Speciation is the evolution of new species. It occurs when a population of interbreeding individuals is split up into separate populations. These separate populations then continue to evolve independent of each other. Over time, they may become separate species and be unable to breed with the populations from which they were initially separated. For animals that reproduce without sex, species are defined by arbitrary decisions based on how genetically distinct groups have become. Speciation can occur if a geographic barrier prevents gene flow between two populations of the same species.