And the Evolutionary Beat Goes On . . .
Monday, July 24, 2006; Page A07
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.
"If you met a human from 10,000 years ago," Pritchard quipped, "they may look a little different, but if you dressed them right, they would probably blend in. Gould's talking about changes in body plan and broader changes."
To spot natural selection at work, Pritchard and Bruce Lahn, also a geneticist at the University of Chicago who has conducted independent research in the same area, first look for places along the human genome to identify sites that show changes in some people but not in others. Then they look at the genetic material surrounding the changed part.
If the surrounding area looks very different from one person to the next, the particular change probably occurred a long time ago, because the general area has had time to accumulate other changes in the DNA. If there are not many differences in the surrounding genetic sequence, that indicates the particular change is relatively new.
Then scientists figure out how widespread that particular change is in large populations. Changes that are both new and widespread reveal the hand of natural selection -- since advantageous genetic changes will quickly spread through the population.
Next, scientists try to guess what the genetic change is accomplishing. If the change is in a part of the genome known to be involved in the immune system, the change may have something to do with responding to new diseases. Other changes may have to do with brain functioning or skin color.
Europeans, for example, show strong changes over the past 10,000 years in genes that affect skin color -- as humans moved into northern Europe, where there was less ultraviolet light, there was a strong evolutionary advantage to having lighter skin to allow in more ultraviolet light, which is needed to synthesize Vitamin D.
Lahn found changes in two genes, dubbed ASPM and MCPH1, that are known to be involved in brain development. He published his results recently in the journal Science.
While genetic changes, especially related to the brain, may prompt people to think different populations are evolving different mental abilities, both Lahn and Pritchard pooh-poohed this idea. For one thing, they pointed out, biology is complex, and the same genes often play multiple roles in the body. A gene that affects brain development may also play a role in the immune system, so it is not possible to say with certainty that natural selection has favored the change because of its effect on the brain.
Besides, Pritchard added, scientists found about the same number of changes in all three groups they studied, suggesting that evolution is taking place everywhere, adapting different groups to the particulars of their ecological niches.
Come to think of it, the late Stephen Jay Gould might have been upset with the above illustration. Contrary to the popular imagination, evolution is not a linear process that culminates in the triumphal ascent of humans at the top of the genetic heap. The process is analogous to a bush, where twigs and leaves push out in every direction.
When biologists talk about evolution and the survival of the fittest, they do not necessarily mean the strongest, fastest or smartest. Fitness is whatever works in a particular environment, and the new research shows that as environments change, notions of fitness change, too.