THE PARTICULAR MATERNAL EFFECT OF PROPAGULE SIZE, ESPECIALLY EGG SIZE- PATTERNS, MODELS, QUALITY OF EVIDENCE AND INTERPRETATIONS

Propagule size is perhaps the most widely recognized and studied mater nal effect in ecology, yet its evolution is not well-understood. The l arge body of extant optimality theory treats parental investment solel y as an ecological problem, largely from the perspective of progeny. T his approach has had limited success explaining the ubiquitous variati on in propagule size within and among natural populations at most temp oral and spatial scales. This problem aside, an unassailable gap in pr opagule size theory is that it pays little heed to the fact that offsp ring size is a joint phenotype of two individuals- the offspring and i ts mother. Hence, the ecology of mothers is decidedly as important in shaping the evolution of propagule size phenotypes. There are two reas ons to suspect that this gap may account for the lack of success of op timality theory to explain variation in nature. The first is that opti mality models of propagule size make no allowance for, nor can they ex plain, widespread, multivariate correlations between maternal characte rs and clutch parameters, namely the positive phenotypic covariances o f maternal age, size, fecundity, and per-propagule investment found in many organisms. If per-propagule investment is optimized by selection based on the expectation of offspring fitness, then why should that p henotype be a function of maternal age or size when the ecological cir cumstances of progeny are not changing as a function of maternal age o r size? The second gap in current theory is that, like all optimizatio n theory, it is patently non-genetic in that it is assumed that the ph enotypes optimized are evolutionarily accessible. Recent maternal effe cts theory indicates that traits subject to maternal influence behave in unanticipated ways. Specifically, there may be time lags in respons e to selection, and hence, selection away from the optimum phenotype. This paper explores a suite of issues pertaining to the evolution of p ropagule size from the broader perspective of propagule size as a mate rnal effect (PSME) with a goal of widening the lens through which prop agule size is viewed by evolutionary ecologists. Two themes are develo ped. First, I suggest that, to understand egg size variance and its im plications for both maternal and offspring fitness, it is necessary to consider explicitly the ecological context in which a mother is produ cing eggs, not just that into which offspring will enter. I argue that some of the variables that have only been incorporated in pairwise fa shion (or not at all) into studies of propagule size might account for the lack of agreement about how this important life history feature e volves. Further, I suggest that failure to consider other sources of s election on maternal phenotypes, driven by a narrow adaptationist view that has historically been taken of PSMEs, has obfuscated many intere sting questions surrounding their coevolution with maternal characters . Thus, the second theme is that it is necessary to consider other exp lanations for why propagule size varies apart from those pertaining to offspring fitness per se'. Based on a detailed review of the empirica l literature, I conclude that the concept of an optimal propagule size is not only an insufficient construct to explain the evolution of pro pagule size, but that continued reliance on an optimization approach i s likely to stifle development of more realistic and predictive theory for the evolution of this key life history trait. Novel theory should incorporate realities from physiology, development and genetics and s hould accommodate the dynamic nature of the selective environments in which propagule size evolves, all of which have been shown by empirici sts to play a role in determining propagule size phenotypes. A key fea ture of this theory should be the explicit treatment of propagule size as a maternal effect.