Although the human and chimpanzee genomes are distinguished by 35 million differences in individual DNA "letters," only about 50,000 of those differences alter the sequences of proteins. Of those 50,000 differences, an estimated 5,000 may have adaptive consequences in the evolutionary divergence between these two species, according to a study published in the March 6, 2007, issue of the Proceedings of the National Academy of Sciences.
Before such a new and beneficial mutation can take its place in the human genome it has to pass through a rigorous two-stepnegative and positivescreening process, say the study authors, evolutionary geneticists from the University of Chicago, the University of Tokyo and the University of Washington. Both steps focus on the most radical changes.
In step one, mutations, the genetic equivalent of typographical errors, are randomly introduced. When these mutations are still rare in the population, only strongly deleterious ones get weeded out through negative selection. The more radical mutations are more likely to be harmful and quickly removed. For those that are only slightly harmful, neutral or beneficial, the selective forces are weak and luck determines their fates.
In step two, a new mutation that has been fortunate enough to survive the initial elimination process, and confers some benefit, can then spread quickly through positive selection. The more radical the mutation, at this point, the faster it is likely to spread.
"We found that the same genetic changes that are unlikely to survive early negative selection are the ones that spread most quickly once they gain a foothold," said the studys senior author Chung-I Wu, Ph.D., professor of ecology and evolution at the University of Chicago. "To reach that conclusion, we needed to decouple the two steps of evolution and classify mutations into many different kinds."