetics. Chapman & Hall, New York, New York, USA. 493 pp. [ Links ]
ROFF DA (2002) Life history evolution. Sinauer Associates Inc., Sunderland, Massachusetts, USA. 527 pp. [ Links ]
ROSENMANN M & P MORRISON (1975) Metabolic level and limiting hypoxia in rodents. Comparative Biochemistry and Physiology 51A: 881-885. [ Links ]
RUPERT JL & PW HOCHACHKA (2001) Genetic approaches to understanding human adaptation to altitude in the Andes. Journal of Experimental Biology 204: 3151-3160. [ Links ]
SCHLICHTING CD & M PIGLIUCCI (1993) Control of phenotypic plasticity via regulatory genes. American Naturalist 142: 366-370. [ Links ]
SCHLICHTING CD & M PIGLIUCCI (1995) Gene-regulation, quantitative genetics and the evolution of reaction norms. Evolutionary Ecology 9: 154-168. [ Links ]
SNYDER LRG, JP HAYES, & MA CHAPPELL (1988) Alpha-chain hemoglobin polymorphisms are correlated with altitude in the deer mouse, Peromyscus maniculatus. Evolution 42: 689-697. [ Links ]
SPICER JI & KJ GASTON (1999) Physiological diversity and its ecological implications. Blackwell Science, Oxford, United Kingdom. 241 pp. [ Links ]
SPICER JI & WW BURGGREN (2003) Development of physiological regulatory systems: altering the timing of crucial events. Zoology 106: 91-99. [ Links ]
STEARNS SC (1992) The evolution of life histories. Oxford University Press, Oxford, United Kingdom. 249 pp. [ Links ]
STORZ JF & MW NACHMAN (2003) Natural selection on protein polymorphism in the rodent genus Peromyscus: evidence from interlocus contrasts. Evolution 57: 26282635. [ Links ]
STORZ JF & JM DUBACH (2004) Natural selection drives altitudinal divergence at the albumin locus in deer mice, Peromyscus maniculatus. Evolution 58: 1342-1352. [ Links ]
STROHL KP & AJ THOMAS (1997) Neonatal conditioning for adult respiratory behavior. Respiratory Physiology 110: 269-275. [ Links ]
TRACY RL & GE WALSBERG (2001) Developmental and acclimatory contributions to water loss in a desert rodent: investigating the time course of adaptive change. Journal of Comparative Physiology 171B: 669-679. [ Links ]
WEATHERS WW, CL DAVIDSON, CR OLSON, ML MORTON, N NUR & TR FAMULA (2002) Altitudinal variation in parental energy expenditure by white-crowned sparrows. Journal of Experimental Biology 205: 2915-2924. [ Links ]
WILSON RS & CE FRANKLIN (2002) Testing the beneficial acclimation hypothesis. Trends in Ecology and Evolution 17: 66-70. [ Links ]
WYNNE-EDWARDS KE (1998) Evolution of parental care in Phodopus: conflict between adaptations for survival and adaptations for rapid reproduction. American Zoologist 38: 238-250. [ Links ]
ZAMUDIO S (2003) The placenta at high altitude. High Altitude Medicine and Biology 4:171-191. [ Links ]
ZAMMUTO RM & JS MILLAR (1985) Environmental predictability, variability, and Spermophilus columbianus life-history over an elevational gradient. Ecology 66: 1784-1794. [ Links ]
Associate Editor: Francisco Bozinovic
Received December 2, 2004; accepted March, 2005


text:
Latest News
Lauryn Williams' wild ride but Olympian at brink of history
Lauryn Williams is a bobsledding rookie on the brink of winning a gold medal. She could make all kinds of Olympic history Wednesday if she and driver Elana Meyers hold on to first place.
But Williams doesn't want to get ahead of herself, especially when she's going 80 mph. She did that when she hit the brakes improperly Sunday and almost crashed out of the Winter Games.
Miami's Williams, pusher of the U.S. team's No. 1 sled, jolted the custom-built 400-pound carriage into a wall at the end of a practice run. It was not a fender bender. The nose was smashed, forcing BMW mechanics who helped design the sleek racing machine to make major repairs.
"The sled was totaled, and they had to stay up all night rebuilding the steering mechanism," said Meyers, who hurt her neck and head in the collision. "That hit was the hardest I've ever taken."
Yet Meyers and Williams were smiling Tuesday as they recalled the mishap. A track record did wonders for their morale. They and their sled recovered in time to rocket down the icy curves of the Sanki Sliding Center course in 57.26 seconds in the first of four heats (two on Tuesday, two more on Wednesday). Their combined two-run time of 1:54.89 put them .23 seconds ahead of Canadians Kaillie Humphries and Heather Moyse, the defending gold medalists, and .56 ahead of teammates Jamie Greubel and Aja Evans.
"I think the accident was a bonding moment," Williams said. "Elana didn't yell at me. She just took a deep breath and handled it like a pro. It was my fault, but to see someone have that much faith in you really makes you feel that camaraderie."
The irrepressible Williams is always finding silver linings. In her new career as a bobsled brakeman she doesn't whine about how cold and obscure the sport is. She jokes about how her thin-bloodedness makes her run faster so she can get back inside.
She and Meyers also set a start record of 5.12 seconds in the second heat, breaking the record of 5.13 they set in the first heat, marks due in large part to Williams' power and pedigree as a world-class sprinter.
Before she took up bobsledding six months ago at the suggestion of track and field friend Lolo Jones, Williams was known for her fast times in the summer. She was Olympic silver medalist in the 100-meter dash at the 2004 Athens Olympics and won gold in helping the U.S. 400-meter relay team qualify for its triumph at the 2012 London Games. Williams, an NCAA champ at the University of Miami, is also a three-time world championship gold medalist.
Williams, ready to retire from track in July, decided to try her legs at bobsled, a sport that has welcomed a host of former sprinters, jumpers, heptathletes, hurdlers like Jones and Willie Davenport, football players like Brian Shimer and Herschel Walker and softball players like Meyers.
At 5-3, Williams is short for a bobsledder, but she makes up for it with her explosiveness off the block.
"Start speed is the killer, and that's why we have these track stars behind us," said Meyers, who explained that humidity from a steady rain made the covered track "a little sticky" on Tuesday. "I made a few mistakes, we've got some work to do, but we're excited to attack."
Williams said the nervousness she felt all day harkened back to her prime.
"When I got on that line, I knew something good was going to happen because I was jumping out of my skin, and that's a feeling I hadn't had for a while in track and field," she said.
Williams, 30, adapted to bobsledding so quickly she even surpassed Jones, who is most famous for tripping on the penultimate hurdle at the 2008 Beijing Olympics when she was in the lead of the sprint, and for her photo shoots. Jones placed fourth at the 2012 Olympics. Jones and No. 3 U.S. driver Jazmine Fenlator were in 11th place at the halfway point, 1.84 seconds out of first.
"I knew we were going to have to fight," said a frustrated Jones, who went on a glutton's diet to add 25 pounds in the fall. She lobbied for the No. 1 slot, and her celebrity has definitely raised the profile of the sport, but Williams has more burst in the 30 meters that push athletes run before they jump into the back of the sled.
Jones is still looking for her first Olympic medal while Williams could become just the second person to win gold in both Winter and Summer Games, joining American Eddie Eagan, who was a boxer in 1920 and a bobsledder in 1932. Only four athletes have won medals of any color at both Winter and Summer Games.
But Williams insists she didn't do this to make a trivia list. No one makes the sacrifices — physical, financial, personal — to be a bobsledder unless they find a deeper satisfaction in sliding down a mile-long chute.
"Any time I step on any track, whether it's ice or some other surface, my goal is to win," Williams said.
The teamwork of bobsledding was a revelation after her years as a solo sprinter with occasional relay duty. Some veterans taught her so well, they didn't make the team.
"I didn't come here for fame or glory or to get rich," she said. "I did it to help my teammates. And I've thoroughly enjoyed the ride."


text:
Ideal presents for passionate cyclists - all our products are inspired by the sport of cycling and make a perfect gift for discerning riders. From luxury tools to stocking filling notebooks, we have you covered whether you are shopping for a father, mother, wife or husband.
Rouleur Cap — Tour Yellow
Regular Price: £15.00
Special Price £8.00
Rouleur T-Shirt – White£30.00
Climbs Collection T-Shirt - Vuelta a España
Regular Price: £30.00
Special Price £15.00
Rouleur T-Shirt – Gloss Black£30.00
Rouleur Logo Women's T-Shirt - Grey£30.00
Service Des Courses - T-shirt£30.00
Mapei Roundel - T-Shirt£30.00
Gel in a Box - T-Shirt£30.00
Rouleur Logo T-Shirt – Grey£30.00
But Ride - Merckx T-Shirt£30.00
Le Parcours - T-Shirt£30.00
We love Paris-Roubaix T-shirt£30.00


text:
We use cookies to make your experience better. By continuing to browse the site you are agreeing to our use of cookies. Learn more.
Acer XB Satin Wedge-Left-AW (52°)
As configured:
The Acer XB combines the tradition, beauty and elegance of the past you would only expect with a blade style wedge.
The Acer XB combines the tradition, beauty and elegance of the past you would only expect with a blade style wedge. Starting with a classic teardrop shape, tapered hosel, very minimal offset, thin top line and narrow sole, no attention to detail is left out.
The four-way cambered sole and moderate bounce angles allow for maximum playability from a variety of situations you may encounter on the course. The versatility of the D-grind sole allows for the face to be opened when needed and is good from any lie to produce creative face angle positions for shots like "bump and run" and "the flop" around the green.
Perhaps the greatest attribute is that no other set of wedges are as complete and versatile as the Acer XB wedges. There are as many as 8 different lofts to choose from providing you with total short-game management. Many golfers are content with the standard 52, 56 and 60 degree set up, which we have you covered. We also have the harder, if not impossible to find higher lofted wedges like the 64 and the 68 degree (satin finish only) for the high little delicate shots when you need to have the ball land softly.
With so many pitching wedges being manufactured with 46° of loft, for many golfers it makes more sense to carry 50, 54 and 58 degree lofts instead. This way they have the nice separation in distance without gaps from their game-improvement irons and still cover the majority of shots they will encounter on the course.
The appearance is equally as superb as its performance. The Acer XB Wedges are precision cast in four different finishes: lustrous pearl chrome, an elegant non-glare matte black, brilliant high polish and maintenance-free satin.
|1 mm
|Hosel ID
|Standard Clublength
|Club Type
|Player Type
|Men, Women
|Low, Mid, High, Beginners
|Weight (g)
|294 g
|294 g
|Head Shafting Depth
|35 mm
Free shipping for orders more then 175 € (excl. shipping cost). We normally ship your order with Schenker, UPS or as a letter, delivery time is 2-4 days. You will also receive a tracking number from us so you can track your package online.
When you order custom golf clubs from us you can try them out at your own driving range. If you're not pleased with your custom golf clubs you can return them within 30 days, and get a vaucher to use in our golf shop. If they are not used you can return the golf clubs within 14 days and get the money back.
Delivery cost: Letters from 5.9 € | DHL from 9.9 € | UPS from 14.9
We offer custom fitted golf clubs at no extra cost, you also have a large selection of golf grips and golf shafts to choose from. Enter you personel data in the custom fit guide and it will recommend you how long golf clubs you should have, grip size and which type of shaft you should use. 20-30% of all golfers play with golf clubs that are not adapted to their needs. After placing your order with us, we will build your golf clubs with the settings you have choosen. Of course you don't need to make any adjustments to the settings if you have played with standard golf clubs and are pleased with that, then just click on the buy button and your clubs will soon be shipped to you.
We send all orders within 48 hours. Nothing could be more annoying than to sit and wait for something you ordered. Therefore, we do our utmost to send your order as quick as possible. The reason it doesn't go faster depends on that we build the golf clubs from scratch – assemble the golf grip, club head and golf shaft to your specific settings. Golf components (golf grips, golf shafts etc), golf balls and accessories are shipped the same day or the day after. Shipment takes between 2-4 days.