Most organisms in nature are faced with the conflicting demands of trying to acquire enough food for themselves while simultaneously trying not to become food for some other organism (e.g., a predator). Because both resource acquisition and predator avoidance contribute to growth and survival, respectively, the conflict between these two performance measures usually causes a trade-off between two important fitness components. This trade-off has been frequently described in phenomenological terms but, like many ecologically-important features of organisms, its underlying functional and genetic determinants are unknown. Consequently, we know very little about what characters actually influence growth and predator avoidance, and thus we know almost nothing about the evolution of such networks of characters. In this proposal, we describe research to examine the phenotypic and genetic architecture of characters that are hypothesized to determine growth and predator avoidance in bullfrog and green frog tadpoles (Anura: Ranidae). Bullfrogs (Rana catesbiana) and green frogs (Rana clamitans) are most closely related to one another, and they tend to segregate between habitats with different suites of predators. Bullfrogs occur primarily in lakes that support predatory fish, while green frogs occur primarily in ponds that do not support fish but do support predatory dragonflies and salamanders. This segregation appears to be strongly influenced by differences between the two species in their levels of activity, foraging, and their defenses against predatory fish and dragonflies.
This pair of species, therefore, offers an excellent opportunity to 1) identify the properties that determine growth and predator avoidance success in a pair of closely related species, 2) examine how the patterns of variation in these properties have changed as these species have adapted to their respective environments, and 3) explore the evolutionary dynamics of these traits and predict their responses to natural selection. To do this, we will conduct two major experiments, both of which examine variation in growth and antipredator performance at the species, population, and individual levels. In the first experiment, we will measure a suite of behavioral and morphological characters that we hypothesize are related to growth and antipredator success. We will make these measurements on a large number of individuals from six populations of each species, and examine patterns of variation among traits. In the second experiment, we will estimate magnitudes and patterns of genetic variation and heritability for most of these traits in a single population of each species. Analyses will test hypotheses about functional and genetic constraints on phenotypic evolution and how couplings among characters have changed during adaptation to different environments.
