What kinds of differences are required to consider two organisms different species? A widely used definition of species is the biological species concept, first fully developed by evolutionary biologist Ernst Mayr in 1942. Mayr was primarily interested in the biological characteristics that are responsible for separating organisms into independently evolving units. He defined a species as a group of organisms whose members are capable of interbreeding with one another but are reproductively isolated from the members of other species. In other words, members of the same species have the biological potential to exchange genes, and members of different species cannot exchange genes. Because different species do not exchange genes, each species evolves independently.

Not all biologists adhere to the biological species concept, and there are several problems associated with it. For example, reproductive isolation, on which the biological species concept is based, cannot be determined from fossils, and in practice, it is often difficult to determine whether even living species are biologically capable of exchanging genes. Furthermore, the biological species concept cannot be applied to asexually reproducing organisms, such as bacteria. In practice, most species are distinguished on the basis of phenotypic (usually anatomical) differences. Biologists often assume that phenotypic differences represent underlying genetic differences; if the phenotypes of two organisms are quite different, then they probably cannot and do not interbreed in nature. Because of these problems, some biologists have proposed alternative definitions for a species. For example, the morphospecies concept defines a species based entirely on phenotypic (morphological) similarities and differences. The phylogenetic species concept defines a species as the smallest recognizable group that has a unique evolutionary history. Here, we will use the biological species concept because it is widely used and is based on genetic differences.