<P> Charles Darwin's famous book On the Origin of Species (1859) offered an explanation as to how species evolve, given enough time . Although Darwin did not provide details on how species can split into two, he viewed speciation as a gradual process . If Darwin was correct, then, when new incipient species are forming, there must be a period of time when they are not yet distinct enough to be recognized as species . Darwin's theory suggested that there was often not going to be an objective fact of the matter, on whether there were one or two species . </P> <P> Darwin's book triggered a crisis of uncertainty for some biologists over the objectivity of species, and some came to wonder whether individual species could be objectively real--i.e. have an existence that is independent of the human observer . </P> <P> In the 1920s and 1930s, Mendel's theory of inheritance and Darwin's theory of evolution by natural selection were joined in what was called the modern synthesis . This conjunction of theories also had a large impact on how biologists think about species . Edward Poulton anticipated many ideas on species that today are well accepted, and that were later more fully developed by Theodosius Dobzhansky and Ernst Mayr, two of the architects of the modern synthesis . Dobzhansky's 1937 book articulated the genetic processes that occur when incipient species are beginning to diverge . In particular, Dobzhansky described the critical role, for the formation of new species, of the evolution of reproductive isolation . </P> <P> Ernst Mayr's 1942 book was a turning point for the species problem . In it, he wrote about how different investigators approach species identification, and he characterized their approaches as species concepts . He argued for what came to be called the Biological Species Concept (BSC), that a species consists of populations of organisms that can reproduce with one another and that are reproductively isolated from other populations, though he was not the first to define "species" on the basis of reproductive compatibility . For example, Mayr discusses how Buffon proposed this kind of definition of "species" in 1753 . Theodosius Dobzhansky was a contemporary of Mayr and the author of a classic book about the evolutionary origins of reproductive barriers between species, published a few years before Mayr's . Many biologists credit Dobzhansky and Mayr jointly for emphasizing reproductive isolation . </P>

When does the biological species concept not work