Up to now we have assumed discrete generations. Under this assumption, all individuals are born at the same time and then reproduce at the same time followed by complete reproductive senescence or death. Such a model approximates reality for some species. For example, many insects and plants have only one generation per year that is synchronized by the seasons, and the discrete-generation model can approximate their evolution. However, as pointed out in Chapter 2, individuals in many species can reproduce at multiple times throughout their life, can mate with individuals of different ages, can survive beyond their age of reproduction, and can coexist with their offspring and other generations. We do not have to look far to find such a species; our own falls into this category of overlapping generations. In species with overlapping generations, an important component of population structure is age structure, the distribution of the ages of the individuals found in the population at a given time. The age distribution can have a large impact on whose gametes are transmitted to the next generation, particularly when an individual's chances of survival, mating, and reproducing are all influenced by age, as they are in humans. In this chapter we will examine the evolutionary impact of age structure and age-dependent fitness components in demes with overlapping generations. Such models also lie at the interface of population genetics and population ecology, a field that deals extensively with age-structured populations. We therefore start this chapter by an examination of some of the fundamental demographic parameters that ecologists have used to characterize populations with overlapping generations. We will then introduce genetic variation that influences the phenotypic variation in these demographic parameters to look at the impact of selection in age-structured populations. Finally, a detailed example of such selection will be given, an example that makes use of many of the concepts developed in the previous chapters.
Population Genetics and Microevolutionary Theory, By Alan R. Templeton Copyright © 2006 John Wiley & Sons, Inc.
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