Alternative Concepts of Reproductive Effort, Costs of Reproduction, and Selection in Life-History Evolution

Abstract

An outline for an organismic theory of reproductive tactics is presented to develop the demographic theory of optimal reproductive tactics into a more realistic theory of life-history evolution. Reproductive effort—denned as the proportion of resources invested in reproduction—and the costs in somatic investment do not automatically result in survival costs. Both the conditions where survival costs are produced and the conditions where reproduction can take place without survival costs are specified. Compensation and threshold hypotheses are put forward to allow weaker correlations between reproduction and survival than the trade-off hypothesis, which assumes direct impacts by reproductive effort on survival. Furthermore, reproductive tactics are unlikely to be moulded by the demographic forces of selection only. An empirical example is shown where residual reproductive value played no significant role in the evolution of reproductive tactics. Selection probably operates not on separate life-history traits but on whole organisms through their entire life-history. The structural and physiological intercouplings between separate traits can result in phenotypic opportunity sets where selection can mould life-history traits only within the constraints of the opportunity sets. Optimization theory has provided an efficient technique for modelling and making predictions. However, organismic selection does not necessarily optimize adaptive strategies but eliminates unfit strategies. Life-history theory, and evolutionary theory in general, can be developed along alternative logical lines when different hypotheses are generated on how selection operates.