26 September 2011

Common ancestry, bottlenecks, and human evolution

Human evolution has been in the news quite a lot recently.

  • New genetic data suggest that ancient humans included both Neanderthals and Denisovans, which colonized different parts of the world but subsequently interbred with so-called modern humans and left telltale traces of this history in the genomes of living humans.
  • New analysis of current genetic diversity suggests that human population size underwent interesting fluctuations throughout the history of our species, but concludes that the population never dipped below a few thousand reproducing individuals.
Unsurprisingly, these findings have been discussed in the context of Christian views of human origins. In the context of some of these discussions (among Catholics, for example), I have noticed some confusion regarding the implications of common ancestry. I will illustrate the error with a stylized example, then explain why it is an error.

Consider an important and human-specific genetic feature. For example, consider the human-specific version of the FoxP2 gene. This gene is thought to play an important role in human speech, and the data indicate that the gene was mutated at some point to create the human version, since the gene itself is not specific to humans. In other words, at a key juncture in human evolution, the human-specific FoxP2 gene form came to be.

Since that time, that gene form became the only gene form in humans. There might have been more than one occurrence of the mutation, but it can't be that the mutation is terribly common, since it isn't found in any other mammal or in other primates. Therefore, the existence of the human-specific FoxP2 gene is overwhelming evidence that all humans (past and present) trace their ancestry through one or just a few ancestors who first acquired the mutation.
So far, so good.
Now, if we all trace our ancestry to just one or two ancestors, then it must be that the human population must have gone far below a few thousand. It must be that the human population declined to near zero, and specifically it must have declined to the number of those common ancestors. That's the only way that all humans could currently have that genetic mutation.
That's completely wrong. It might make sense superficially, but it's wrong, and some careful thinking about how inheritance works should make that obvious.

Let's assume that the mutation provided some advantage to the first animals who bore it. In fact, let's assume that the benefit was huge. And let's assume that it occurred in a small population of 100 individuals. (In reality, those are probably unrealistic assumptions; mutations rarely confer an instantaneously huge benefit, and the human population isn't thought to have gone as low as 100.) The outcome of such a scenario is this: the next generation will include more individuals bearing the mutation. Considering human reproduction rates, let's say that the next generation includes 5 of those individuals, and let's allow those individuals to interbreed. The generation following that one would include, say, 15 individuals (3 females with 5 kids each). And the following generation might include 40-ish, and so on. Soon, every member of the population would have the mutation, and every one of those individuals and all of their descendants would share ancestry with the first animal with the mutation. But the population never shrank; in fact, it could have grown during the process and the march of the mutation would have occurred just fine.

This is almost certainly what occurred hundreds or thousands of times in the evolution of our species, and the result is that we have hundreds or thousands of human-specific features that we all inherited from one or a few common ancestors. That fact alone does not mean that our entire population ever contracted to include only those common ancestors.

Evidence for common ancestry is not evidence for genetic bottlenecking. Think about it.

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