Chapter 9 - Natural Selection

Abstract

Individuals have the phenotypes of viability, mating success, and fertility. These phenotypes are combined into a single phenotype called fitness. Natural selection alters allele frequencies in the gene pool proportional to an allele's average excess of fitness. Natural selection is not “survival of the fittest,” but rather favors gametes with positive average excesses for fitness. Because selection operates through average excesses, any factor that influences average excesses, such as system of mating and initial allele frequencies can alter the course of evolution due to natural selection. When genotypes are not measured, fitness is assigned to other traits, such as morphological or behavioral traits. The rate of change of average fitness is proportional to the additive variance of the phenotype of fitness when genotypes are not measured. The additive variance of fitness is a measure of the gamete's impact on fitness variation in the next generation, so both the measured and unmeasured genotype approaches demonstrate that natural selection operates from the perspective of gametes and not individuals. Selection always operates to increase average fitness, but epistasis and pleiotropy often create multiple average fitness peaks. Natural selection only causes evolution toward the nearest local optimum, but it does not ensure evolution toward the global optimum. Indeed, natural selection can prevent the evolution of a population to a global optimum from many initial conditions. Natural selection does not optimize individual traits that contribute to fitness unless their contribution is strictly linear. Sometimes pleiotropy results in a deleterious trait being positively correlated with a selectively favored trait, and in this case the deleterious trait can actually increase in the population because of natural selection. The most common genetic diseases and disease risk factors in human populations are likely due to the action of natural selection acting on correlated traits.