On Breeding More Than Once

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Abstract
L.C. Cole proved, for a particular population model, that a semelparous population with mean fecundity (b + 1) would have the same rate of increase as an indefinitely iteroparous population with mean fecundity (b). The evolution of iteroparity will be favored by a number of factors and particularly by delayed maturity and by reduction in the rate of survival during the 1st yr of life. Iteroparity is not a simple phenomenon; individuals may breed only once in a given year, they may breed several times, or they may breed almost continuously. The relative fitness of these strategies is defined, and almost continuous breeding is not likely to evolve in animals which mature early in life and produce very large numbers of eggs. The consequences of variation in the age at maturity are explored. An equation is given which yields r for an individual maturing at any given age and is used to define conditions that r should increase with delayed maturity. As the result of selection, the age at maturity may vary within and among populations and may also change with time. Identical phenomena may be observed even if the age at maturity has no heritable component. Variation in this character must be interpreted cautiously. Attention is drawn to the deficiencies of the simple population models used. The time dependence of certain terms in the general equations, the effects of variance in population size and differences in the genetics of semelparity and iteroparity combine to make general predictions from simple models naive and precise predictions from general models difficult to identify. Ideas such as these can be tested adequately only by detailed reference to particular situations.

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