Speciation is the process by which two groups of organisms become reproductively isolated from each other. Because this process requires genetic isolation between the diverging populations and because geography is the easiest way to ensure physical and therefore genetic isolation, many models of speciation focus on geography.

The process usually begins with the creation of allopatric (literally, “different place”) populations, populations that are geographically separated from each other. Clearly, physical separation will not immediately cause reproductive isolation. If a single population is split in two by a geographic barrier, and, after a couple of generations, individuals from each population are allowed to interbreed again, they will still be capable of producing fertile offspring and therefore still be members of the same species. What’s also required for speciation to occur is time. Time allows for different mutations to become fixed in the two separated populations so that eventually they become reproductively isolated from each other.

Because genetic divergence is typically gradual, we often find allopatric populations that have yet to evolve even partial reproductive isolation but which have accumulated a few population-specific traits. This genetic distinctness is sometimes recognized by taxonomists, who call each geographic form a subspecies by adding a further designation after its species name. For example, Sri Lankan Asian elephants, subspecies Elephas maximus maximus, are generally larger and darker than Indian ones, subspecies Elephas maximus indicus (see Fig. 22.2).