Adaptive Radiation, Convergence and the Marsupials
Metatheria (Marsupalia) originated upon Laurasia in the Cretaceous with the oldest fossilised remains having been dated to 125 Ma. The first period of metatherian diversification occurred in the late Cretaceous. The K-T mass extinction created an ecological void in which mammalian radiation occurred in the Palaeocene. With South America isolated from North America throughout the Tertiary, there was the second great period of metatherian diversification and there were numerous examples of placental and marsupial convergence. In the Eocene, one line of metatheria radiated across the Antarctic to Australasia and from that lineage the modern Australasian marsupials are descended. There are numerous examples of convergence within this marsupial population, which are now becoming evident with advances in DNA analysis. In the Pliocene the Great American Interchange occurred when the Isthmus of Panama was created and many South American marsupial fauna became extinct. Introduction
To examine the relationship between marsupials and placentals throughout the tertiary one must first understand the concepts of adaptive radiation and convergent evolution. In 1953, G.G. Simpson defined adaptive radiation as the ‘more or less simultaneous divergence of numerous lines from much the same adaptive type into different, also diverging adaptive zones.’ Adaptive radiation is the process by which a single common ancestor differentiates into myriad species, which have morphological and ecological adaptations to aid their survival in a wide range of environments. These adaptations arise through both speciation and phenotypic adaptation (Schluter, 2000). Convergence is the process by which two or more non-monophyletic organisms evolve analogous traits due to occupying similar ecological niches. Simon Conway Morris has argued that a set number of potential ecological niches exist rather than evolution and natural selection leading to the creation of new ecological niches. From this one could infer that there are also a finite number of morphological adaptations that promote the successful functionality of an organism in available ecological niches (Ruse, 2008). Conway Morris cites the shear number of examples of convergence as proof that evolution is constrained by natural selection along certain evolutionary pathways, which lead to the successful occupation of ecological niches. Geographical isolation can also lead to the duplication of certain traits through both convergence and parallel evolution (Springer et. al, 1997). The Cretaceous-Tertiary mass extinction event left an ecological void. Many modern eutheria and metatheria can trace their origins back to the late Cretaceous but it was not until the early Cenozoic that the true mammalian adaptive radiation occurred. In modern times there are approximately four thousand three hundred extant species of placental mammals (Wilson & Reeder, 2005). This number is factors of ten larger than the number of species of marsupials. The marsupial biodiversity in this smaller number of species is therefore astounding. This biodiversity, both morphological and ecological, can be attributed to the geographical isolation of the Gondwanan continents from the more northern continents throughout the majority of the Cenozoic. The marsupial radiation throughout the previously Gondwanan continents has been a result of at least two periods of adaptive radiation, followed by periods of progressive occupation and the convergence of not only placentals and marsupials but also within the marsupial population itself (Springer et al., 1997). It is tempting to view the placental sister group of marsupials as an alternative evolutionary pathway of Mammalia but this comparison is of limited use. Metatheria did not evolve to fill as an extensive an array of ecological niches as the placental mammals did throughout the Tertiary and therefore any...
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