user3340: We do get a lot of questions that ask for why evolution did not lead to situation-X?
Recent examples:

Why don't mosquitoes evolve towards muting themselves?
https://biology.stackexchange.com/q/35528/3340

The answer to all such questions is that there is not enough selection pressure towards situation-X, and evolution does not try to find global optimum. Should we keep repeating the same answer again and again? What do you think should be done for these questions? If you close them as a duplicate of some other question, the OP will argue that their question is about a different situation. 
I think that these should be closed unless the OP asks for a specific hypothesis, but should we keep retyping the same statement for the custom close? An option is to clearly mention this rule in the tag-wiki for evolution and cite that as a close reason.
What is your take?

Based on terdon's suggestion, I have created a community wiki post:
Why do some bad traits evolve, and good ones don't? 
Those who are good at the subject, feel free to refine the post or add more information.

user71680: I understand and agree with the purpose of this thread. However, I believe I found a case where this thread might encourage users to misunderstand newcomers that are still learning to ask professional questions.
Let me explain:
In my experience, undergrads/amateurs/newcomers often times use a rhetoric like this:

"Why does A exist, while there is B ?"


"Why did A evolve, while there is already B ?"


"Why did A exist, although B would save energy ?"

These questions can be misunderstood as speculations about why traits did or didn't evolve.
HOWEVER what they actually mean is:

"How does A compare to B ?"

A simple example:
QUESTION:

"Why did life evolve on carbon and not silicon, since both engage in four chemical bonds?"

USEFUL ANSWER:

"Silicon-oxygen bonds require too much energy to be broken down, while most carbon compounds offers chemical plasticity and flexibility"

USELESS ANSWER:

"Silicon seems to have had selective benefits. And also, I am closing this thread as a duplicate, since this very same answer can be found here: Why do some bad traits evolve, and good ones don't?. "

I did see this happen not with 'carbon vs. silicon', but with ‘transcriptional vs. post-transcriptional gene regulation', which can be tricky topic, even for biologists. To me, the question was valuable, because I wish I had known the answer earlier in my career. Still, the question did have that "horrible amateurish stink". Nevertheless, I found it downright condescending that it was closed as a duplicate of a thread that was completely beside the point.
My point is: Not every question that sounds like "trait speculation" is actually about traits, so please act responsibly and think again if a question deserves to be closed as a duplicate of Why do some bad traits evolve, and good ones don't?.

user22057: I have only just chanced on this Meta Question (because of recent activity) although I have often noticed the reference to “Why do some bad traits evolve, and good ones don’t?”. I have never actually looked at the Community Question until today. Why not?

Because the reference to “bad traits” never seemed relevant and I
assumed that it related to something different. Indeed, it annoyed me intensely and I suspect posters too.

However, the answers on the community question are very good (I may add my own two penn’orth), and the page is very useful.
But please could we change the title, and farm out the separate question of bad traits elsewhere?

user1578: Beside the canonical question (which I support), I think that we can do a bit more. Specifically, somebody who asks this type of question probably can benefit from a tutorial on the basics of evolution written for laymen. Not everybody will read them, but some will. And the more people learn, the more we can expect them to come back with interesting questions. 
Corvus suggested such a resource some time ago, and I promised that I'll make a community ad suggestion out of it. Now I made the graphic and posted it in the Meta thread. Your upvotes, downvotes and improvement suggestions are welcome!
The site also has a nicely compiled list of misconceptions about evolution. The direct link to that could be added to an auto-comment used when closing this type of question as a dupe to the cannonical one. This may improve the chances of it being spotted and read by the people who need it. 

user1306: Write a general question and post a general answer, then close them as dupes of it. For example:

Q: If trait X would be advantageous to an organism, why hasn't it evolved?

Then, just write a general answer explaining that evolution is not guided and has no knowledge of the "perfect" state. Nor is there such a thing as a perfect state, etc.
Basically, we need a question that's broad enough to apply to any trait X and an answer that explains the basic reasoning behind why not everything that would be good for a species will necessarily occur. 

3340: <p>We do get a lot of questions that ask for why evolution did not lead to situation-<strong>X</strong>?</p>

<p>Recent examples:</p>

<ul>
<li><a href="https://biology.stackexchange.com/q/35278/3340">Why don&#39;t mosquitoes evolve towards muting themselves?</a></li>
<li><a href="https://biology.stackexchange.com/q/35528/3340">https://biology.stackexchange.com/q/35528/3340</a></li>
</ul>

<p>The answer to all such questions is that there is not enough selection pressure towards situation-<strong>X</strong>, and evolution does not try to find global optimum. Should we keep repeating the same answer again and again? What do you think should be done for these questions? If you close them as a duplicate of some other question, the OP will argue that their question is about a different situation. </p>

<p>I think that these should be closed unless the OP asks for a specific hypothesis, but should we keep retyping the same statement for the custom close? An option is to clearly mention this rule in the tag-wiki for <a href="https://biology.stackexchange.com/questions/tagged/evolution" class="post-tag" title="show questions tagged &#39;evolution&#39;" rel="tag">evolution</a> and cite that as a close reason.
What is your take?</p>

<hr>

<p>Based on terdon's suggestion, I have created a community wiki post:</p>

<p><a href="https://biology.stackexchange.com/questions/35532/if-a-trait-would-be-advantageous-to-an-organism-why-hasnt-it-evolved">Why do some bad traits evolve, and good ones don&#39;t?</a> </p>

<p>Those who are good at the subject, feel free to refine the post or add more information.</p>
 71680: <p>I understand and agree with the purpose of this thread. However, I believe I found a case where this thread might encourage users to misunderstand newcomers that are still learning to ask professional questions.</p>
<p>Let me explain:</p>
<p>In my experience, undergrads/amateurs/newcomers often times use a rhetoric like this:</p>
<blockquote>
<p>&quot;Why does <em>A</em> exist, while there is <em>B</em> ?&quot;</p>
</blockquote>
<blockquote>
<p>&quot;Why did <em>A</em> <em><strong>evolve</strong></em>, while there is already <em>B</em> ?&quot;</p>
</blockquote>
<blockquote>
<p>&quot;Why did <em>A</em> exist, although <em>B</em> would <em><strong>save energy</strong></em> ?&quot;</p>
</blockquote>
<p>These questions can be misunderstood as speculations about why traits did or didn't evolve.</p>
<p><strong>HOWEVER what they <em>actually</em> mean is:</strong></p>
<blockquote>
<p><strong>&quot;How does <em>A</em> <em><strong>compare</strong></em> to <em>B</em> ?&quot;</strong></p>
</blockquote>
<p><strong>A simple example:</strong></p>
<p>QUESTION:</p>
<blockquote>
<p>&quot;Why did life <em><strong>evolve</strong></em> on carbon and not silicon, since both engage in four chemical bonds?&quot;</p>
</blockquote>
<p>USEFUL ANSWER:</p>
<blockquote>
<p>&quot;Silicon-oxygen bonds require too much energy to be broken down, while most carbon compounds offers chemical plasticity and flexibility&quot;</p>
</blockquote>
<p>USELESS ANSWER:</p>
<blockquote>
<p>&quot;Silicon seems to have had selective benefits. And also, I am closing this thread as a duplicate, since this very same answer can be found here: <a href="https://biology.stackexchange.com/questions/35532/why-do-some-bad-traits-evolve-and-good-ones-dont">Why do some bad traits evolve, and good ones don't?</a>. &quot;</p>
</blockquote>
<p>I did see this happen not with 'carbon vs. silicon', but with ‘transcriptional vs. post-transcriptional gene regulation', which can be tricky topic, even for biologists. To me, the question was valuable, because I wish I had known the answer earlier in my career. Still, the question did have that &quot;horrible amateurish stink&quot;. Nevertheless, I found it downright condescending that it was closed as a duplicate of a thread that was completely beside the point.</p>
<p><strong>My point is:</strong> Not every question that sounds like &quot;trait speculation&quot; is actually about traits, so please act responsibly and think again if a question deserves to be closed as a duplicate of <a href="https://biology.stackexchange.com/questions/35532/why-do-some-bad-traits-evolve-and-good-ones-dont">Why do some bad traits evolve, and good ones don't?</a>.</p>
 22057: <p>I have only just chanced on this Meta Question (because of recent activity) although I have often noticed the reference to “Why do some bad traits evolve, and good ones don’t?”. I have never actually looked at the Community Question until today. Why not?</p>
<blockquote>
<p>Because the reference to “bad traits” never seemed relevant and I
assumed that it related to something different. <em>Indeed, it annoyed me intensely and I suspect posters too.</em></p>
</blockquote>
<p><strong>However, the answers on the community question are very good</strong> (I may add my own two penn’orth), <strong>and the page is very useful.</strong></p>
<p>But please could we change the title, and farm out the separate question of bad traits elsewhere?</p>
 1578: <p>Beside the canonical question (which I support), I think that we can do a bit more. Specifically, somebody who asks this type of question probably can benefit from a tutorial on the basics of evolution written for laymen. Not everybody will read them, but some will. And the more people learn, the more we can expect them to come back with <em>interesting</em> questions. </p>

<p>Corvus <a href="https://biology.meta.stackexchange.com/a/3003/1578">suggested</a> such a resource some time ago, and I promised that I'll make a community ad suggestion out of it. Now I made the graphic and posted it in the <a href="https://biology.meta.stackexchange.com/a/3120/1578">Meta thread</a>. Your upvotes, downvotes and improvement suggestions are welcome!</p>

<p>The site also has a nicely compiled list of <a href="http://evolution.berkeley.edu/evolibrary/misconceptions_faq.php#b7" rel="nofollow noreferrer">misconceptions about evolution</a>. The direct link to that could be added to an auto-comment used when closing this type of question as a dupe to the cannonical one. This may improve the chances of it being spotted and read by the people who need it. </p>
 1306: <p>Write a general question and post a general answer, then close them as dupes of it. For example:</p>

<blockquote>
  <p>Q: If trait X would be advantageous to an organism, why hasn't it evolved?</p>
</blockquote>

<p>Then, just write a general answer explaining that evolution is not guided and has no knowledge of the "perfect" state. Nor is there such a thing as a perfect state, etc.</p>

<p>Basically, we need a question that's broad enough to apply to any trait X and an answer that explains the basic reasoning behind why not everything that would be good for a species will necessarily occur. </p>
