Character Displacement
Character displacement is the term used to describe an evolutionary change that occurs when two similar species inhabit the same environment. Under such conditions, natural selection favors a divergence in the characters--morphology, ecology, behavior, or physiology--of the organisms. The concept has been understood at least since the English naturalist Charles Darwin (1809-1882) was putting together his argument for evolution, but the idea was only formalized in the middle of the twentieth century.
In 1956, the American entomologists William L. Brown (1922-1997) and Edward O. Wilson (1929-) compared the characters of a number of species living together--or, in technical terms, living in sympatry--with characters in the same species living apart, or in allopatry. They found that sympatric species possessed many different characters although these same species were sometimes indistinguishable when living allopatrically. Brown and Wilson concluded that these situations resulted from competition: because the species were similar, they competed for the same resources and natural selection favored those species that competed less. Thus, the characters diverged. Character displacement also resulted from reinforcement of reproductive barriers. In some cases, the two species might not be able to produce young or the young might be unviable, and natural selection would favor differences between the species so that they could recognize one another and not mate.
Two decades later the concept was revisited by the American ornithologist Peter Grant (1936-). Grant studied the finches of the Galapagos, called Darwin's finches, in large part to test the idea of character displacement. In the process, he refined understanding of the concept. Grant pointed out that putative cases of character displacement might be, instead, character convergence, as two species that originally evolved in the presence of one another moved into new areas and, without the pressure of competition, broadened the range of their characters, a process that he called character release. Grant also noted that putative cases of character displacement might have nothing to do with competition or reinforcement of reproductive isolation, but might result from other causes altogether. These arguments cleared the way for a new definition of character displacement, decoupling the concept from a simple comparison of sympatric and allopatric populations. Instead, ecologists had to look at the range of factors influencing the evolution of character states in both sympatric and allopatric populations--testing to see if competition could be proven to be the driving force.
Using this new definition, Grant found that many of the presumed examples of character displacement could be explained by other factors. In rock nuthatches, for example, Grant showed that what was believed to be divergence was, in fact, an example of another phenomenon, geographical variation. Animals and plants belonging to the same species, it had long been known, vary over their range. The nuthatch species in this example showed a wide diversity in their beak shapes, even in areas where they did not live together. Using statistical methods, Grant suggested that the differences between the nuthatches in their area of sympatry were well within the range of variation that the birds exhibited in areas of allopatry. Thus, character displacement could not be used to explain the differences between the nuthatch species.
Despite his criticisms, Grant did find some cases of character displacement among Darwin's finches. He studied thirteen pairs of species that lived together on various islands. In eleven cases, he found that the birds differed more in the shape of their beaks than when they lived alone on separate islands. Through a series of experiments and mathematical manipulations, Grant concluded that these differences resulted from competition. Character displacement has also been found operating among ants, frogs, fruit flies, and snails.
The introduction of character displacement turned ecologists' attention away from the principle of competitive exclusion--in which one species drives a competitor to extinction within their zone of sympatry--and focused them instead on learning how organisms survived together. The American ecologist G. Evelyn Hutchinson (1903-1991), for example, used character displacement to partially explain how the earth supports so many organisms: because of character displacement, even similar organisms can live together. Other ecologists and evolutionary biologists noted that character displacement may be the engine driving adaptive radiations, in which a single species gives rise to many others, filling a variety of ecological niches. The process is not as ubiquitous as once believed but, according to E. O. Wilson, character displacement still "represents one process by which communities can be organized, mediating a rise in general biological diversity."
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