Ape Biogeography

Evolutionary biogeography addresses the historical relationship between geographic space and the processes of biological differentiation, such as speciation and adaptation. Darwin observed that the evolution of related species in different locations required that they also share a common ancestral location he called the “center of origin.” Darwin thought this requirement was so obvious that it constituted a self-evident truth and to call it into question was to appeal to the agency of a miracle. The occurrence of related species in different locations, especially those considered to be separated by geographic or environmental barriers, was explained as the result of their having migrated away from their original centers of origin according to their individual abilities to disperse (walking, flying, rafting, floating, and so on). Dispersal ability was seen to be the key to geographic distribution, and biogeographic evolution was simply a compendium of unique, individual, and unrelated dispersal events. This perspective justified each group being explored in isolation, whether upon the static geography of Darwin's time or the currently accepted plate tectonic theory of geological history.

Evolutionists following in Darwin's footsteps did not question his theory of evolution through centers of origin and dispersal. The science of biogeography was reduced to the practice of creating historical narratives or stories about imagined centers of origin and dispersal routes for each individual group of organisms. These stories were constructed according to prevailing beliefs about evolutionary age, dispersal ability, geological and ecological history, and most important, particular beliefs about how the center of origin could be identified. A variety of contradictory criteria were theorized to identify the center of origin, among the most popular being the location of the oldest fossil or the most primitive (and therefore oldest) member of the evolutionary group. Biogeographic narratives were always a product of prevailing beliefs and knowledge never advanced beyond what was already presumed to be known from geological or ecological history. In this role, biogeography is rendered, at best, a subdiscipline of ecology or systematics, and not a very informative one at that.

Darwin's theory of geographic evolution faced its first serious challenge from Leon Croizat, who was perhaps the first biogeographer to formally recognize geographic location as an independent source of historical information about the evolution and origin of species. Croizat's unique approach was first developed in the 1950s and became known as panbiogeography. His research program analyzed the geographic relationships between different taxa at different localities using line graphs or “tracks.” Tracks are generally drawn to connect localities over the shortest geographic distance, since that provides the minimum amount of geographic space and therefore the minimum number of ad hoc geographic hypotheses required to explain the spatial relationships. The line graphs allow direct comparison of spatial geometry for groups of organisms and tectonic features (such as ocean basins and geosynclines) associated with earth history. These biological and geological components comprise the raw data of biogeography. Darwin's predication that dispersal ability would be the key to understanding the evolution of geographic distribution was not supported by this approach. When Croizat compared tracks, he found that supposedly “poor” dispersers could be as widely distributed as “good” dispersers. He also found that animal and plant distributions could be correlated with geomorphologic features, and this suggested that geological history had more to do with the evolution of a distribution than with dispersal ability.

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