Sweeps weak in human evolution

Rapid spread of beneficial mutations relatively rare

Humans probably didn’t get swept up in evolution.

Scientists have favored a model of evolution in which beneficial gene mutations quickly and dramatically sweep through a population due to the evolutionary advantages they confer. Such mutations would become nearly universal in a population. But this selective sweep model may not be accurate for humans, a new study indicates. Human evolution likely followed a more subtle and complicated path, say population geneticists Molly Przeworski of the University of Chicago and Guy Sella of Hebrew University of Jerusalem and colleagues.

Computational analysis of 179 genomes belonging to people from Europe, Asia and Africa reveal that selective sweeps have been rare in human evolution, the researchers report in the Feb. 18 Science.

“I’m convinced,” says Andrew Clark, a population geneticist at Cornell University. Clark was among the first to find evidence that selective sweeps can shape evolution. The idea of a favored gene sweeping in to save the evolutionary day is so attractive that other forms of natural selection have been largely ignored, he says. The new study could change that. “I think this will be taken to heart and people will take a step back and start asking what other signatures of selection may be present.”

In the study, the researchers based their analysis on the idea that when a gene containing a beneficial mutation becomes more common over successive generations it drags along big swaths of neighboring DNA. A sweep would happen so quickly that individual changes in the nearby DNA wouldn’t have time to accumulate, so everyone in a population would end up with essentially the same genetic signature in the DNA regions surrounding the beneficial mutation.

The researchers searched for such troughs of genetic diversity around genes carrying mutations that would change an amino acid building block in the protein built from the gene — a sign of functional importance. The team reasoned that if the genetic changes were really beneficial, they ought to have deeper troughs than mutations that don’t alter amino acids.

“But in fact, we found very little difference,” says Sella. That could indicate that “very few of these mutations came into the population in the mode of a selective sweep.” The researchers didn’t find evidence of selective sweeps in regions of the genome that change how genes are turned on and off either.

It may have been difficult for selective sweeps to take hold in humans because of demographics, Clark says. People are scattered throughout the globe, so a beneficial mutation would have a long way to spread. Such a mutation would have to have dramatic effects on evolutionary fitness to go global.

Good evidence does exist for some mutations that did undergo selective sweeps in humans, such as those for skin pigmentation, hair and teeth morphology and the genetic change that allows adults in some populations to digest the milk sugar lactose. But those examples are the exception rather than the rule in human evolution.

“We have beautiful examples of selective sweeps. But there are not many of them, and our results suggest [there are] not many more to come,” Przeworski says. “Our results do not suggest that adaptation was rare. Many protein changes in humans may well have been adaptive. What our results indicate is that the dominant mode of adaptation was not the classic sweep,” she says.

“In looking for the genetic mechanisms of adaptation in the human lineage we’ll have to turn to more elaborate models,” Przeworski says.

Selective sweeps may have been more important in the evolution of some other species, though. Sella and colleagues recently reported evidence that selective sweeps happen often in some fruit flies.

Tina Hesman Saey is the senior staff writer and reports on molecular biology. She has a Ph.D. in molecular genetics from Washington University in St. Louis and a master’s degree in science journalism from Boston University.

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