Allopatry and Sympatry
Allopatry and sympatry are terms used in biogeography to describe the comparative distributions of populations and species. Species with sympatric distributions overlap in their geographical range to some degree. In contrast, allopatric species do not overlap in their distributions. Evolutionary biologists are particularly interested in these sorts of distributions, because they are believed to be one of the most important factors affecting the evolution of distinct, new populations and species.
Ecologists and evolutionary biologists define a population as: "a group of individuals of the same species, which are capable of interbreeding with each other and producing fertile offspring." When populations of a particular species become physically isolated, they are effectively prevented from breeding with each other. Such an allopatric distribution might occur because the populations live on different islands, or because they inhabit opposite sides of a mountain chain. In either case, because of their physical isolation, the isolated, allopatric populations cannot easily breed with each other, if at all. Because the populations occur in habitats characterized by different environmental conditions, they are exposed varying to regimes of natural selection. Consequently, over a long period of time the populations may evolve to become different in their physiology, morphology, behavior, and other attributes. (If one or more of the isolated the populations are small, their genetic characteristics will also change (i.e., evolve) because of the phenomenon of genetic drift.) Eventually, if the allopatric populations become different enough in their genetic attributes, they may no longer be able to interbreed successfully if they come into contact again (that is, if their distributions become sympatric). If this was the case, evolutionary biologists would consider them to have diverged sufficiently to be characterized as different species.
Allopatric distributions of populations are believed to be one of the most critical factors allowing the evolution of new species to occur. In contrast, populations that overlap to any significant degree will experience a relatively free exchange of genetic information. Even though partially sympatric populations may be experiencing quite different environmental conditions (at least at the extremes of their ranges), and therefore differing regimes of natural selection, the occasional interbreeding is sufficient to prevent them from diverging genetically to the degree that they can no longer successfully interbreed.
The mode of speciation just described, involving genetic divergence of allopatric populations, is believed to explain the occurrence of different, but closely related species in remote oceanic archipelagos. For example, the Galapagos Islands of the eastern, equatorial Pacific Ocean sustain about 13 species of so-called warbler finches (also known as Darwin's finches). These include 6 species of ground finches (in the genus Geospiza), 6 species of tree finches (genus Camarhynchus), and the warbler finch (Certhidea inornata). These are all believed to have originated from the long-ago colonization of one or several of the Galapagos Islands by a few, probably storm-blown finches from the South American mainland. Because there were no competitors for these small, seed-eating birds, the tiny founder population increased in size. Eventually populations occurred on each of the islands of the archipelago, where they were effectively isolated and could not interbreed with each other. These locally allopatric distributions allowed the isolated populations to diverge evolutionarily, and to eventually develop new species that occur nowhere else. Because there are few other kinds of landbirds on the Galapagos Islands, the original founder population of seed-eating finches was able to take advantage of various, empty ecological niches. In this manner, species of warbler finches evolved with very heavy bills for feeding on large seeds (e.g., the large ground finch, Geospiza magnirostris), while medium-billed species specialize on smaller seeds (e.g., the small ground finch, G. fuliginosa), and thin-billed ones on small insects (e.g., the small insectivorous tree finch, G. parvulus. One species, the tool-using finch (Camarhynchus pallidus), has a specialized behavioral trait involving the use of a cactus spine to probe small tree-cavities so that insect larvae can be extracted. All of these distinct, unusual species are thought to have evolved because their originally homogeneous, founder population was able to diverge genetically in the isolated environments of the various islands of the Galapagos Archipelago.
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