Spontaneous mutations and the origin and maintenance of quantitative genetic variation

A key challenge in evolutionary biology is to understand how genetic variation – differences in the DNA of individuals in a population – is generated and maintained to create the enormous diversity that exists in nature. Mutations to the DNA introduce new variation, but these are constantly removed from populations by two other evolutionary forces: natural selection and genetic drift. Natural selection removes harmful genetic mutations that affect an organism’s fitness and reproduction, and genetic drift is the random increase in, or loss of, a genetic variant from a population over time. However, disentangling the effects of these evolutionary forces is challenging because the genetic variation we observe is often the final product of a long history of interaction between them. Huang et al. have now investigated genetic variation by breeding fruit flies in the laboratory. Natural selection was minimized for these flies; genetic drift was therefore the main force that removed variation. Huang et al. then sequenced the DNA of the flies to estimate the rate at which genetic mutations spontaneously occur. The sequences contained many more “high-impact” mutations (which directly affect how proteins in the fly’s cells work) than seen in sequences taken from a natural fly population. Traits that are produced by the cumulative actions of many genes and the environment are known as quantitative traits. By examining how much variation genetic mutations introduced into the quantitative traits of each generation of the laboratory-grown flies, Huang et al. estimated how much variation should occur in a natural population whose quantitative traits evolved without natural selection. This estimate was much higher than the levels of genetic variation seen in nature, suggesting that natural selection acts to eliminate mutations that significantly affect quantitative traits. Simple theoretical models cannot explain the relatively high spontaneous mutation rate and low genetic variation seen in the quantitative traits of natural populations. Therefore, further work is now required to understand more about the balance of evolutionary forces that maintain quantitative genetic variation.