evolutionary history. The third section applies the framework developed in the first two parts to understanding major historical trends in human fertility, parental investment, and mating regimes. The transition from hunting and gathering to farming and pastoralism is considered first. Land- and power-based stratified societies are then discussed, followed by an analysis of wage-based competitive labor markets and demographic transition. The chapter concludes with a discussion of the new research questions and approaches to research design suggested by this framework.


Fundamental Trade-Offs in Life History Theory

Natural selection acts on variability in the traits of individual organisms within populations. Traits (and the genes that code for them) increase in frequency relative to other traits when their average effects on the individuals possessing those traits act to maximize their long-term production of descendents through time.1 Fertility is the most direct contributor to an organism’s fitness (i.e., the number of descendents it produces). In fact, all other fitness components, such as mortality, only affect fitness through their effects on fertility (e.g., mortality rates affect fitness by affecting the probability of living to the next reproductive event). All else constant, any increase in fertility increases an organism’s fitness. However, there are two trade-offs affecting natural selection on fertility.

The first is the trade-off between present and future reproduction. An organism can increase its energy capture rates in the future by growing and thus increasing its future fertility. For this reason, organisms typically have a juvenile phase in which fertility is zero until they reach a size at which some allocation to reproduction increases fitness more than growth. Similarly, among organisms that engage in repeated bouts of reproduction (humans included), some energy during the reproductive phase is diverted away from reproduction and allocated to maintenance so that it can live to reproduce again. The general expectation is that natural selection on age of first reproduction and on the adult reproductive rate will tend to maximize total allocations of energy to reproduction over the life course.


Selection acts on the “inclusive fitness” of genes coding for traits. Inclusive fitness includes effects on both the reproductive success of the individual bearing the gene and other individuals, related by common descent, who also bear the gene. For example, selection on genes affecting alarm calls in response to predators depends both on their effects on the reproductive fitness of the caller (who may risk a greater threat of predation) and on relatives bearing those genes (whose lives may be saved by the call).

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