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 (1904–2005) 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.
Subjects: Zoology and Animal Sciences.