An aim of comparative biology is to identify model organisms that illustrate general principles of evolution. Mammals are especially useful for this purpose because they demonstrate the principle of convergent (repetitive) evolution and can be used to test the proposition that similar ecological demands evoke parallel evolutionary adaptations. In this context, the marsupials are particularly important because they evolved in isolation in South America and Australia, and came to include ecological and anatomical "mimics" of more familiar (placental, or eutherian) mammals. Moreover, given the fact of continental drift, relationships among marsupials themselves reflect the detachment and increasing separation of the continents on which they occur and there are apparently convergent similarities among geographically separated marsupials as well as between these and eutherians; for example, some of the American opossums are almost indistinguishable from Australian marsupials which lead similar lives. Biologists have long argued whether the resemblances of certain South American and Australian marsupials are in fact convergent or represent special relationships, perhaps indicating additional "invasions" of one continent from the other. To settle that dispute, and to develop marsupials as a model group, more reliable genealogical trees are needed. The problem in obtaining such trees is that anatomy can be deceiving, and ideally relationships should be established using methods which measure internal or chemical characteristics as well. Over the long run, the genetic material itself (DNA) seems to change at a roughly constant rate; comparison of the DNAs of related species can therefore give a direct measure of the time of the species' separation from a common ancestor. DNA is made up of two strands held together along their lengths by chemical bonds. In this study, DNA will be extracted and purified from several species, after which the strands will be separated by boiling. Single strands of DNA from one species will then be allowed to hybridize, or pair, with their counterparts from another. The stability of hybrid DNA molecules is a measure of the genetic similarity between the two species. If measurements of the thermal stability of hybrids between many pairs of marsupial species are made, evolutionary relationships among all of them can be calculated. In this manner, a reliable phylogenetic tree will be constructed and correlated with the geographic history and other features of marsupial biology.
