The concept of coevolution has been defined in various ways, but it generally refers to an evolutionary change in one species that is a result of evolutionary change in another species. Evolutionary change can be physiological or involve a fixed behavioral pattern, and commonly it involves a morphological structure—but it always implies a genetically based change. Evidence of coevolution derives from the study of fossils, the phylogeny of modern species, and their ecology. There is a spectrum of types of coevolution: escape and radiate, guild coevo-lution, arms race or escalation, and cospeciation or parallel cladogenesis.
Escape and radiate coevolution involves the radiation of one species into many as a result of an adaptive breakthrough that frees it from the selection pressures of predation or parasitism. An example is the radiation of many kinds of mollusks as a result of the thickening of their shells over time. By evolving thicker shells in response to various predatory fish and gastropods, certain mollusks were able to become ecologically dominant and diverse. In response to thickening shells, some predatory gastropods evolved a radula (a filelike structure with teeth) that is drill-like, capable of penetrating a very thick shell and thereby allowing its inhabitant to be consumed.
Guild coevolution involves evolutionary change in a species or group of species in response to a suite of predators, competitors, parasites, or other interacting species. Excellent examples of guild coevolution are plants and the insects that feed on their leaves or pollinate their flowers. Many plants defend themselves against herbivory with noxious or toxic chemicals that repel most insects. Some insects, though, have evolved defenses against the poisons, which are physiological and even behavioral. Milkweeds (Family Asclepi-adaceae) derive their name from the sticky latex that oozes from wounds to the stems or leaves. The latex gums up the mandibles of chewing insects, and it contains poisonous cardiac glycosides. As a result, there are suites of insect species—such as certain chrysomelid beetles, plant bugs, and monarch butterfly caterpillars—that feed on particular species of milkweeds. These specialized insects can detoxify the poisons and even sequester them in their bodies for use in defending themselves against predators. Some milkweed-feeding insects even chew a cut into the base of the main vein of the leaf to prevent the latex from reaching the more distal parts of the leaf, where they will then feed. Guild coevolution is predominant among angio-sperms and their insect pollinators. Flowers have evolved "syndromes" of color, structure, and fragrance to lure particular types of pollinators. Pollinators, in turn, have evolved various behaviors and structures that make them particularly efficient at harvesting pollen and nectar from certain kinds of flowers—and therefore at transferring the pollen. Flowers with long spurs containing nectar, for example, are usually pollinated only by certain moths, flies, and bees with extremely long tongues.
Arms race, or escalation, coevolution refers to multiple adaptations in interacting species, each one a response to adaptations in the other species. Cheetahs, for example, are uniquely adapted among all cats for running down their prey during the daytime in highspeed chases. All other cats are usually nocturnal, ambushing predators. Cheetahs are inefficient at overwhelming larger animals but highly specialized for preying on Thompson's gazelles ("tommies"), particularly by tripping them up during the chase. Tommies, in turn, flee extremely fast. Their small size makes them proficient in darting, which makes it particularly difficult for lions and leopards to catch them. The cheetah's specialization is evolutionarily precarious, should tommy populations crash, say, from a virus, but it does allow a coexistence with predators that kill zebra, wildebeest, and the other large ungulates of East African plains.
In tropical regions of South America lives a diverse genus of understory nymphalid butterflies, Heliconius. The caterpillars feed on plants in the Passifloraceae, or family of passion fruit vines. Species of Heliconius usually live very specifically on particular species of Passiflora, having become physiologically adapted to detoxifying certain glycosides that make the foliage of Passiflora inedible to most herbivores. Female butterflies will not lay eggs on Passiflora vines that have caterpillars or even eggs on them. The Passiflora plants have evolved, in response, specialized glands and stipules that mimic Heliconius eggs and often deter ovipositing butterflies.
Cospeciation, also called parallel cladogen-esis, occurs when speciation in one organism results in the speciation of another. Cospe-ciation can lead to situations in which the phylogeny of, say, a group of host species matches the phylogeny of the parasites that live on them. Indeed, cospeciation is found in situations where species are intimately associated, usually obligate (where the existence of at least one is dependent upon the other). These especially include organisms that are parasites, or symbiotic mutualists. One of the best examples of cospeciation involves a basidiomycete fungus that lives exclusively in the nests of leaf-cutting ants of the New World tropics, the Attinae. The ants provision their underground nests with chewed vegetation, but they actually feed on the fungus that grows on the rotting plant material. The ants and fungi are codependent, and the phylogeny of the ants matches the phylogeny of the fungus: as the ants speciate, the fungus genetically diverges accordingly. Certain groups of lice on their mammal and bird hosts show similar patterns, as do some tapeworms and other parasites.
See also: Adaptation; Ecological Niches; Evolution; Herbivory; Natural Selection; Phylogeny; Pollination; Positive Interactions; Speciation
Futuyma, Douglas J. 1998. Evolutionary Biology, 3d ed. Sunderland, MA: Sinauer; Futuyma, Douglas J., and M. C. Keese. 1992. "Evolution and Coevolution of Plants and Phytophagous Arthropods." In Herbivores: Their Interactions with Secondary Plant Metabolites, edited by G. A. Rosenthal and M. A. Berenbaum, pp. 439-475. New York: Academic; Futuyma, Douglas J., and Montgomery Slatkin, eds. 1983. Coevo-lution. Sunderland, MA: Sinauer.
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