CONSTRAINTS IN THE EVOLUTION OF LIFE HISTORIES

The life history favoured by natural selection maximizes fitness, and this implies maximization of fecundity and survival at all ages. The observed diversity in life histories suggests that there are constraints on what can be achieved in practice. Functional constraints occur if only certain combinations of age-specific fertility and survival are possible, either because of the physiology of the organism or because of the ecological impact of its environment. The resulting constrained optimization means that the organism is involved in making trade-offs between life-history characters. A major task for the future is the measurement of trade-off functions in the environment in which the life-history evolved. Natural variation between individuals and populations, genetic studies and experimental manipulations have all been used to detect trade-offs. The last two methods are the most satisfactory, and can be complementary. Experimental manipulations are at their best when based on sound physiological understanding of the traits under manipulation. Constraints can also operate on the long-term. Local optima, evolutionary lags and irreversible evolution may all have contributed to the diversity of life histories.