WIKELSKI, Martin C; Princeton Univ., Princeton: MORPHOLOGY, PERFORMANCE AND FITNESS IN GALAPAGOS MARINE IGUANAS

Patterns of selection in a natural population of vertebrates had deterministic and stochastic elements in the short term. Over longer time scales, this combination of selective forces resulted in directional evolutionary change of morphology as well as in the formation of apparently broad reaction norms for performance. Marine iguanas (Amblyrhynchus cristatus) differ in body mass by more than one order of magnitude among island populations. Common garden experiments on a related iguanid, Green iguanas (Iguana iguana) from Panama and Curacao, show that body size is to a large extent heritable. Algae pasture height (food abundance) and environmental temperature sufficiently explain the maximum body size reached on each island. The mechanistic reason for absolute size limits is that the performance of individuals decreases sharply above a certain body size threshold per island. Nevertheless, a 100-year museum record documents that maximum body size on each island increased consistent with the predictions from increased global temperatures, and despite the fact that single events (El Ni�os) introduce a high level of stochasticity in the temporal pattern of selection. High environmental stochasticity is countered by broad reaction norms for organismal performance: Individuals react to changes in their (social) environment using simple endocrine feedback loops that adjust performance to current situations. For example, individuals can completely change their reproductive ecology or reverse their growth pattern in response to environmental cues. I suggest that biologists should give more emphasis to the study of the evolution of reaction norms in organismal performance in nature.