system
Given the following post, respond as if you're commenting on Reddit:
human
What differences (if any) are there between Darwin's theory of evolution and today's current theory on evolution?
gpt
Very little of his work has been abandoned, we have just added and expanded. There are two big changes. First we learned how inheritance worked. Darwin did not know and guess there was some sort of blending involved. Turns out blending leads to really slow change. The other biggest change is called Neutral Theory. And this Darwin did anticipate a bit. Neutral theory says that sometimes a allele will go to fixation randomly.

OK, let me unpack that sentence. An allele is a variation of a gene. So if eye color is determined by a gene there is a blue allele, a green allele, a brown allele. (In reality eye color is more complex than that, most things are.)  Fixation means that you get enough organisms with that allele that it will stick around a long time. 

So let me put back together Neutral and Selection. Suppose we have a population of organisms that magically start off genetically identical. With the next generation there are mutations, gene variations, new alleles. OK? Our question is which of these will we see in say 20 generations. Let us look at three particular alleles, A, B, and C. C slightly increases metabolic efficiency (how much energy the organisms gets from food), B slightly hurts it, C just does things different with no advantage or disadvantage. OK so far? 

So what happens? An ultraselectionist like Dawkins will say everyone gets A since it is the best. Kumra tells us no. There is a greater chance of seeing lots of As, it is unlikely (but possible) to see Bs, and we might see some C. Evolution is not simply selection. So why would we not see all As? Well maybe A is in the area where a big storm kills everyone. Maybe A was asleep when a predator came around. Maybe lots of things. How can we see the deleterious Bs? Well maybe B is part of an important family (hemophilia in the European royals). Maybe B mates with someone with a really good allele for another gene and it gets carried along. 

Last bit. Let us now look at a different population and we find some alleles X, Y, and Z. We can't simply look at trait T(Z) and say it was selected for. Maybe it was, maybe not. Maybe it just sort of wandered into place. If we want to say there is selective advantage to something we need to actually show the advantage. Simply existing does not show it.