Читать книгу Ecology - Michael Begon - Страница 20

biospecies: the Mayr–Dobzhansky test

Cynics have said, with some truth, that a species is what a competent taxonomist regards as a species. On the other hand, back in the 1930s two American biologists, Mayr and Dobzhansky, proposed an empirical test that could be used to decide whether two populations were part of the same species or of two different species. They recognised organisms as being members of a single species if they could, at least potentially, breed together in nature to produce fertile offspring. They called a species tested and defined in this way a biological species or biospecies. In the examples that we have used earlier in this chapter, we know that melanic and normal peppered moths can mate and that the offspring are fully fertile; this is also true of Anthoxanthum plants from different positions along the gradient at the Trelogan mine. They are all variations within species – not separate species.

In practice, however, biologists do not apply the Mayr–Dobzhansky test before they recognise every species: there is simply not enough time or resources, and in any case, there are vast portions of the living world – most microorganisms, for example – where an absence of sexual reproduction makes a strict interbreeding criterion inappropriate. What is more important is that the test recognises a crucial element in the evolutionary process that we have met already in considering specialisation within species. If the members of two populations are able to hybridise, and their genes are combined and reassorted in their progeny, then natural selection can never make them truly distinct. Although natural selection may tend to force a population to evolve into two or more distinct forms, sexual reproduction and hybridisation mix them up again.