What makes a species? Members of the same species have a similar appearance, at least when compared to members of a different species. They presumably share a common ancestry, and are more closely related to conspecifics than to members of a different species. Biologists have used a variety of ways of thinking about the concept of "species", including these criteria; the most prevalent conceptualization in the evolutionary biology literature, though, is called the Biological Species Concept.

One of the first to articulate the Biological Species Concept was Ernst Mayr in 1940, who defined a species as

"groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups."
Here are the main points of the Biological Species Concept:
  • Populations. A species is made up of populations, not of individuals.
  • Interbreeding. Populations that are part of the same species interbreed, or at least could if they were in spatial proximity. A species represents a single gene pool.
  • Reproductive isolation. Mechanisms exist that prevent gene exchange between populations of different species that coexist.
The Biological Species Concept overcomes problems that arise with a purely morphological definition of species -- sexual dimorphism, polymorphism, and simple variation within a population can result in conspecifics with large differences in morphology. However, with its emphasis on interbreeding as a mechanism for gene exchange, the Biological Species Concept doesn't work well with types of organisms that do not reproduce sexually.