Friday, September 04, 2009

Modern Human Evolution

Daily Galaxy has an article on how human evolution has proceeded in the recent past and how it might continue into the future.

A team led by University of Wisconsin-Madison anthropologist John Hawks estimates that positive selection just in the past 5,000 years alone -dating back to the Stone Age - has occurred at a rate roughly 100 times higher than any other period of human evolution. Many of the new genetic adjustments are occurring around changes in the human diet brought on by the advent of agriculture, and resistance to epidemic diseases that became major killers after the growth of human civilizations.

"In evolutionary terms, cultures that grow slowly are at a disadvantage, but the massive growth of human populations has led to far more genetic mutations," says Hawks. "And every mutation that is advantageous to people has a chance of being selected and driven toward fixation. What we are catching is an exceptional time."

While the correlation between population size and natural selection is nothing new - it was a core premise of Charles Darwin, Hawks says - the ability to bring quantifiable evidence to the table is a new and exciting outgrowth of the Human Genome Project.

This is the convergence of population genetics, comparative anatomy and palaeontology to explain how humans have evolved. About natural selection, he writes:

Another recently discovered gene, CCR5, originated about 4,000 years ago and now exists in about 10 percent of the European population. It was discovered recently because it makes people resistant to HIV/AIDS. But its original value might have come from obstructing the pathway for smallpox.

"There are many things under selection that are making it harder for pathogens to kill us," Hawks says.

Population growth is making all of this change occur much faster, Hawks says, giving a tribute to Charles Darwin. When Darwin wrote in "Origin of the Species" about challenges in animal breeding, he always emphasized that herd size "is of the highest importance for success" because large populations have more genetic variation, Hawks says.

This is contrary to popular wisdom that shows that evolution proceeds faster in smaller populations because traits are expressed more often. Typically, larger populations slow evolution down because they are much closer to Hardy-Weinberg equlibrium. I also wonder about the statement that human evolution is proceeding faster now than at any time in the past. I do not think we have the resolution to determine that, even genetically. The human "bush" has gone through some remarkable changes even in the last 200 thousand years, with the successive appearance of archaic Homo sapiens and modern Homo sapiens, with regional characteristics present in many populations. It is doubtful that any of these changes would have happened had the population been as global as it now is.

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