Sympatric speciation

From EvoWiki

Sympatric speciation is the splitting of one species into two in the presence of gene flow. Speciation used to be defined in terms of geographical isolation (now called allopatric speciation), but what constitutes an 'isolating barrier' varies greatly between species. For example, a small mountain can be an unsurmountable obstacle for gene flow to a wingless beetle, while other beetles don't seem to have a problem crossing over continents and oceans.

Models of sympatric speciation usually invoke assortative mating, and very strong diverging selection between the speciating populations to counter the effects of gene flow and linkage disequilibrium.