Within the last decade, our ability to probe our own species for evidence of selection has increased dramatically due to the flood of genetic data that have been generated. Investigators are interested in the types of signals that can be detected in a selective sweep, as well as their properties and technical challenges (Nielsen, 2005 Sabeti et al., 2006). ![]() Thus, while the selected allele and hitchhiking alleles increase in prevalence in a selective sweep, at the same time, the segment that includes the selected allele is slowly reduced in size by recombination. Note that the entire chromosome is not passed down as a unit, however rather, because of recombination, segments of the chromosome are inherited. Through succeeding generations, the descendants of this copy, including the selected allele and nearby " hitchhiking" alleles, become more and more common through a process called a " selective sweep" (Figure 1). Now, say that an advantageous allele arises as a mutation on one copy of a chromosome. Consider a population of individuals for which, before selection, there are hundreds of thousands of varied chromosomes in the population, all with different combinations of genetic variants. Moreover, when selection does occur, it is most often in the form of negative, or purifying, selection, which removes new deleterious mutations as they arise, rather than promoting the spread of new traits (Kreitman, 2000).Īs advantageous alleles that are under positive selection increase in prevalence, these alleles leave distinctive signatures, or patterns of genetic variation, in the DNA sequence. ![]() The frequency of some of these neutral genetic variants ( alleles) increases simply by chance, and the resulting " genetic drift" is thought to be the most common process in human evolution (Kimura, 1968). In humans, at least, the great majority of mutations are thought to be selectively neutral, conferring neither benefit nor cost on their bearers (Hellmann et al., 2003). Positive selection is by no means the only component of evolution, however. At the molecular level, selection occurs when a particular DNA variant becomes more common because of its effect on the organisms that carry it.Ĭharles Darwin and Alfred Wallace (1858) famously proposed that positive selection could explain the many marvelous adaptations that suit organisms to their environments and lifestyles, and this simple process remains the central explanation for all evolutionary adaptation yet today. If a trait results in more offspring who share the trait, then that trait is more likely to become common in the population than a trait that arises randomly. ![]() Beneficial traits are extremely varied and may include anything from protective coloration, to the ability to utilize a new food source, to a change in size or shape that might be useful in a particular environment. Second, the trait must be heritable so that it can be passed to an organism's offspring. First, the trait must be beneficial in other words, it must increase the organism's probability of surviving and reproducing. For a trait to undergo positive selection, it must have two characteristics. ![]() Positive natural selection, or the tendency of beneficial traits to increase in prevalence ( frequency) in a population, is the driving force behind adaptive evolution.
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