Under the concept of natural selection, individuals should respond to their environment in such a way as to increase their genetic fitness. Individuals in a population may respond differently, however, to identical environmental conditions; there are often alternative, equally adaptive responses to selection pressure. The proposed research will study spadefoot tadpoles which occur as two discrete phenotypes: a large, rapidly developing carnivore morph and a smaller, slower developing omnivore morph. This system provides a unique opportunity to examine how an animal's ecology can profoundly affect its development. The system poses interesting problems on the population level as well. The first goal of this study will be to ascertain how the dimorphism is maintained evolutionarily. Carnivores appear to do better in highly ephemeral pools (because of their more rapid development), but omnivores may do better in more permanent ponds (because of their greater postmetamorphic survivorship and reduced susceptibility to carnivore predation). The hypothesis that these fitness trade-offs balance will be tested by examining whether survivorship (both pre- and postmetamorphic) of the two morphs is equal within a given pond. If this experiment shows that the two morphs do equally well, this would provide the opportunity to analyze the strategies of development. The second goal is to explain what factors favor the spread of this and other similar dimorphisms. Research will be done on the extent to which this variability represents genetic adaptation to different environmental conditions at opposite ends of the species' range as opposed to environmentally induced phenotypic variation. The third goal is to explain the physiological basis of morph determination. Whether the ingestion of naturally occurring thyroxine (the endocrine "trigger" of amphibian differentiation at metamorphosis) causes young tadpoles to develop into carnivores will be examined. If thyroxine is the trigger of morph differentiation, the underlying physiological mechanisms will be studied. This study will provide much needed information, at both the proximate and ultimate levels, concerning how alternative strategies of development can persist in a population. Scaphiopus with its dimorphic tadpole is an excellent model animal for studying the evolution of developmental plasticity. The results of this study may have potential applications. Tadpoles have long been used in the study of animal development and spadefoots may provide developmental biologists with a unique opportunity to study large scale facultative shifts in development in a vertebrate. Finally, the carnivore's ability to switch to omnivory has already interested a German scientist attempting to increase yields in commercial fish hatcheries, where fish are typically fed a more expensive carnivorous diet.
