Equations 11.5 and 11.22 offer many common insights into natural selection and its attributes:
1. Natural selection can only operate when there is genetic variation associated with phenotypic variation for fitness in the population. If there were no genetic variation influencing the phenotype of fitness, fitness could not possibly be heritable and therefore equation 11.22 implies no evolution due to natural selection. Likewise, without genetic variation associated with phenotypic variation of fitness, there can be no nonzero average excesses of fitness, and therefore equation 11.5 also implies no evolution by natural selection.
2. The only fitness effects that influence the response to natural selection are those transmissible through a gamete. Equation 11.5 shows that there is no response to selection unless the average excess for fitness is not zero, and equation 11.22 shows that there is no response to selection unless the additive genetic variance for fitness is not zero. The additive genetic variance can only be nonzero if the average effects are nonzero, and the average effects are nonzero if and only if the average excesses are nonzero (Chapter 8). Hence, both equations demonstrate that the response to natural selection depends upon how individual fitness variation is funneled through a gamete and transmitted to the next generation. Natural selection can only be understood by taking a gamete's perspective.
3. The adaptive outcome represents an interaction of fitness variation with population structure. As shown in Chapter 8, both the average excesses and average effects are explicitly functions of the rules by which gametes are paired together to form genotypes, that is, population structure. Since it is average excesses and average effects that exclusively determine the response to natural selection (equations 11.5 and 11.22, respectively), population structure plays a direct role in determining the adaptive response to an environment. This reinforces the fact that natural selection cannot be survival of the fittest. The impact of the fitness of an individual upon the response to selection can only be evaluated in the context of a population structure. Even complete knowledge of the fitness of every individual in a population is insufficient to determine the response to selection.
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