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：哺乳动物四肢进化约束的研究凯伦·西尔斯和约翰·弗林有袋动物仅占现存哺乳动物的百分之六，并且身体多样性不如胎盘哺乳动物。传统上，人们认为有袋动物多样性低是由于其独特的繁殖方式限制了其进化。刚出生后，新生有袋动物仅使用前肢，爬行其身体长度一百倍的距离，到达乳头，并在那里完成发育。这项研究的主要目标是检验以下假设：有袋动物前肢和肩带在将新生儿推向乳头时的义务限制了其进化。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，因此变异越大的物种或谱系就越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，但尚未得到充分测试。除了说明约束在哺乳动物肢体进化中的作用之外，了解约束在该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。将采用三层方法，结合来自现存和灭绝哺乳动物的遗传、发育身体和成年身体数据，来测试对有袋动物前肢发育的假设限制。遗传数据将为观察到的有袋动物和胎盘动物发育事件时间差异提供解释。发育时间的变化很重要，因为它们提供了一种约束机制。物理数据将用于检查个体内、物种内以及有袋动物和胎盘动物内的变异模式。如果有袋动物相对于胎盘动物受到限制，那么它们在发育过程中，特别是在出生前，应该表现出较小的身体变化，并且成年后的身体多样性也较低。结合不同类型的数据将有助于对进化过程以及约束在其中发挥的作用产生独特的见解。

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.