biological species concept The view that the species comprises populations (or groups of populations) that are reproductively isolated from each other, i.e. that species should always be biospecies. This has been mistakenly interpreted as implying that either they cannot interbreed, or that hybrids between them are sterile; but there are many reproductive isolating mechanisms. The concept was proposed by Ernst Mayr in the 1940s, but many biologists have come to regard it as too restrictive. For example, species that are not sister groups may interbreed in nature. Further, the professional taxonomist, usually working in a museum, must make (often unwarranted) judgements about whether two taxa might or might not be capable of interbreeding in nature. Other species concepts (the recognition, cohesion, and, especially, the phylogenetic species concepts) have come to be employed more and more in recent years.

biological species concept (BSC) The concept of a species as a group of populations whose members are capable of interbreeding successfully and are reproductively isolated from other groups. This concept became influential during the late 19th and early 20th centuries, largely replacing the typological species concept favoured by pioneer naturalists. Central to the concept is the role of sexual reproduction. This maintains the broad uniformity of species' members through genetic recombination and sharing of a common gene pool. Isolating mechanisms prevent breeding, and hence gene flow, between different groups, thus ensuring genetic divergence between groups. However, the concept cannot be applied to exclusively asexual organisms, such as certain groups of fungi and bacteria. Nor does it account satisfactorily for the many instances in which interspecies mating does occur, especially in plants, fungi, and prokaryotes.