Demographic parameters at evolutionary equilibrium' PETER

TAYLOR, P. D., and G . C. W I L I ~ I A M S . 1984. Demographic parameters at evolutionary equilibrium. Can. J . Zool. 62: 2264 -227 1 . We reason that natural selection acting under conditions imposed by physical factors (e.g., geometric constraints on growth rate) and community organization (e.g., persistent finite population sizes, equal total biomass of organisms in proportionately equal size ranges) should lead to certain life history features. The initial size of resource-capturing young should be the smallest that permits growth rate to exceed mortality rate so that the age cohort will start to increase in biomass. Production of such young must be an inefficient use of biomass because of metabolism, predation of embryos, the cost of males, and other losses. Mortality rate during juvenile growth should be a power function like growth rate, but always a bit lower so that the age cohort continues to increase and ultimately to compensate for the inefficiency of reproduction. In a constant environment, the individual should stop growing at the size of greatest expected excess of future reproductive resources over size, and thereafter devote all expendable resources to reproduction. Any given size range of animals, such as 1-2 mg or 1-2 kg, should consist of both mature and immature individuals in any community and should be devoting about a third of its investable resources to reproduction and the rest to growth. We use our equations to generate sample life histories. The simple form of our equations organizes these life tables into families of similar schedules with variable generation time.