Why Human Evolution Accelerated
Ever had a creationist talk about how there is no proof of evolution? This post dates back to 2007 but is very detailed and goes into depth about recent and continuing human evolution.
n. b. This is a story about my work on recent human evolution, describing some of the main results and how the work came about. The story refers to my paper (with Gregory Cochran, Eric Wang, Henry Harpending, and Robert Moyzis), “Recent acceleration of human adaptive evolution,” which came out in December, 2007.
Like most good stories in biology, this one begins with Darwin. Darwin was always very interested in animal breeding, which he considered the best analogy for the process of natural selection. Of course, if you’re breeding livestock and want to select for some characteristics, it is important to select from as large a herd as possible, because large populations have more variation in them. Darwin recognized this as an important condition for natural selection, which relies on sufficient variation in natural populations.“[A]s variations manifestly useful or pleasing to man appear only occasionally, the chance of their appearance will be much increased by a large number of individuals being kept…. Hence, number is of the highest importance for success.”
These words from the Origin, “number is of the highest importance for success” were influential.
This is a quick review of the research, based on a presentation I gave earlier this year. It is not complete, and glosses a number of very important details. A close reader looking for how to do genomics would be better served reading the actual research paper. Here, I’m trying to express the science for everyone else.
By 1930, R. A. Fisher picked up Darwin’s idea about numbers, predicting that evolution in large populations could be faster than in small populations. However, this is not in all circumstances, but only where the number of new adaptive mutations is quite small — in other words, where evolution is “mutation-limited”:
The great contrast between abundant and rare species lies in the number of individuals available in each generation as possible mutants…. The importance of the contrast lies with the extremely rare mutations, in which the number of new mutations occurring must increase proportionately to the number of individuals available.A long history of research in plant genetics (corn breeding), microbial chemostat experiments, and the examination of pesticide resistance in insects support Fisher’s concept. For example, flies subjected to low doses of pesticide in the laboratory tend to acquire very complicated patterns of resistance — involving slight changes in many different genes. These usually aren’t transmitted perfectly and often have fitness costs; it’s a very imperfect adaptation. But if pesticide is sprayed over a large area, flies sometimes appear very quickly with a single mutation that confers very complete resistance. Here, the very advantageous resistance mutation is incredibly rare — it only occurs in maybe one in a billion flies. It would never occur in the small laboratory population.
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Lactose tolerance, gluten tolerance, and so forth - we’re still an evolving species. Hopefully we won’t short circuit it by killing ourselves off.