species

species (sing. and pl.) Literally, a group of organisms that resemble one another closely. The Latin word species means ‘appearance’ or ‘semblance’. In taxonomy, it is applied to one or more groups (populations) of individuals that can interbreed within the group but that do not, under natural conditions, exchange genes with other groups (populations); it is an interbreeding group of biological organisms that is isolated reproductively from all other organisms (see BIOSPECIES). A species can be made up of groups in which members do not actually exchange genes with members of other groups (though in principle they could do so), as e.g. at the two extremes of a continuous geographical range. However, if some gene flow occurs along a continuum, the formation of another species is unlikely to occur. Where barriers to gene flow arise (e.g. physical barriers, such as sea, or areas of unfavourable habitat) this reproductive isolation may lead either by local selection or by random genetic drift to the formation of morphologically distinct forms (‘races’ or ‘subspecies’). These could interbreed with other races of the same species if they were introduced to one another. Once this potential is lost, through some further evolutionary divergence, the races may be recognized as species, although this concept is not a rigid one. Most species cannot interbreed with others; a few can, but produce infertile offspring; a smaller number may actually produce fertile offspring. The term cannot be applied precisely to organisms whose breeding behaviour is unknown. See MORPHOSPECIES; PALAEOSPECIES; REPRODUCTIVE ISOLATING MECHANISMS.