Species | Research & Encyclopedia Articles

The term species, in the most general sense, refers to the various kinds of living things. Thus, species are generally recognized as distinct, fully differentiated groups of organisms. However, most modern definitions of species also recognize them as reproductive communities and acknowledge that mating occurs between members of each species but does not occur (or occurs only rarely) among members of different species. Species are therefore generally recognized as genetically differentiated, reproductive communities within which there is a pattern of ancestry and descent among organisms. Although most scientists accept this general definition, there are two somewhat different criteria that are often employed in the recognition of species, and the application of the two criteria does not always lead to the same conclusions.

The first major criterion for the designation of species is the actual occurrence of interbreeding among the various organisms and populations within a species and the absence of such interbreeding between species. However, patterns of interbreeding are difficult to observe directly, particularly among plants that may live for hundreds of years. For this reason, indirect evidence regarding patterns of interbreeding is usually provided by the study of the patterns of differentiation among populations in genetically determined characteristics. In a trivial sense, oak trees and daisies are regarded as belonging to different species because they are distinguished by numerous characters and because hybrids between them are never observed. Thus, it is reasonable to conclude that they are not part of the same reproductive community. However, there are many recognized species of daisies, and many recognized species of oaks, and these species often are delimited by subtle differences. Consequently, one might examine several populations of daisies and observe that the plants are identical except for one character: the occurrence of a line of hairs along the undersides of the leaves. If all of the individuals in some populations have the line of hairs, and all of the individuals in other populations lack these hairs, there is evidence that two population systems exist and that there is no gene flow between them. In contrast, if each population that is examined includes individuals with the line of hairs and other individuals without the line of hairs, it can be concluded that this is simply a character that varies within a single species, like blood types in humans. Under this criterion, two species can be recognized even when the differences between them are not readily observable. For example, there are many documented cases in which twoor more population systems differ from each other by genetically determined differences that can only be detected by biochemical tests. Though they are difficult to distinguish, the species are recognized as distinct genetic communities.

The second major criterion of species status is the existence of a genetically determined barrier to gene flow between species. Such a barrier, known as a reproductive-isolating mechanism or a reproductive isolating barrier (RIB), prevents members of two different species from interbreeding, even if they occur in the same location. For example, the pollen that is produced by plants of one species may not germinate when placed on the stigmas of plants of another species and, thus, there can be no reproduction or gene flow between them. In this case, the RIB is the pollen/stigma incompatibility. Because this barrier is genetically determined, the two are regarded as reproductively isolated, and, as a result, two species are recognized.

Generally, any species boundary due to a reproductive isolating barrier also serves to prevent interbreeding as defined by the first criterion, but there are many instances in which the first criterion is satisfied while the second is not. The line of hairs on the underside of the leaves, which distinguishes two species of daisies under the first criterion in the example just described, does not by itself prevent interbreeding from occurring between the two kinds of daisies. The two daisy species may fail to exchange genes not because of a genetic mechanism but because they occur on different sides of a mountain range. Some biologists argue that if a RIB is not identified, the two kinds of daisies (one with the line of hairs, the other lacking it) should be grouped together and recognized as belonging to the same species. Others argue that the two populations are, indeed, persisting as separate and fully differentiated reproductive communities. Although they may have the potential to interbreed, the available evidence suggests that this does not occur, so they should be recognized as different species. Whatever position one takes on this matter, it should be noted that most species that have been recognized by science have, in fact, been delimited according the first criterion.

Cultivar; Hybrids and Hybridization; Speciation; Taxonomy; Variety.

Futuyma, D. J. Evolutionary Biology. Sunderland, MA: Sinauer Associates, 1997.

Grant, V. Plant Speciation, 2nd ed. New York: Columbia University Press, 1981.

Mayr, E. The Growth of Biological Thought. Cambridge, MA: Belknap Press of Harvard University Press, 1982.

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