A close look at the human genome shows the slow and steady beat of adaptation.

Scientists looked at 40,000 genetic changes to determine the pace of human evolution.

Here’s how scientists have typically explained the emergence of a new genetic trait: A genetic mutation randomly occurs that gives its carrier an advantage in reproducing. Over a handful of generations, the mutation becomes more prevalent in the population, quickly becoming ubiquitous. It’s called a “selective sweep” and has been the predominant explanation for how most new human genes have surfaced. Now here’s Molly Przeworski’s take: evolution is slow and complex and few human traits have ever emerged through such a speedy takeover.

Przeworski, an HHMI early career scientist at the University of Chicago, relied on the fact that if a gene mutation moved that quickly across the human population, most everyone would have inherited the identical genetic material bordering the mutation. If a mutation spread slowly, on the other hand, or arose more than once, it would gradually pick up other mutations in surrounding genes as it broadened throughout the population.

Using this reasoning, Przeworski and her team analyzed 179 human genomes collected through the 1000 Genomes Project. They looked at 40,000 genetic changes that set humans apart from their primate ancestors—some that might change a protein’s function, others that are essentially silent. If the “selective sweep” model had dominated throughout human history, Przeworski’s team would see more highly conserved regions around mutations that had functional effects. Yet they saw no differences between the variability surrounding functional mutations compared with the rest of the genome, they reported on February 18, 2011, in Science.

This finding must mean that “not many adaptations in our history have proceeded through sweeps,” says Przeworski. “Selective sweeps must be really rare.”

She suggests two alternatives that could explain how adaptations might spread more often. Preexisting mutations across the population can face a new selective pressure from a change in the environment—this process is called “selection on standing variation.” Or, a trait can rely on many gene changes rather than one change with a large effect. “Height, for example, is controlled by thousands of loci,” says Przeworki. “If the environment is selecting for height, that will happen through hundreds of gene locations.”

Photo: Omikron / Photo Researchers, Inc.