DISSERTATION RESEARCH: The Evolutionary History of Marsupial and Placental Mammals: A Study of Evolutionary Constraints in Mammalian Limbs

论文研究：有袋类和胎盘类哺乳动物的进化史：哺乳动物肢体进化制约因素的研究凯伦·西尔斯和约翰·弗林有袋类动物只占现存哺乳动物的6%，身体上的多样性低于胎盘类哺乳动物。传统上，人们认为有袋动物的低多样性是由于它们独特的繁殖模式限制了它们的进化。新生的有袋类动物在早期出生后，只用它们的前肢，爬一段100倍于它们身体长度的距离到它们附着的乳头，完成它们的发育。这项研究的主要目的是检验这一假设，即有袋动物前肢和肩带在推动新生儿到乳头的过程中的预定承诺限制了它的进化。约束被定义为限制物理变化的因素。变异是自然选择的原材料，因此变异较多的物种或谱系可以更容易地进化出新的物理形式。长期以来，制约因素一直被认为是进化史上的重要因素，但还没有得到充分的检验。除了阐明约束在哺乳动物肢体进化中的作用外，了解约束在这个系统中的作用将为理解约束在其他系统中的作用提供基础。一种结合了来自现存和灭绝哺乳动物的遗传、发育和成年身体数据的三级方法将被用来测试对有袋类前肢发育的假设限制。遗传数据将为观察到的有袋类和胎盘类发育事件的时间差异提供解释。发育时间的变化很重要，因为它们提供了一种约束机制。物理数据将被用来检查个体内、物种内以及有袋动物和胎盘动物之间的变异模式。如果有袋类相对于胎盘受到限制，它们在发育过程中，特别是在出生之前，应该表现出较少的身体变异，并实现较少的成年身体多样性。将不同类型的数据组合在一起，将使人们能够对进化过程以及限制因素在其中所起的作用有独特的见解。

DISSERTATION RESEARCH: The Evolutionary History of Marsupial and Placental Mammals: A Study of Evolutionary Constraints in Mammalian LimbsKaren Sears and John Flynn Marsupials compose only six percent of living mammals and are less physically diverse than placental mammals. Traditionally, it has been assumed that low marsupial diversity is due their unique mode of reproduction constraining their evolution. Immediately after their early birth, newborn marsupials, using only their forelimbs, crawl a distance one hundred times their body length to the teat where they attach and complete their development. The primary goal of this study is to test the hypothesis that the obligate commitment of the marsupial forelimb and shoulder girdle in propelling the newborn to the teat has constrained its evolution. Constraints are defined as factors that limit physical variation. Variation is the raw material of natural selection, and therefore species or lineages with more variance can evolve new physical forms more easily. Constraints have long been cited as important factors in evolutionary history but have not been adequately tested. Aside from illustrating the role of constraints in the evolution of mammalian limbs, an understanding of the role of a constraint in this system will provide a base for understanding the role of constraints in other systems. A three-tiered approach incorporating genetic, developmental physical and adult physical data from living and extinct mammals will be used to test the hypothetical constraint on marsupial forelimb development. Genetic data will provide explanations for the observed differences in timing of developmental events in marsupials and placentals. Changes in developmental timing are important because they provide a mechanism for constraint. Physical data will be used to examine patterns of variation within individuals, within species and within marsupials and placentals. If marsupials are constrained relative to placentals, they should show less physical variation during development, especially before birth, and achieve less adult physical diversity. Combining different types of data will allow unique insights into the process of evolution and the role that constraints play in it.