The evolution of reproductive rates is usually modelled by variants of Lack's (1947) hypothesis on the evolution of an optimal clutch size in birds. This hypothesis suggests that the survival of parents and offspring will decline with the number of offspring produced so that number of offspring that will survive and reproduce is optimal at an intermediate reproductive rate.

Lack's hypothesis seems to holds at a local scale, where the organism has a fixed trade-off between the rate of reproduction and the rate of offspring and/or parent survival. But on a longer time scale, where the trade-off evolves by natural selection, the hypothesis predicts a continuous decline in the trade-off and a continuous increase in reproductive rate. This paradox of a continuously increasing reproductive rate may be solved by selection by density dependent competitive interactions. The latter model predicts a limited rate of reproduction that is balanced against the level of extrinsic mortality. The hypothesis also suggests that the trade-off between reproduction and survival is evolutionarily dependent upon the optimum to the population dynamic growth rate. This goes contrary to Lack's hypothesis that suggests that it is instead the optimal growth rate that is determined by the trade-off between reproduction and survival.

References

• Lack, D. (1947). The significance of clutch size. Ibis 89, 302--352.