The Role of Development in the Evolution of Life Histories

The aspects of development that interest students of life history evolution are plasticity, canalization, and constraint. Plasticity has both an adaptive component modifiable by gene substitutions and a chemically and structurally inevitable component. Plasticity may be either continuous or discrete. Continuous plasticity in life history traits is shaped by major trade-offs among growth, reproduction, survival, and differentiation. Allocations among these competing needs can be understood as problems in optimization within constraints set by the developmental system. More is known about what evolution ought to produce, in this respect, than about what developmental systems can produce. Discrete plasticity — the environmentally induced production of alternative forms, such as environmental sex determination — seems to be selected when there are discontinuities in the environment or where intermediate shapes will not work, and when environments vary mostly between, rather than within, generations. In different ways both plasticity and canalization uncouple the gene pool from selection, implying that the organism is a privileged level of selection and promoting stasis within species. We know little about developmental constraints and their implications for evolutionary ecology. Organisms appear to be a mosaic of relatively recent adaptations which we can understand in terms of optimality theory, embedded in a framework of relatively old constraints which we would like to understand in terms of developmental mechanisms.