Natural Selection

Natural selection is the nonrandom and consistent differences between different genotypes (that is, in survival and reproduction). Natural selection acts to preserve genotypes that have higher fitness (that is, viability or fertility) and to eliminate genotypes that have lower fitness. Natural selection is viewed as a creative force that directs the course of evolution by preserving genotypes or traits best adapted in the face of environmental conditions and competition. Natural selection causes adaptation that is improvement in function, and it can act on tiny differences in fitness. This is illustrated in Figure 3 a, which shows the changes in the frequency of a slightly advantageous mutant that is initially rare in a hypo thetical asexual population. It is assumed that the ancestral genotype has a relative fitness of 1. (In this example, "fitness" means "viabil-ity"—that is, the probability of surviving to the age of reproduction.) The mutant genotype has slightly higher fitness (w = 1.001) and a very low initial frequency (p = 0.00001). Figure 3 a shows that by approximately generation 18,000, the mutant genotype replaces the ancestral genotype.

Another illustration is provided in Figure 3b using a hypothetical asexual population that initially has five different genotypes at equal frequencies. In this figure, five different lines describe the frequencies of five different genotypes with fitnesses w1 = 0.96; w2 = 0.97; w3 = 0.98; w4 = 0.99; and w5 = 1.00. Here, the differences in fitness between different genotypes are larger than in the previous example, meaning that natural selection is stronger. The least fit genotypes (genotypes 1 and 2) practically disappear from the population by generation 100. The fittest genotype (genotype 5) dominates by generation 500. Figure 3b shows that if natural selection is strong, biological evolution can be very rapid. In large populations, natural selection will quickly establish advantageous genotypes and eliminate disadvantageous ones (as illustrated in Figure 3). In contrast, in small populations random genetic drift will interfere with natural selection, resulting in the possibility of a stochastic loss of advantageous genotypes and establishment of slightly deleterious genotypes. Accumulation of deleterious mutations in small populations may result in their extinction.

Figure 3 a

Effects of Natural Selection on Allele Frequencies in an Asexual Population

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Generation number

Establishment of a slightly advantageous mutant that is initially rare

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Generation number

Establishment of a slightly advantageous mutant that is initially rare

Figure 3b ot0.4

Figure 3b ot0.4

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Strong selection in a polymorphic population

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Strong selection in a polymorphic population

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