The prevention of gene flow between all the members of a population due to the development of reproductive barriers (isolating mechanisms), which result in the formation of distinct noninterbreeding groups that may in time develop into separate species. Isolating mechanisms can arise in many ways. Populations may be separated geographically (see allopatric
). Populations occupying the same area (see sympatric
) may not interbreed because they have different flowering times or different pollinators, or because they occupy different niches. Such populations may also be isolated by the inviability or sterility of any hybrids that do form.