Sympatric Speciation - Background
In sympatric speciation, divergence occurs without geographic separation. Although the two subpopulations freely intermix, they must choose to mate with their own type if the two types are to diverge genetically. This may occur when two different selection pressures exist within one geographic range.
Hybrids will be selected against because they are not well suited for either environment. Selection will favor, therefore, mating within and not between subpopulations. This type of mating (postitive assortative mating) reinforces the divergence between the two subpopulations and may eventually lead to speciation.
One good example of sympatric speciation comes from Apple and Hawthorne flies (Rhagoletis pomonella) in the Northeastern US. These flies choose to mate and lay their eggs on either apple or hawthorne fruits, which exist within the same geographic range. Although the two races of flies appear identical, the are have been shown to be different genetically.