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  Speciation (from Latin species, ‘kind’) is the evolutionary mechanism by which a species, the basic unit of biological classification, is formed. Most species are distinguishable from others because members have a high degree of similarity with one another and produce offspring which are similar to both parents. This similarity exists because it is inherited, while differences between species are retained because interspecies mating is rare if not impossible. Humans have long been capable of recognizing those species which are important to them, but an understanding of the evolutionary mechanism and significance of speciation is relatively modern. The process of speciation cannot be observed as it takes place over a period of time which far exceeds the duration of human civilization. However, it is a continuous process, and the evolutionary process by which populations change are observable and fairly well understood.

It is probable that a new species arises as a result of the reproductive isolation of a group of individuals from an existing population. The most obvious, and possibly most common, type of isolation is geographical, and speciation which occurs by such a mechanism is termed allopatric speciation; that which occurs by other means is termed sympatric speciation. Two geographically isolated groups of a single species will evolve independently because no two habitats are identical, and because evolution involves the appearance of variation by random mutation. Thus two isolated groups will eventually lose the ability to mate with each other and will subsequently diverge still further as separate species. In geological terms, events which might divide populations are very frequent and need not be cataclysmic; changes in sea level result in bodies of water and land being subdivided, along with the organisms they are home to. Sympatric speciation is less readily envisaged but is theoretically possible where, for example, a species forms two loci of population around two similar food sources. The individuals in each sub-population may tend to breed with nearby individuals: and this effect, coupled with evolutionary pressure to become more specialized to the chosen food, may eventually lead to the two groups becoming separate species. RB

See also biogeography; hybridization; niche.



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