Species

Species, category in the classification of plants and animals, individuals of which can interbreed but are unable to breed with other such groups. Species ranks below genus and family, and contains subspecies.

Ernst Mayr, one of the founders of modern evolutionary theory, defined such biological species as “groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups”. On this basis the species is the fundamental unit in the classification system. Categories above it recognize different degrees of similarity of organisms; they constitute a formal system which allows us to picture the hierarchical patterns of organization of living things. Within the species, there may be geographical variation, so that the species consists of more or less distinct subspecies occupying different parts of the range, which can nevertheless interbreed (a polytypic species). In taxonomy, the term race is used interchangeably with subspecies.

There are many difficulties in the application of the biological species concept. Capacity to interbreed cannot always be tested, nor can the potential for interbreeding. It may not be possible to distinguish between a single polytypic species and a group of similar species occupying neighbouring areas. The museum taxonomist normally has to deal with dead material, with no information on breeding behaviour. The definition cannot readily be used for organisms, such as bacteria, which reproduce asexually for long periods, or plants which reproduce vegetatively. It cannot be applied in palaeontology, where there is no evidence concerning breeding potential. In addition, the fossil sequence often lacks evidence of discontinuity within lineages, although different sections differ as much as modern species.

As a result, practical systematics continues to be carried out using what is known as the typological species concept. The approach, going back to Plato and Aristotle, and established for modern taxonomic purposes in the 17th century by John Ray, is to identify a set of morphological features (supplemented by any available information on distribution, behaviour, ecology, etc), which characterize a particular coherent group. Individuals falling outside the range so defined then belong to another species. The intention nowadays is to define groupings which correspond as nearly as possible to those recognized as biological species in sexual organisms in nature. This procedure has been formalized in cladistic taxonomy in which evolution is seen as a process of progressive bifurcations of lineages displaying different combinations of characters. Every species therefore has a sister species, whether we can recognize it or not. The pair are derived from a third, ancestral, species, which, in its turn, has or had a sister species.

The process of evolution involves the origin of new species—distinct and reproductively isolated groupings that are nevertheless descended from other such groupings. Species are described as allopatric, if they occupy different ranges, sympatric if they coexist and parapatric if their ranges abut in a zone of contact. There is no general agreement as to how one species gives rise to another. Perhaps the most common pattern is allopatric species formation. Races of a polytypic species may become isolated as a result of some geological accident, or some individuals may colonize an isolated island and thereafter be cut off from the parent stock. From then on progressive divergence is likely to ensue. Accidental fluctuations in gene frequency occur in the two groups an different patterns of selection my operate. If divergence goes far enough for there to be reproductive isolation, should their ranges rejoin or one of them invade the territory of the other, then new species have been formed. If interbreeding can still take place, but the hybrid progeny are less fit than either of the parental types then characteristics which favour mating between the parental types will be selected. Initial separation was partial, but selection has driven it to completion. See Natural Selection.

Thus, there are two components to species formation: isolation and disruptive or diverging selection. According to the allopatric model, isolation initiates the process. It is a random process, comparable to the mutation which may initiate the establishment of an adaptive trait by selection. Thereafter, selection will consolidate the isolation of new species and tend to lead them along diverging paths of adaptation.

Sympatric speciation is a process perceived as resulting from selection for alternative adaptive modes. G. L. Bush had studied fruit flies in the genus Rhagoletis. There are species which exploit the fruits of different trees living together, such as plums and cherries. Adults are attracted to ripe fruits, mate and lay eggs on them. The larvae then develop within the fruit. Selection for alternative genotypes conferring high larval fitness on either plum or cherry, but not both, could start the speciation process off. Selection of other genes operating in the adults to attract them to plums or cherries respectively would reinforce the ensuing genetic separation. What was initially a single species with a capacity to colonize each type of fruit would become two, showing host-plant specialization without the aid of geographical isolation. On this model, disruptive selection initiates speciation, eventually leading to reproductive isolation. In the original population the alternative alleles favouring larval survival on the two types of fruit would have to coexist long enough to allow selection of adult preference to be achieved.

Disruptive selection is also the starting point for the parapatric model. Here, races are perceived as occupying adjacent territories where they are adapted to the local conditions. There is a zone of interbreeding between them. Progeny of interracial crosses will be less well adapted than either of the parental types, so that any inherited mechanism resulting in like-to-like matings will be favoured. Given suitable characters and sufficiently strong selection, the consequence could eventually be separation of the races as a pair of new species.

Reproductive isolation, which is necessary before good biological species can arise, may occur before or after mating. Premating isolating mechanisms are characteristics such as seasonal differences in flowering time or differences in the courtship signals which precede mating. The classical example of postmating isolation is the infertility of the mule, the offspring of a horse and a donkey. Both types of mechanism are equally effective in preventing genetic mixture.

Species formation is a major field of interest in evolutionary biology, and there is little consensus on the mechanisms involved. The majority view is that allopatric speciation is probably the most common, although the sympatric and parapatric models have their adherents. Premating isolation is sometimes held to be of paramount importance, and chromosomal reorganization is sometimes stressed as a mechanism for achieving rapid isolation. The fundamental problem is to reconcile a continuous process (evolution) with a discontinuous classification (species) which has ecological validity when we study the lives of sympatric species.