SPECIES

biological classification comprising related organisms that share common characteristics and are capable of interbreeding. The term speciation designates the process by which one species of organism splits into two or more species. Speciation is one of the fundamental processes of evolution. The classification of species is helped by the international system of nomenclature, which assigns new species a standard two-part name of genus and species. The genus is the generic name that includes closely related species; the Gila monster, for example, is classified as Heloderma suspectum and is a close relative to the lizard found in Mexico called Heloderma horridum, their relation being indicated by their sharing the same genus name, Heloderma. Similarly, genera that have shared traits are classified in the same family; related families are placed in the same order; related orders are placed in the same class; and related classes are placed in the same phylum. Organisms are grouped into species partly according to their morphological, or external, similarities, but more important in classifying sexually reproducing organisms is the organisms' ability to successfully interbreed. Individuals of a single species can mate and produce viable offspring with one another but not with members of other species. Separate species have been known to produce hybrid offspring (for example, the horse and the donkey producing the mule), but, because the offspring are almost always inviable or sterile, the interbreeding is not considered successful. Interbreeding only within the species is of great importance for evolution in that individuals of one species share a common gene pool that members of other species do not. Within a single pool there is always a certain amount of variation among individuals, and those whose genetic variations leave them at a disadvantage in a particular environment tend to be eliminated in favour of those with advantageous variations. This process of natural selection results in the gene pool's evolving in such a way that the advantageous variations become the norm. Because genetic variations originate in individuals of a species and because those individuals pass on their variations only within the species, then it is at the species level that evolution takes place. The evolution of one species into others is called speciation. Subspecies are groups at the first stage of speciation; individuals of different subspecies sometimes interbreed, but they produce many sterile male offspring. At the second stage are incipient species, or semispecies; individuals of these groups rarely interbreed, and all their male offspring are sterile. Natural selection separates incipient species into sibling species, which do not mate at all but which in morphology, or appearance, are nearly indistinguishable. Sibling species then evolve into morphologically (and taxonomically) different species. Speciation may occur in many ways. A population may become geographically separated from the rest of its species and never be rejoined. Through the process of adaptive radiation (q.v.) this population might evolve independently into a new species, changing to fit particular ecological niches in the new environment and never requiring natural selection to complete its reproductive isolation from the parent species. Within the new environment, populations of the new species might then radiate into species themselves. A famous example of adaptive radiation is that of the Galpagos finch. Quantum speciation is a rapid process of reproductive isolation often brought about by chromosomal mutation in one population of a species. When the mutant population interbreeds with the parent population, unfit hybrid offspring are produced; then, as in geographic speciation, natural selection completes the isolation of the two groups. Polyploidy is a form of quantum speciation that creates the beginnings of a new species in two or three generations. It is most common among flowering plants. As a consequence of abnormal cell division at the gamete stage, a hybrid plant may develop not two sets of chromosomes (one inherited from each parent) but four or more in each cell nucleus. Through self-fertilization at the flowering stage, the hybrid may produce its own viable offspring, which, being polyploid (of multiple chromosomes), would be reproductively isolated from the parent species that is diploid (of double chromosomes). The evidence for speciation was formerly found in the fossil record by tracing successive changes in the morphology, or structure and form, of organisms. Genetic studies now show that morphological change does not always accompany speciation, as many apparently identical groups are, in fact, reproductively isolated.