The Natural History Reader in Evolution

By Niles Eldredge | Go to book overview

PART 4
BIOGEOGRAPHY AND EVOLUTION

The geographic distribution of animals and plants has an old, and straightforward, connection with the very idea of evolution. If all organisms had been created in the Garden of Eden, later to be collected by Noah and subsequently released from Mt. Ararat, we might expect the greatest concentration of life to center around the Middle East, with life fanning out from there, becoming less dense and diverse the further we sample from this center of origin and dispersal. In any case, we would not expect to travel the world over and find highly localized flora and fauna that bear no particularly close resemblance to the denizens of the Ark.

Darwin realized that the highly diverse nature of life spread out over the globe, a geographically based diversity that by his day was becoming increasingly appreciated, afforded a powerful boost to the argument that life has had a long and complex history. That similar species seemed to replace one another from region to region led to the notion of "ecological vicars," an idea closely related to interspecific competition and mutual exclusion of closely related species. And the concept of allopatric speciation, long the dominant model of the speciation process, casts geography in the leading role in the differentiation and origin of new species.

Geographic isolation allows elements of the local biota to go their own separate evolutionary ways. The pattern is strongly hierarchical: each major biogeographic region has many faunal and floral entities unique (endemic) to it. As we shall see in some of the following essays, entire families, orders and, in some cases, taxa of even higher rank may be restricted to particular regions. But within any biogeographic area, there are inevitably subregions of endemism. It is the task of all biogeographers to work out the distributions of taxa of an area, and to come to grips with the history of those distributions.

The essays in part 4 cover several approaches to historical biogeography. Analysts of living organisms frequently take a functionalist approach, seeing distributions governed by an interplay of the biological requirements of the organisms and the physical circumstances in which they find themselves. Often their explanations assume a center of origin for a group, with subsequent dispersal leading to patterns of distribution in the present day. This is the central thesis of

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