Species and Speciation, basic concepts in the classification of organisms. In simple terms, a single species is a distinct kind of organism, with a characteristic shape, size, behavior, and habitat that remains constant from year to year. A biological species is defined as a group of natural populations that mate and produce offspring with one another, but do not breed with other populations. This definition includes genealogical relationships as well as physical properties, and emphasizes that species evolve independently of one another. See Evolution.
THE NATURE OF SPECIES
Other conceptions of species exist, the oldest of which is the typological-species concept that originated with Plato and Aristotle. According to this concept, a species represents some ideal form, of which individual variation is merely the imperfect expression. The morphological-species definition, on the other hand, is purely observational: a group of individuals that resemble one another and are separated from other such groups by gaps in morphological variation, that is, variation in structure and form. These concepts are adequate for classification of inanimate objects such as minerals, where a particular degree of similarity reflects the effects of the same degree of similarity in the physical processes that formed the objects. Organisms, however, are also influenced by genealogy (hereditary characteristics from preceding generations), therefore, these definitions are inappropriate. Certain properties of organisms may reflect past history but may be irrelevant to or only partly affected by current environmental conditions. The human vermiform appendix, a classic example, is a vestige of a more herbivorous ancestor.
In addition to being inappropriate, the typological and morphological concepts prove inadequate when the attempt is made to apply them over geological time or over a broad geographical area; a characteristic used to distinguish between two species in one place is often not valid in another. This is because, in space and geologic time, species change in morphology, behavior, and habitats. The biological-species concept takes this into account, but the typological and morphological definitions refer to only one static type.
Speciation is the process whereby new species are formed. The following events are thought to occur in most cases. In the first step, extrinsic isolation, an existing species becomes subdivided by some extrinsic event, such as a climatic change, the formation of a physical barrier (such as a mountain range), or its invasion of a new habitat or island. Subdivision may also occur merely because many hundreds of generations may be required for individuals to disperse from one end of the species' geographic range to another. In the second step, differentiation, the isolated populations diverge genetically, which they can do more rapidly than populations exchanging individuals with other populations. Populations may diverge either at random or as a result of natural selection. In the third step, intrinsic isolation, some form of isolation evolves among the populations; this is dependent on the organisms rather than the environment. Such isolation may result from preferences during courtship or from genetic incompatibility, in which offspring of matings between the differentiated populations are no longer viable or fertile; the mule is an example. In the final step, independence, the newly separated populations continue to evolve independently and may subsequently invade each other's geographic ranges without hybridization. Each of these steps has been demonstrated in the field and laboratory with various organisms.
Two major modes of speciation are theoretically possible: geographic and nongeographic. In geographic speciation, initial isolation results from geographic separation of the populations. In nongeographic speciation, initial isolation results from changes in behavior or genetics of part of a local population. For example, many insects will eat only one species of plant and will use this plant's shape, color, or odor as cues for location of mates and egg laying. If a group of these insects accidentally invades a new plant species and mates there, then it is as isolated as if it were far away. A great deal of controversy exists about the relative frequency of various modes of speciation, but the geographic mode is generally considered more common.