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

University Of Chicago, Chicago IL

Investigators

Abstract

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