The Evolution of Developmental Plasticity in Reproductive Characteristics: An Application of the "Adaptive Coin-Flipping" Principle

Reproductive characteristics such as egg size and clutch size can be highly variable within populations of cold-blooded vertebrates. Such variation makes the idea of an optimal mean reproductive characteristic difficult to defend. Using a decision-theoretic approach and data from amphibians it is shown how the concepts of canalizing and decanalizing selection, which result, respectively, in a buffered or unbuffered developmental system, can be responsible for the maintenance of specific levels of intrapopulational variation. We distinguish different types of variation, such as intraclutch and interclutch variation in egg size. We review evidence which suggests that, as an adaptation to an unpredictable offspring environment, a single female may produce a seemingly maladaptive egg as a result of selection operating to maintain developmental plasticity in the vitellogenic process. We have called the developmental mechanisms which generate such seemingly stochastic variation "adaptive coin-flipping." The properties of the developmental "coin" (which can be two-sided or many-sided, fair or weighted) can evolve as a result of differences among individuals in sensitivities of vitellogenesis to environmental variation or as a result of individual differences in unresolvable developmental noise. Evidence is cited for the heritable nature of such variation in the properties of "coins" and the significance of a developmental "coin-flip" for ecological processes in general is discussed.