Seasonality and Patterns of Natural Selection for Life Histories

Abstract

I present a model to investigate the effects of density-dependent resource availability on per capita population growth rate. By considering a continuous time model of inclusive fitness, population losses as well as growth are essential determinants of phenotypic fitness. The outcome of natural selection for various life-history characteristics is dependent upon not only the shape of the life history-specific population growth curves, but also upon the form of the seasonality function. As the magnitude and length of a resource shortage period increases, the rate of decrease becomes a more important component of fitness. Seasonality is often characterized by (1) a period of population decrease, when natural selection favors functions which minimize somatic or embryonic mortality, and (2) a period of high resource availability when high somatic and gametic productivity are favored. Assuming that mean resource availability remains constant, seasonally deterministic and/or random variation in the availability of an essential resource lowers mean fitness and carrying capacity. Life histories differ considerably in adaptedness to various levels and seasonal fluctuations in resource availability. Clearly life histories must assume a compromise between "good" and "bad" times. In resource-limited populations, large body size in homeotherms represents an example of an evolutionary "strategy" which can be neither r- nor K-selected. The usual low fecundity and late maturity of large homeotherms establishes a low r, but larger individuals require more resources for maintenance and thus cannot sustain higher carrying capacities on a given set of resources. Seasonality appears to be a very important factor in the evolution of large body size, especially in homeothermic vertebrates. Seasonality is also suggested as an important factor in the evolution of (1) fat and resource storage mechanisms, e.g., wax ester production in marine zooplankton, (2) geographic variation in clutch or litter size, and (3) rapid seasonal somatic growth patterns.