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Modern understanding of the speciation process emphasizes the considerable interaction between phylogeny and biogeography. New methods of historical biogeographic analysis, such as vicariance theory, refuge theory, and equilibrium theory, have greatly complemented but not supplanted classical dispersal theory. Vicariance theory is a method which infers the existence of ancestral biotas by analyzing the distributions of numerous living organisms and which then interprets historical biogeographic relations by emphasizing the splitting of the ancestral biotas after range extension by dispersal. The application of vicariance theory to the distribution of fossils, rather than extant organisms, is problematical, because the time element associated with paleontologic data provid s both additional information and additional complexities for biogeographic interpretations. A historical biogeographic analysis should give equal consideration to numerous interrelated factors, including inferred ancestral distributions, chronologies of speciation, distributional changes related to paleoenvironmental, paleo-oceanographic, and paleotectonic events, dispersal routes, mechanisms, barriers, and ecologic relations with associated taxa. The acknowledged danger of such a method, of course, is that one may end up with an untestable narrative explanation.
To illustrate these concepts, we consider the Cenozoic biogeographic history and phylogeny of tropical Scleractinian corals. It has been known since the early part of this century that the major evolutionary features and distributional patterns of these corals can be explained by a pan-tropical Tethyan biota which has been subsequently modified by paleo-oceanographic events. Paleontologic, biologic, and geologic data strongly support the following conclusions. (1) Breakup and subsequent disjunction by vicariance of the Oligocene pan-Tethyan coral fauna resulted from changes in marine climate and circulation caused by creation of the Antarctic Convergence and closure of the seaway between the eastern and western Tethys (early Miocene), great restriction and closure of the Panama seaway (middle Miocene), eustatic sea-level fall and other oceanic phenomena associated with the Mediterranean salinity crisis (late Miocene), and closure of the Bolivar seaway (early Pliocene). (2) Both the Indo-Pacific and the Caribbean faunal provinces served as centers of origin for coral genera and species. (3) The Gulf of California Pliocene disjunct fauna is a result of either the extension of the relict western range of the vicariating Caribbean fauna or long-distance dispersal from a previously differentiated Caribbean fauna into a refuge which ultimately failed. (4) The modern eastern Pacific coral fauna is a mixture of both the pan-Tethyan fauna and long-distance dispersal from the Indo-Pacific fauna as controlled by marine climate and barriers.
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