What Is Selective Sweep?

Selective sweep, or genetic hitchhiking, is a genetics and evolution term that explains how alleles for favorable adaptations, and their associated alleles near them on chromosomes, become more frequently seen in a population due to natural selection.

What Are Strong Alleles

Natural selection works to choose the most favorable alleles for an environment in order to keep a species passing down those traits generation after generation. The more favorable the allele for the environment, the more likely the individuals that possess that allele will be to live long enough to reproduce and pass that desirable trait down to their offspring. Eventually, undesirable traits will be bred out of the population and only the strong alleles will be left to continue on.

How a Selective Sweep Happens

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The selection of these preferred traits can be very strong. After a particularly strong selection for a trait that is the most desirable, a selective sweep will happen. Not only will the genes that code for the favorable adaptation increase in frequency and be seen more often in the population, other traits that are controlled by alleles that are close in proximity to those favorable alleles will also be selected for, whether they are good or bad adaptations.

Also called "genetic hitchhiking", these extra alleles come along for the selection ride. This phenomenon may be the reason why some seemingly undesirable traits get passed down, even if it does not make the population the "fittest". One major misconception of how natural selection works is the idea that if only the desirable traits are selected for, then all other negatives, such as genetic diseases, should be bred out of the population. Yet, these not so favorable characteristics seem to persist. Some of this could be explained by the idea of selective sweep and genetic hitchhiking.

Examples of Selective Sweep in Humans

Do you know someone who is lactose intolerant? People who suffer from lactose intolerance are unable to fully digest milk or milk products like cheese and ice cream. Lactose is a type of sugar that is found in milk that requires the enzyme lactase in order to be broken down and digested. Human infants are born with lactase and can digest the lactose. However, by the time they reach adulthood, a large percentage of the human population loses the ability to produce lactase and therefore can no longer handle drinking or eating milk products.

Looking Back at Our Ancestors 

About 10,000 years ago, our human ancestors learned the art of agriculture and subsequently started to domesticate animals. The domestication of cows in Europe allowed these people to use cow's milk for nutrition. Over time, those individuals who had the allele to make lactase possessed the favorable trait over those who could not digest the cow's milk.

A selective sweep occurred for the Europeans and the ability to get nutrition from milk and milk products was highly positively selected. Therefore, the majority of Europeans possessed the ability to make lactase. Other genes hitchhiked along with this selection. In fact, researchers estimate that about a million base pairs of DNA hitchhiked along with the sequence that coded for the lactase enzyme.

Another Example Is Skin Color 

Another example of a selective sweep in humans is skin color. As human ancestors moved from Africa where dark skin is a necessary protection against the direct ultraviolet rays of the sun, less direct sunlight meant that the dark pigments were no longer necessary for survival. Groups of these early humans moved north to Europe and Asia and gradually lost the dark pigmentation in favor of a lighter coloring for the skin.

Not only was this lack of dark pigmentation favored and selected, nearby alleles that controlled the rate of metabolism hitchhiked along. Metabolic rates have been studied for different cultures all over the world and have been found to correlate very closely to the type of climate where the individual lives, much like the skin coloring genes.  It is proposed that the skin pigmentation gene and the metabolic rate gene were involved in the same selective sweep in the early human ancestors.