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

论文研究：有袋类和胎盘哺乳动物的进化史：哺乳动物四肢进化限制的研究凯伦·西尔斯和约翰·弗林有袋类只占现存哺乳动物的6%，而且身体上的多样性不如胎盘哺乳动物。传统上，有袋动物的低多样性被认为是由于它们独特的繁殖方式限制了它们的进化。刚出生不久的有袋动物，只用前肢爬行一百倍于身长的距离到达乳头，并在那里附着完成发育。本研究的主要目的是检验这一假设，即有袋类动物的前肢和肩带在推动新生儿乳头的强制性承诺，限制了其演变。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可以更容易地进化出新的物理形式。长期以来，限制被认为是进化史上的重要因素，但尚未得到充分的检验。除了说明在哺乳动物四肢的进化中的作用的约束，在这个系统中的约束的作用的理解将提供一个基础，了解约束在其他系统中的作用。一个三层的方法，结合遗传，发育的身体和成年人的身体数据，从生活和灭绝的哺乳动物将被用来测试的假设约束有袋类动物前肢的发展。遗传数据将提供解释所观察到的有袋动物和胎盘动物的发育事件的时间差异。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。如果有袋类动物相对于胎盘类受到限制，它们在发育过程中，特别是出生前，应该表现出较少的身体变化，并实现较少的成年身体多样性。结合不同类型的数据将使我们能够对进化过程以及约束在其中所扮演的角色有独特的见解。

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.