Is Evolution Linked to Environmental Change?

 
As a general rule, adaptationists are mostly interested in the results of natural selection and not very interested in evolution by random genetic drift. They tend to use the word "evolution" when what they really mean is "natural selection" or adaptation.

Most adaptationists see evolution as positive natural selection. They focus mostly on changes where the population is becoming more fit with respect to the environment. Some of them think that this eventually results in populations that stop evolving because they have become optimized to a particular niche. In such cases, "evolution" (i.e., positive natural selection) will only start up again if the environment changes.

Whenever I mention this I'm usually confronted with a storm of denials. Apparently none of the adaptationists who comment on Sandwalk are guilty of such fuzzy thinking.

I'm so happy for them. The idea that species have exhausted all possible adaptations and reached the very tip top of their adaptive peak seems incredibly naive. The idea that "evolution" will have halted—as opposed to adaptation—seems even more naive.

Now that I've got that off my chest, let's turn to the Hawks et al. (2007) paper that's getting so much press [Accelerated Human Evolution]. Remember, this is a paper about human evolution.

I've read the paper and I can't really comment because there's no data in the paper. What I mean is that there are no examples of the 11,439 "selective events" that they found. It would have been nice to see a few examples of their data just to get some feel for it's quality.

The paper is complicated because it consists mostly of a discussion of the data, which we can't see. The first author, John Hawks, has made an attempt to simplify the work by posting an explanation on his blog john hawks weblog.

Here's an excerpt from the article titled Why human evolution accelerated [my emphasis-LAM]. The article explains why he expected to see an big increase in evolution adaptation following the development of agriculture about 10-40,000 years ago.

Still, a very small fraction of the mutations in any given population will be advantageous. And the longer a population has existed, the more likely it will be close to its adaptive optimum -- the point at which positively selected mutations don't happen because there is no possible improvement. This is the most likely explanation for why very large species in nature don't always evolve rapidly.

Instead, it is when a new environment is imposed that natural populations respond. And when the environment changes, larger populations have an intrinsic advantage, as Fisher showed, because they have a faster potential response by new mutations.

From that standpoint, the ecological changes documented in human history and the archaeological record create an exceptional situation. Humans faced new selective pressures during the last 40,000 years, related to disease, agricultural diets, sedentism, city life, greater lifespan, and many other ecological changes. This created a need for selection.

Larger population sizes allowed the rapid response to selection -- more new adaptive mutations. Together, the the two patterns of historical change have placed humans far from an equilibrium. In that case, we expect that the pace of genetic change due to positive selection should recently have been radically higher than at other times in human evolution.

Now, if I understand this correctly, here's the scenario. About 40,000 years ago humans had pretty much stopped accumulating adaptations because they were becoming optimized to their environment. This is reflected in the data, which shows a slow rate of adaptation at that time.

Then humans started to live in larger communities as they abandoned the hunter-gatherer mode of existence for one based on farming. This created a new environment that was less fit than the previous one. The human population responded to this less fit environment by expanding rapidly in numbers. This created more opportunity for beneficial mutations that were required under the new environmental conditions. The result was a huge increase in the rate of adaptive evolution.

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