Coevolution Of Competing Species

Darwin argued that competition is an important agent of natural selection for adaptation to different habitats or resources by different species. Indeed, a common theme in community ecology is that coexisting species differ in food or other components of their ecological niches and that such differences are ordinarily necessary for species to coexist in the long term.

Quantitative genetic models of the evolution of competitors assume that in each of two or more species, a heritable, continuously varying trait, such as an animal's body size or mouth size, determines the mean and variance of resources (e.g., size of prey) consumed. Because competition for limiting resources decreases an individual's fitness, genotypes of species 1 that use a resource different from that used by species 2 are likely to increase in frequency, so that the mean phenotype (and resource use) shifts away from that of the other species. At evolutionary equilibrium, the species will still overlap in resource use to a greater or lesser extent, depending on the abundance of different resources, but the variance in each (the breadth of resources used) is likely to be lower than in a solitary species. Three or more species may evolve differences from each other in phenotype (e.g., size) and resource use. Such coevolutionary changes should promote coexistence. However, if competition between species is asymmetrical (e.g., if larger individuals reduce the fitness of smaller ones more than the converse), a species may converge toward the other, use its resources, and "chase" it to extinction.

Considerable evidence, mostly from vertebrates, supports this coevolutionary theory. For example, closely related sympatric species of Darwin's finches, woodpeckers, and some other animals each use a narrower variety of food types or microhabitats than do species that occur singly on islands. Evidence for evolutionary response to competition is provided by some instances of character displacement—a greater difference between two species where they occur together than where each occurs alone. Some lakes left by retreating glaciers in northwestern North America are inhabited by a single species of stickleback fish (Gasterosteus aculeatus complex), which feeds both near the bottom and in open water. In other lakes, two coexisting species have evolved. Relative to the solitary form, the coexisting species have diverged and specialized in morphology and behavior: one feeds on benthic prey and the other on plankton. Experiments have shown that competition among similar phenotypes reduces growth of juveniles more than among dissimilar phenotypes. In one of the few cases of ecological character displacement reported for insects, sympatric populations of two species of rhinoceros beetles (Scarabaeidae: Chalcosoma) overlap less in altitudinal range and differ more in size than allopatric populations. However, it has not been shown that these differences stem from competition for resources.

Coevolution of competitors may explain some patterns in community structure. For example, differences in body size or trophic structures among sympatric pairs of species of bird-eating hawks, carnivorous mammals, and seed-eating Galapagos finches are greater than if the species had been assembled at random. In a remarkable example of coevolutionary consistency, ecologically and morphologically equivalent sets of species of Anolis lizards have evolved independently on each of the four islands of the Greater Antilles.

Bee Keeping

Bee Keeping

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