University of Sussex biologists estimate the pace of evolution

Scientists at the University of Sussex have provided the key to resolving a 30-year-old controversy in evolutionary biology: what proportion of the differences between similar species came about as a result of natural selection, and how many are just the result of ‘random genetic drift’. In a paper in this week’s issue of Nature (28 February), Sussex biologists put the ratio at 45:55.

The DNA sequences of humans and chimpanzees differ by less than 2% but this adds up to about 350,000 amino acid differences. A central problem for evolutionary biologists has been to determine what proportion of this very large number of amino acid differences came about as a result of natural selection, and how many are just the result of random genetic drift. In the example of humans and chimpanzees, how many of the differences actually helped humans or chimpanzees to adapt to their environment?

Nick Smith and Dr Adam Eyre-Walker, from the Centre for the Study of Evolution at the University of Sussex, tackled the problem by looking at the divergence between two species of fruit-fly: Drosophila melanogaster, the vinegar fly (beloved of wine makers), and Drosophila yakuba, a close relative.

These two species are thought to have separated about 6 million years ago (round about when human and chimpanzee ancestors separated) and to have a roughly similar number of differences to those that divide humans and chimpanzees.

Using DNA sequence databases, Smith and Eyre-Walker calculated that about 270,000, or 45%, of all the amino acid differences between these species are adaptive, helping the flies to do better in their own particular environments. Presumably these differences have been maintained in the two genomes as a result of natural selection, which would favour individuals with beneficial mutations and ‘punish’ those with harmful ones. They calculate that, on average, one such adaptive substitution must have occurred every 45 years during the past six million years.

The other 55% of differences seem to be relatively neutral in their effect and are probably due to random genetic drift.

If the proportion is the same in the case of humans, say the Sussex researchers, we would have about 160,000 adaptive differences from chimpanzees: a small part of the total genome but nevertheless a very large number of differences.