The most important factor in the divergence of sexually reproducing lineages from one another is the evolution of reproductive isolation, a state in which two groups of organisms can no longer exchange genes. If individuals of group A mate and reproduce only with one another, then group A constitutes a distinct species within which genes recombine. In other words, group A is an independent evolutionary lineage—a separate branch on the tree of life.

Evolutionary biologist Ernst Mayr recognized the importance of reproductive isolation in maintaining species, and so he proposed the biological species concept: “Species are groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups.” The phrase “actually or potentially” is an important element of this definition. “Actually” says that the individuals live in the same area and interbreed with one another. “Potentially” says that even though the individuals do not live in the same area, and therefore do not interbreed, other information suggests that they would do so if they were able to get together. This widely used species concept does not apply to organisms that reproduce asexually, and it is limited to a single point in evolutionary time.