Given that populations continually evolve, at what point does a population evolve into a new species? The generally accepted definition of the biological species concept is a population of organisms that is reproductively isolated from all other populations. This definition is generally accepted for vertebrate species, although other legitimate alternative definitions exist that take into account the diversity of reproductive biology (sexual, asexual, budding, etc.), and natural histories and life cycles.

Speciation occurs in three stages:

1. A population becomes isolated: Speciation begins when a group of individuals separates into an isolated population that no longer exchanges individuals with the parent population. Physical or geographical barriers to migration can occur from changes in the environment, such as a new stream resulting from a storm, creating allopatric (living separately) species. Isolation can also be due to a change in a trait such as behavior or coloration that prevents individuals from interbreeding with dissimilar individuals in the population, even if they are living together or sympatric.
   
   
2. The isolated population evolves independently: Once isolated, individuals will naturally accumulate random mutations, but they will also be subjected to a different set of selective pressures and/or evolutionary processes (i.e., genetic drift, founder effects) from that of the original population, and thus evolve differently than the parent population.
   
   
3. Reproductive isolating mechanisms evolve: Eventually the separated populations will evolve to a point where they can no longer interbreed because of reproductive isolating mechanisms. These are grouped into two categories: Pre-zygotic mechanisms prevent reproduction, and include physical mechanisms that prevent successful copulation or fertilization; behavioral mechanisms that prevent successful solicitation of a mate; or temporal mechanisms in which mating seasons or fertility patterns are no longer synchronized. Post-zygotic mechanisms result in offspring with gene combinations that are fatal, cause sterility, or otherwise prevent reproduction.