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The species is the fundamental unit of organization of the taxonomic system; of interactions between organisms as described by geneticists and ecologists; and of evolution as studied by phylogeneticists. As a category the term species resists definition; thus, a species concept is adopted as a framework within which biologists of various persuasions delineate the taxa with which they work at the species level. However, no universal concept has been accepted by all biologists for two fundamental reasons: (1) Different groups of organisms in nature are organized differently in terms of reproductive mechanisms and patterns; in degrees of differentiation among species in morphological, genetic, physiological, behavioral, biochemical, and other types of characters; and in the modes of speciation that have given rise to the members of the group. (2) The philosophy, training, working methods, and goals of different of biologists affect the manner in which each perceives the coherence or diversity of the biological world in general and that of the group of organisms in question in particular.
According to the taxonomic concept, a species consists of groups of individuals (populations) that are morphologically similar to one another, and differ morphologically from other such groups. There are several important ideas expressed in this concept. First, there is internal cohesiveness; that is, the members of the species share certain characteristics. Second, there is external distinction because other species have different characteristics, and thus species may be distinguished from one another. Third, the characteristics that a species possesses may be easily observed because they are phenotypic; that is, a species may be identified by its appearance.
Difficulty in applying the taxonomic concept arises with certain groups of organisms. Bacteria are often identified by physiological and biochemical tests requiring sophisticated laboratories and equipment; in addition, the mutation rate in bacteria is so high that the various traits used to identify them can change rapidly. In insects, the morphological differences between species may be very slight and easily overlooked. In certain groups of plants, hybridization and polyploidy have led to a continuous range of variation of characters, in which no discontinuities sufficient to distinguish species can be discerned. Critics claim that the purely phenetic approach of the taxonomic concept may not reflect real genetic or breeding relationships. However, this concept provides guidelines by which species may be recognized by ordinary (nonexperimental) means. The composition of a species so recognized can then be subjected to hypothesis testing within the framework of other concepts. See Bacterial taxonomy, Taxonomic categories
According to the biological concept, a species is composed of groups of individuals (populations) that normally interbreed with one another. The fundamental ideas expressed by this concept are that the internal cohesiveness of a species is maintained by the exchange of genes through sexual reproduction (gene flow) and that the distinctness of the species is maintained by reproductive isolation (barriers to gene flow) from other groups of populations. If two populations do not exchange genes, they belong to separate species regardless of their morphological similarity.
This concept works well in those groups of organisms that are exclusively outbreeding, such as birds and mammals. However, it is difficult to apply to plants, in which interbreeding between morphologically very distinct species and even genera is common. Also, those organisms that do not reproduce sexually present problems of classification. Even in sexually reproducing organisms, populations that are morphologically identical but reproductively separated by geographic distance (disjuncts) present problems of classification within the framework of the concept. The populations might interbreed if they were in contact, but this can be determined only under artificial conditions and not in nature. However, the development of the biological species concept has contributed greatly to making taxonomy an evolutionary science because of its emphasis on the identification of genetic, rather than the very possibly superficial phenetic, relationship among organisms.
According to the evolutionary concept, a species is a lineage of ancestor-descendant populations that maintains its identity from other such lineages and that has its own evolutionary tendencies and historical fate. The important ideas expressed in this concept are the following. (1) All organisms, regardless of their mode of reproduction, belong to some evolutionary species. (2) Species need be reproductively isolated from one another to the extent that they maintain their distinction from other species. (3) There may or may not be a morphological discontinuity between species but, if there is, it is reasonable to hypothesize that more than one species is present. If there is not, other data such as that on breeding relationships may be used to recognize species.
The evolutionary concept encompasses the taxonomic concept, the biological concept, and other more narrowly defined concepts—for example, the ecological species, the genetic species, and the paleospecies. It is operational in that it provides guidelines for the recognition of species and for testing of hypotheses concerning membership in each species; it also is compatible with the Linnaean taxonomic hierarchy. As it becomes more widely used by working systematists, problems and difficulties with the concept may appear that will require its refinement. However, the evolutionary concept may in the long run be more acceptable to a wider group of biologists than any other yet proposed. See Taxonomy