Biogeography is "the study of the distribution of organisms in space and through time". Analyses of the patterns of biogeography can be divided into the two fields of historical biogeography and ecological biogeography (Wiley, 1981 [link]).

Historical biogeography examines past events in the geological history of the Earth and uses these to explain patterns in the spatial and temporal distributions of organisms (usually species or higher taxonomic ranks). For example, an explanation of the distribution of closely related groups of organisms in Africa and South America is based on the understanding that these two land masses were formerly connected as part of a single land mass (Gondwana). The ancestors of those related species which are now found in Africa and South America are assumed to have had a cosmopolitan distribution across both continents when they were connected. Following the separation of the continents by the process of plate tectonics, the isolated populations are assumed to have undergone allopatric speciation (i.e., speciation achieved between populations that are completely geographically separate). This separation resulted in the closely related groups of species on the now separate continents. Clearly, an understanding of the systematics of the groups of organisms (i.e., the evolutionary relationships that exists between the species) is an integral part of these historical biogeographic analyses.

The same historical biogeographic hypotheses can be applied to the spatial and temporal distributions of marine biota. For example, the biogeography of fishes from different ocean basins has been shown to be associated with the geological evolution of these ocean basins (see Stiassny and Harrison, 2000 [link] for examples with references). However, we cannot assume that all existing distribution patterns are solely the product of these past geological processes. It is evident, for example, that the existing marine fauna of the Mediterranean is a product of the complex geological history of this marine basin, involving separation from the Indian and Atlantic Oceans, periods of extensive desiccation followed by flooding and recolonization from the Atlantic (Por, 1989 [link]). However, there is also good evidence that the eastern end of the Mediterranean has been colonized more recently by species that have dispersed from the Red Sea via the Suez canal.

Thus, the field of ecological biogeography first examines the dispersal of organisms (usually individuals or populations) and the mechanisms that influence this dispersal, and then uses this information to explain the spatial distribution patterns of these organisms. For further discussion see the module on "Biogeography" and see Wiley, 1981 [link], and Humphries and Parenti, 1999 [link].