Prezygotic isolation evolves much faster in sympatric species pairs than it does in allopatric species pairs Prezygotic isolation evolves much faster in sympatric species pairs than it does in allopatric species pairs . These graphs plot degree of prezygotic isolation versus genetic distance in a variety of sister-species pairs from the genus Drosophila. Prezygotic isolation is estimated from mate-choice tests performed in the laboratory.
Genetic distance is estimated from differences in allele frequencies found in allozyme surveys.
Sibling species with the same degree of overall genetic divergence show much more prezygotic isolation if they live in sympatry.