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Washington, Feb 18 (ANI): A new study has found that selective sweeps, which were thought to drive human evolution, may not have done so. In fact, such events may have been rare, with little influence on the history of our species.

Ryan Hernandez at the University of California at San Francisco, and his colleagues examined the sequences of nearly 200 human genomes to find new evidence arguing against selective sweeps as the dominant mode of human adaptation.

The find suggests that new models are needed to retrace the genetic steps of evolution.

“Our findings suggest that recent human adaptation has not taken place through the arrival and spread of single changes of large effect, but through shifts of frequency in many places of the genome,” said Molly Przeworski at the University of Chicago.

“It suggests that human adaptation, like most common human diseases, has a complex genetic architecture.”

Under the classic selective sweep model, a new, advantageous gene appears and quickly spreads through the population. Geneticists have used this model to look for genetic segments surrounded by “troughs” of low variation, the theoretical footprint of a selective sweep.

“It is fair to say that it is the model behind almost every scan for selection done to date, in humans or in other organisms,” said Przeworski.

The research team looked at genes with human-specific substitutions, where the nucleotide sequence is different from close primate relatives. In some cases, the new sequence switches an amino acid in the protein the gene encodes, a replacement that likely improved the protein’s function. In other genes, the sequence change is “synonymous,” coding for the same amino acid as before and leaving the protein’s function unperturbed.

But when the two groups were compared, the troughs of low diversity were similar for genes that produce functional changes and genes with synonymous substitutions that do not.

“The idea that human adaptation might proceed by single changes at the amino acid level is quite a nice idea, and it’s great that we have a few concrete examples of where that occurred, but it’s too simplistic a view,” said Hernandez.

Przeworski said, “These findings call into question how much more there is to find using the selective sweep approach, and should also make us skeptical of how many of the findings to date will turn out to be validated.”

The study is published in the February 18, 2011 issue of Science. (ANI)