Environmental variables can also interact in their effects on the life history. For instance, adult female Drosophila that had been reared under standard conditions were exposed to different levels of nutrition, chosen to span the level that the females usually encountered, and to two levels of opportunities for mating. Excess mating can reduce the life span of female Drosophila (Fowler and Partridge, 1989; Chapman et al., 1995). Nutrition and opportunity for mating interacted in their effect on female lifetime reproductive success, because the cost of mating reduced lifetime reproductive success only at the highest level of nutrition (Chapman and Partridge, 1996).
Life histories, like any other trait evolving under natural selection, are expected to evolve toward a form optimal only for the environments in which selection occurs. There has been a great deal of theoretical and empirical work on how sensitivity to the environment of life-history traits, their "norm of reaction" or "phenotypic plasticity," is expected to evolve for the range of environments naturally encountered (e.g., Gomulkiewicz and Kirkpatrick, 1992; Scheiner, 1993; de Jong, 1995). The conclusions of such models depend, to some extent, upon their assumptions. For instance, increased plasticity might or might not carry a cost. The trait could be one like insect body size, which is fixed at the onset of adulthood, so that plasticity is expressed only once during the life history, or one like egg-laying rate in female Drosophila, which can vary throughout the adult period. Different environments could be encountered equally or unequally frequently. The general theoretical findings are that phenotypic plasticity will evolve under a wide range of circumstances. Further, where different environments are encountered unequally frequently, the pattern of plasticity will produce the most appropriate expression of the trait and minimum additive genetic variance for it in the environment most frequently encountered.
The environments encountered by present-day human populations undoubtedly differ widely from those in which their life history evolved. Factors such as nutrition and disease are probably particularly important. Another peculiarity of human populations is that as their economic status rises, their fecundity declines by design. We are therefore probably dealing with fecundity schedules for both males and females that are very different from those that applied, on average, in the evolutionary past. In humans, changed environment can alter circumstances during development, especially for growth, in a way that is known to affect age at first breeding and adult phenotype. Within the adult period, both fecundity schedule and age-specific death rates are affected by environment. Many peculiarities of modern human populations could be manifestations of adaptive phenotypic plasticity that has evolved in response to the various levels of resources encountered in the past. Some insight into the sorts of plasticity patterns we might expect in response to these variables could, therefore, come from a knowledge of the effects of social and economic structuring in human populations before the demographic transition. The effects on postreproductive survival patterns would be particularly interesting. Whether the effects of the demo-