Dissertation Research: Evolution and Development of Alternative Amphibian Morphologies

在自然选择的概念下，个体应该以一种增加其遗传适应性的方式对环境做出反应。然而，种群中的个体可能会对相同的环境条件做出不同的反应；对于选择压力，往往有不同的、同样适应性的反应。这项拟议的研究将以两种不同的表型出现：一种是快速发育的大型食肉动物形态，另一种是发育较慢的较小的杂食动物形态。这个系统提供了一个独特的机会来研究动物的生态如何深刻地影响它的发育。这一制度也在人口层面上提出了一些有趣的问题。这项研究的第一个目标将是确定二形性是如何在进化中保持的。肉食动物似乎在高度短暂的池塘中表现得更好(因为它们发展得更快)，但杂食动物可能在更持久的池塘中做得更好(因为它们在变质后的生存能力更强，对肉食动物捕食的敏感性更低)。通过检查在给定池塘内两种变种的存活率(变态前和变态后)是否相等，来检验这些适合度权衡的假设。如果这个实验表明这两个变种做得一样好，这将为分析发展战略提供机会。第二个目标是解释什么因素有利于这种和其他类似的二态现象的传播。将研究这种变异性在多大程度上代表了物种范围两端不同环境条件的遗传适应，而不是环境引起的表型差异。第三个目标是解释形态决定的生理学基础。摄取自然产生的甲状腺激素(变态时两栖动物分化的内分泌“触发器”)是否会导致年轻的蝌蚪发育成食肉动物将被研究。如果甲状腺激素是形态分化的触发因素，其潜在的生理机制将被研究。这项研究将在最近的和最终的两级提供急需的信息，说明替代发展战略如何在人口中持续存在。Scaphiopus及其二态蝌蚪是研究发育可塑性进化的优秀模式动物。这项研究的结果可能具有潜在的应用价值。蝌蚪长期以来一直被用于动物发育的研究，而平足可能为发育生物学家提供了一个独特的机会来研究脊椎动物发育过程中大规模的兼性变化。最后，这种肉食性动物转变为杂食性动物的能力已经引起了一位德国科学家的兴趣，他试图提高商业鱼类孵化场的产量，在那里，鱼类通常以更昂贵的肉食性饮食喂养。

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.