Natural Selection

A final process that brings about changes in allelic frequencies is natural selection, the differential reproduction of genotypes (see p. 000 in Chapter 22). Natural selection takes place when individuals with adaptive traits produce more offspring. If the adaptive traits have a genetic basis, they are inherited by the offspring and appear with greater frequency


23.14 Populations diverge in allelic frequency and become fixed for one allele as a result of genetic drift. In this experiment, Buri examined the frequency of two alleles (bw75 and bw) that affect Drosophila eye color in 107 replicate populations. Each population consisted of 8 males and 8 females; each population began with the frequency of bw75 equal to .5.

in the next generation. A trait that provides a reproductive advantage thereby increases over time, enabling populations to become better suited to their environments—to become better adapted. Natural selection is unique among evolutionary forces in that it promotes adaptation (< Figure 23.15).

Fitness and selection coefficient The effect of natural selection on the gene pool of a population depends on the fitness values of the genotypes in the population. Fitness is defined as the relative reproductive success of a genotype. Here the term relative is critically important: fitness is the reproductive success of one genotype compared with the reproductive successes of other genotypes in the population.

Fitness (W) ranges from 0 to 1. Suppose the number of viable offspring produced by three genotypes is:

Genotypes: Mean number of offspring produced:


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