Sympatric speciation

Sympatric speciation is the evolution of a new species from a surviving ancestral species while both continue to live in the same geographic area, so that, unlike other modes of speciation, it occurs without any geographic separation. Reproductive isolation instead arises through ecological, behavioral or genetic differences, commonly under a model of disruptive selection in which different ecological niches within one population create diverging pressures; as individuals adapt to different niches they can develop barriers to interbreeding, such as different mating signals, that separate them from the rest of the population despite sharing the same range. The medium ground finch on Santa Cruz Island illustrates the process: its beak sizes diverge into two distinct categories, with intermediate sizes selected against, and because beak shape affects both feeding and the calls used in mate choice, it works as a trait that produces reproductive isolation directly. Sympatric speciation is considered a real and documented process, though how often it occurs is not well established; it appears more common in plants through polyploidy and may be more frequent in bacteria.