Back in the '70s, researchers using mathematical calculations came up with the idea that recent human evolution — within the last 250,000 years, or since the emergence of modern man — was likely governed by selective sweeps. During a sweep, an adaptation would emerge and quickly spread throughout the population, driving evolution and leaving behind traces in the human genome. Some studies of selective sweeps, in humans and other organisms, suggest that they underlie much of human evolution.
And yet, the University of Chicago's Molly Przeworski and her colleagues found little evidence of selective sweeps in the human genome, as they recently reported in Science. Using data from the 1,000 Genomes Project, the team looked for human-specific targets of evolution and searched nearby for the evidence of a selective sweep. "If you look at all the changes in the genome that, a priori, are most likely to be functional, you do not see the footprint of selective sweeps," Przeworski says. She and her colleagues then wondered whether they were just missing the footprint. "The answer we found is no. If they were frequent, we'd see it," she says.
Selective sweeps, it seems, were not important for recent human evolution; something else has to have driven it, but what? "We know that our findings show that sweeps are rare, but unfortunately it doesn't tell us. We know that humans adapted in myriad ways, but we don't know how they adapted," she says.
Similar to what medical geneticists are finding in their search for variants associated with disease risk, it could be that that there are many small, polygenic drivers that add up to the total, Przeworski says. "Maybe adaptations are changes of small effects rather than changes of large effect. If so, the genetic architecture of adaptations may be similar to that of common diseases."
Evidence of selective sweeps, though, has been seen in other organisms. The Hebrew University of Jerusalem's Guy Sella, a co-author on this study, recently reported in PLoS Genetics that the fruit fly Drosophila simulans shows these classic selective sweeps. "[They] are prominent enough to leave a clear footprint in the genome," Przeworski says. "We've become quite intrigued by this difference; humans on the one hand show no evidence for sweeps and Drosophila at least shows some evidence for sweeps." The team is following up on that inconsistency, looking into ecological and life histories that may have caused adaptation to arise through different mechanisms in the two species. In addition, the researchers plan to delve in to the genomes of chimpanzees to study how adaptations took hold in that species, and are collecting data toward that end.
It is the data, however, that make asking questions about how humans evolved possible, Przeworski adds. "These are unprecedented data sets. Even the data set in Drosophila is dwarfed by this data set in humans," she says. "With the drop in price of sequencing, we're going to start seeing data sets across a wide range and we're going to collect more to ask these questions."