Biogeography as Evidence That Evolution Accounts for Diversity of Life

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Biogeography is one source of evidence that evolution accounts for the diversity of life. Biogeography is the study of the geographic distribution of species and has contributed evidence for descent from common ancestors, which was hypothesized by Charles Darwin. Darwin and Alfred Wallace were both very interested in biogeography, which provided Darwin with evidence for evolution. Species distribution can be accounted for by ecological factors or by historical factors. The three major historical factors affecting geographic distribution are dispersal, extinction and vicariance. Island biogeography has been extensively studied to show the evolution of species due to geographic barriers. Biogeography along with the history of the earth lends supporting evidence to evolution and the diversity of life on earth. Darwin showed that biogeographic facts make sense if a species has a definite site of origin, achieves a broader distribution by dispersal, and becomes modified giving rise to descendent species in the regions in which it migrates. He noticed that unrelated organisms inhabit parts of the world with similar climates and habitats, such as Old and New World organisms. Darwin also found that organisms of various regions may be different due to barriers or obstacles that may limit migration. He also stated that there is a relation between inhabitants of the same continent or sea but that species differ from place to place. An example of this is aquatic rodents of South America are related to mountainous and grassland rodents of South America, but not to aquatic rodents of North America. Alfred Wallace noticed that several higher taxa had similar distributions and that the composition of biota is more uniform within certain regions then between them. He had collected specimens in the Malay Archipelago and had thought of natural selection. He did extensive field work in the Amazon and noticed that geographic barriers, such as the Amazon River itself, separated the ranges of closely related species. These observations led him to designate several biogeographic realms. He observed that the fauna of Australia and Asia were different and the break between them is known as the Wallace Line.

Historical factors affecting geographic distributions of species include extinction, dispersal, and vicariance. Extinction of certain populations reduces the distribution of species but it also allows for diversification. Extinctions are selective in that some species are more likely to survive then others. For example, gastropods with wide geographic and ecological distributions and those with many species survived the end-Permian extinction. Extinction can lead to diversification in that it resets the stage for evolutionary radiations, perhaps by permitting the appearance of new community structures. Futuyma suggests that the extinction of one group permits the efflorescence of others, which is also shown in the fossil record. Dispersal and vicariance are the major hypotheses attributing to a taxon’s distribution. An example of vicarance is taxa that have members on different land masses in the Southern Hemisphere, which is hypothesized to be due to the breakup of Gondwanaland isolating descendents of common ancestors. America, Africa, Madagascar, and India are all home to the freshwater fishes, cichlids. Molecular phylogenetic analyses has shown that two sister clades of cichlids have been found, one consisting of Madagascan and Indian species, and the other of two monophyletic groups, one in Africa and one in South America. However, the splits between the clades are more recent than the breakup of Gondwanaland which suggests that perhaps the cichlids achieved their distribution by dispersal. Both vicariance and dispersal could be the likely cause of the geographic distribution of cichlids. Species expand their ranges by dispersal which is a critical process for geographic isolation in evolution and...
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