species defined in 1951 yearspecies - species;
species - The smallest unit of classification commonly used; i.e. the group whose members have the greatest mutual resemblance. The common names of familiar animals and plants often denote species e.g. man, fox, beech. For the great majority of animals and many plants, a species is roughly a group of individuals able to breed among themselves (if one disregards geographical separation) but not to breed with organisms of other groups. Consequently the members of a species form a reproductively isolated group, whose genes do not combine with those of outsiders, but are able to re-combine continually by sexual reproduction within the group. As a result no striking differences in genetic composition and in the characters controlled by genes occur within the species, though local differences, which are recognized in classification as a subspecies, may arise through reproductive isolation which is only partial or has recently occurred. Many of these sub-species are no doubt on the road to becoming full species, but are still capable of interbreeding within their species. Reproductive isolation however admits of degrees and so it is not easy to apply rigorously to the problem. Furthermore, it is usually unknown whether two kinds of organism are in fact able to breed together in natural conditions. Consequently species are in practice usually determined by experts in the particular group, on the basis of degree of difference, which is often, though not always, a close reflexion of reproductive isolation. Reproductive isolation, however, cannot be applied at all as a criterion to many organisms. Unless, for instance, sexual reproduction with cross-fertilization freely occurs, the conception of species as a breeding unit breaks down; it does so therefore for some plants. Species must in such cases always be determined on the basis of difference, according to convenience; and while numerous species of such organisms are well-marked and undisputed, there can exist no general rule as to the minimum degree of difference separating any two species. A comprehensive definition of species applying to all kinds of organisms is in fact hardly possible. For naming of species See also: Binomial Nomenclature.
near species in Knolik
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