And the Evolutionary Beat Goes On . . .
Monday, July 24, 2006
Stephen Jay Gould would have been pleased.
No, not about his mug shot at the endpoint of evolution in the illustration above, but about the growing evidence that evolution is not just real but is actually happening to human beings right now.
"From 1970 to 2000, there was a widespread view that although natural selection is very important, it is relatively rare," said Jonathan Pritchard, a geneticist at the University of Chicago. "That view was driven largely because we did not have data to identify the signals of natural selection. . . . In the last five years or so, there has been a tremendous growth in our understanding of how much selection there is."
That insight has only deepened as scientists have gained the ability to read the entire human genome, the chain of "letters" that spell out humanity's genetic identity.
"Signals of natural selection are incredibly widespread across the human genome," Pritchard said. "Everywhere we look, there appears to be very widespread signals of natural selection in many genes and many processes."
Pritchard helped write a recent paper that identified some of those changes. The paper was published in the public access journal PLoS Biology.
The research offers a fascinating snapshot into how the human genome has continued to change as humans adapted to new circumstances over the past 10,000 years. As people went from hunter-gatherers to agricultural societies, for instance, there is evidence of genetic adaptations to new diseases and diets.
Europeans seem to be adapting to the increased availability of dairy products, with genetic changes that allow the enzyme lactase, which breaks down lactose in milk, to be available throughout life, not just in infancy. Similarly, East Asians show genetic changes that affect the metabolism of the sugar sucrose, while the Yoruba people in sub-Saharan Africa show genetic changes that alter how they metabolize the sugar mannose.
Where starvation was once widespread in humans' evolutionary history, making it genetically advantageous to conserve calories as much as possible, the abundance of food in many countries today has led to the opposite problem -- risk factors and diseases related to metabolic overload, including obesity and diabetes -- suggesting these could be areas in which natural selection may currently be active, as genetic variations that help protect against such disorders gain selective advantage.
There are also a host of changes at the genetic level that scientists do not yet understand -- they are probably useful, but it is not clear how.
Several changes seem related to fertility and reproduction, areas of very high relevance to natural selection. The basic protein structure of sperm may have changed in East Asians and the Yoruba; East Asians also show genetic changes related to sperm motility; and Europeans show genetic changes related to egg viability, fertilization and the female immune response to sperm.
Pritchard said his research does not speak directly to Gould's "punctuated equilibrium" hypothesis that suggests that evolution progresses in leaps and starts. That is because Gould focused on large changes in form or structure, whereas Pritchard studies subtler changes at the genetic level.