Allopatric speciation: a population forms a new species while
geographically isolated from its parent population, reducing gene
flow. Sympatric speciation: a population becomes a new species by
habitat differentiation without geographic separation.
Sympatric speciation: sexual
Under normal light, two cichlid species in Lake Victoria are different
in coloration, and females mate only with males of their own species.
Under orange light, the two species appear identical, and females mate
with males of either species, yielding fertile hybrids.
Intersexual selection by females, based on coloration, is the
reproductive barrier here.