No, we’re not talking about brooms here…

When a mutation arises that confers some sort of advantage, those individuals with the mutation have more kids than those without it. Over time, of course, the mutated gene will become more prevalent in a population (this is simply natural selection).  In some cases, other pieces of DNA will hitch-hike along with the advantageous mutant gene because they are linked (i.e., close-by) on the same chromosome and will thus also increase in frequency. A SELECTIVE SWEEP occurs when the positively selected gene and all its neighbors (called a haplotype) become the only variant in a population. So, the result of a selective sweep is a reduction in overall genetic diversity in that region of the genome. 

Selective sweeps have certainly occurred in recent human evolution: for example, the genes (and associated DNA neighbors) for skin pigmentation and lactose tolerance appear to have arisen among modern human populations in a manner consistent with a selective sweep.     

According to a newly published study in Science, selective sweeps were considered to be a relatively common occurrence among humans. However, the new research suggests that this is not so. From a summary in ScienceNews:

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.

It may have been difficult for selective sweeps to take hold in humans because of demographics…[p]eople 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.

References

Hernandez, R.D., et al. (2011). Classic selective sweeps were rare in recent human evolution. Science 331: 920-924.

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