Population genetics textbooks, for obvious didactical reasons, also tend to concentrate on scenarios akin to simple viability selection, with the relative viabilities independent of the population density and genetic constitution. In contrast with the early tradition, the textbooks refer to the viabilities as fitnesses, thus making in the minds of new generations of population geneticists fitness effectively synonymous with viability. The advantage of the so restricted universe of discourse is that it allows population genetics to concentrate on cataloging the dynamic consequences of the variety of genotype to fitness maps, as well as mating and transmission rules. Another advantage of the restriction to simplified life histories is that one can consider fitnesses on an instantaneous and local basis. One may thus consider models in which fitnesses vary in time, over space, or with the local gene frequencies or population densities.
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