Adaptive value, entropy and survivorship curves

NATURAL survivorship curves fall into three main types^1,2. Type I, or rectangular distribution, describes the situation in which all individuals attain the maximum physiological longevity of the species. Here the maximum age at death and the mean life expectancy coincide. The Type II life table describes a mortality which is independent of age; that is, no single age group is favoured at the time of dying. The Type III table is characterised by a high mortality early in life and a life expectancy which increases with the age of survivors. The three types of curve are illustrated in Fig. 1. The general shape of the survivorship curve is fixed for each species, however, the convexity of the curve is highly sensitive to environmental conditions and the genetic constitution of the population. This fact suggests that the general shape of the life table may provide information about the genetic variability in the population, the range of environmental factors that impinge on the population or the incidence of random events in the lifetimes of different individuals³. This note characterises in terms of two new demographic parameters the relative effect genetic and environmental factors play in the evolution of the life table. The analysis we give revolves around two demographic variables, the entropy of a population and the adaptive value. Entropy H measures the variability of the mortality distribution. The measure of variability also describes the convexity of the life table. The adaptive value ψ is a measure of the correlation between the variability of the mortality distribution and the environmental variability. Thus the adaptive value reduces to the entropy when the environment is constant.