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

论文研究：有袋动物和胎盘哺乳动物的进化史：哺乳动物肢体进化约束的研究有袋动物仅占现存哺乳动物的6%，其物理多样性低于胎盘哺乳动物。传统上，人们一直认为有袋动物多样性低是由于它们独特的繁殖模式限制了它们的进化。刚出生的有袋动物，只用它们的前肢，就能爬上一段相当于身体长度一百倍的距离，到达它们附着的乳头，完成发育。本研究的主要目的是验证一种假设，即有袋类动物的前肢和肩带在推动新生儿到达乳头时的义务承诺限制了其进化。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，因此具有更多变异的物种或谱系更容易进化出新的物理形态。长期以来，约束一直被认为是进化史上的重要因素，但尚未得到充分的检验。除了说明约束在哺乳动物肢体进化中的作用外，理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。一种结合遗传、发育生理和成年生理数据的三层方法将被用于测试有袋类动物前肢发育的假设约束。遗传数据将为有袋动物和胎盘在发育时间上的差异提供解释。发育时间的变化很重要，因为它们提供了一种约束机制。物理数据将用于检查个体内、物种内、有袋动物和胎盘内的变异模式。如果有袋动物相对于胎盘受到限制，它们在发育过程中应该表现出较少的身体变化，特别是在出生前，并且在成年后实现较少的身体多样性。将不同类型的数据结合起来，可以对进化过程和约束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.