How do populations become allopatric? There are two ways. The first is by dispersal, in which some individuals colonize a distant place, such as an island, far from the main source population. The second is by vicariance, in which a geographic barrier arises within a single population, separating it into two or more isolated populations. For example, when sea levels rose at the end of the most recent ice age, new islands formed along the coastline as the low-lying land around them was flooded. The populations on those new islands suddenly found themselves isolated from other populations of their species. This kind of island formation is a vicariance event.

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Regardless of how the allopatric populations came about—whether through dispersal or vicariance—the outcome is the same. The two separated populations will diverge genetically until speciation occurs.

Often, vicariance-derived speciation is the easier to study because we can date the time at which the populations were separated if we know when the vicariance occurred. One such event whose history is well known is the formation of the Isthmus of Panama between Central and South America, shown in Fig. 22.7. This event took place about 3.5 million years ago. As a result, populations of marine organisms in the western Caribbean and eastern Pacific that had formerly been able to interbreed freely were separated from each other. After a period of time, the result was the formation of many distinct species, with each one’s closest relative being on the other side of the isthmus.

Dispersal is important in a specific kind of allopatric speciation known as peripatric speciation (that is, in a peripheral place). In this model, a few individuals from a mainland population (the central population of a species) disperse to a new location remote from the original population and evolve separately. This may be an intentional act of dispersal, such as young mammals migrating away from where they were raised, or it could be an accident brought about by, for example, an unusual storm that blows migrating birds off their normal route. The result is a distant, isolated island population. “Island” in this case may refer to a true island—like Hawaii—or may simply refer to a patch of habitat on the mainland that is appropriate for the species but is geographically remote from the mainland population’s habitat area. For a species adapted to life on mountaintops, a new island might be another previously uninhabited mountaintop. For a rain forest tree species, that new island might be a patch of lowland forest on the far side of a range of mountains that separates it from its mainland forest population.

The island population is classically small and often in an environment that is slightly different from that of the mainland population. The peripatric speciation model suggests that change accumulates faster in these peripheral isolated populations than in the large mainland populations, both because genetic drift is more pronounced in smaller populations than in larger ones and because the environment may differ between the mainland and island in a way that results in natural selection driving differences between the two populations. These mechanisms cause genetic divergence of the island population from the mainland one, ultimately leading to speciation.

It is possible to glimpse peripatric differentiation in action (Fig. 22.8). Studies of a kingfisher, Tanysiptera galatea, in New Guinea and nearby islands show the process under way. There are eight recognized subspecies of T. galatea, three on mainland New Guinea (where they exist in large populations separated by mountain ranges) and five on nearby islands (where, because the islands are small, the populations are correspondingly small). The mainland subspecies are still quite similar to one another, but the island subspecies are much more distinct, suggesting that genetic divergence is occurring faster in the small island populations. If we wait long enough, these subspecies will probably diverge into new species.

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Because dating such dispersal events is tricky—newcomers on an island tend not to leave a record of when they arrived—we can sometimes use vicariance information to study the timing of peripatric speciation. For instance, we know that the oldest of the Galápagos Islands were formed 4–5 million years ago by volcanic action. Sometime early in the history of the Galápagos, individuals of a small South American finch species arrived there. Conditions on the Galápagos are very different from those on the South American continent, where the mainland population of this ancestral finch lived, and so the isolated island population evolved to become distinct from its mainland ancestor and eventually became a new species.

The finches’ subsequent dispersal among the other islands of the Galápagos has promoted further peripatric speciation (Fig. 22.9). In the graph at the bottom of the figure, we see that the number of finch species is correlated with the number of islands in the archipelago. This is clear indication of the importance of geographic separation (allopatry) in speciation: The availability of islands provided opportunities for populations to become isolated from one another, allowing speciation. The result was the evolution of 13 different species of finches, collectively known today as Darwin’s finches.

The Galápagos finches and their frenzy of speciation illustrate the important evolutionary idea of adaptive radiation, a bout of unusually rapid evolutionary diversification in which natural selection accelerates the rates of both speciation and adaptation. Adaptive radiation occurs when there are many ecological opportunities available for exploitation. Consider the ancestral finch immigrants arriving on the Galápagos. A wealth of ecological opportunities was open and available. Until the arrival of the colonizing finches, there were no birds on the islands to eat the plant seeds, or to eat the insects on the plants, and so on. Suppose that the ancestral finches fed specifically on medium-sized seeds on the South American mainland (that is, they were medium-seed specialists, with bills that are the right size for handling medium-sized seeds). On the mainland, they were constrained to that size of seed because any attempt to eat larger or smaller ones brought them into conflict with other species—a large-seed specialist and a small-seed specialist—that already used these resources. In effect, stabilizing selection (Chapter 21) was operating on the mainland to eliminate the extremes of the bill-size spectrum in the medium-seed specialists.

When the medium-billed immigrant finches first arrived on the Galápagos, however, no such competition existed. Therefore, natural selection promoted the formation of new species of small- and large-seed specialists from the original medium-seed-eating ancestral stock. With the elimination of the stabilizing selection that kept the medium-billed finches medium-billed on the mainland, selection actually operated in the opposite direction, favoring the large and small extremes of the bill-size spectrum because these individuals could take advantage of the abundance of unused resources, small and large seeds. It is this combination of emptiness—the availability of ecological opportunity—and the potential for allopatric speciation that results in adaptive radiation.

Quick Check 2 Why do we see so many wonderful examples of adaptive radiation on mid-ocean volcanic archipelagos like the Galápagos?