Effects of developmental plasticity and antagonistic selection on phenotypic variation in spiders.

摘要

In this dissertation, I ask whether developmental plasticity can explain the variation of phenotypic distributions as an adaptive response to variation in population demography and changes in the strength and direction of selection. I use three spider species to examine this question. Using a combination of field attraction and mark-recapture experiments, I first ask how male spiders locate females and the choices they make while mate searching. I demonstrate that males can distinguish between females of different species, populations and ages using long-distance pheromones, and that males increase mate searching risks because they are searching for specific females. As these long-distance pheromones can provide cues of female density, I next examine whether juvenile males alter their development when reared in the presence or absence of female's pheromones (cues of high and low female density, respectively). My results demonstrate that males alter their ontogeny to mature the phenotype that is most beneficial in the competitive context they are likely to experience at maturity. Males develop significantly faster when females are present, and are significantly larger and in better condition when females are absent. Since small size is apparently the result of a decision that is independent of resource availability, I next examine whether male fitness is phenotype specific. By testing small and large males in the competitive environments in which they mature, I show that although larger males are superior in direct competitions, smaller males have higher fitness when tested in the context that leads to their more rapid development. These results challenge the concept of male quality as a fixed trait value, demonstrating the necessity of taking life-history traits into consideration. I next use field populations to demonstrate that demographic variables fluctuate within a season, and depend on the scale of the examination. The strength of selection pressures also varied significantly throughout the breeding season. As a result, males are likely to experience different competitive challenges and selection pressures at different spatial and temporal scales, making a single phenotypic optimum unlikely. I end with a discussion regarding the adaptive nature of developmental plasticity, and when it is likely to evolve.