Summary: Ahearn et al. 1974

Ahearn et al. (1974) identified two kinds of reproductive isolating mechanisms: 1) premating isolation barriers (in the form of behavioural/ethological isolation); and 2) postmating isolation barriers (male hybrid sterility). They suggest that reproductive isolation arises as a by-product of genetic divergence or is induced by natural selection as a gene exchange barrier. Thus, premating isolation should occur more often between sympatric species. Drosophila heteroneura and D. silvestris are sympatric fly species on the island of Hawaii, while D. planitibia is allopatric on nearby Maui. Ahearn et al. (1974) found pronounced behavioural isolation between the sympatric, but not the allopatric, species. They did note that D. planitibia females do discriminate against D. heteroneura males. Male bybrid sterility is seen in allopatric crosses and is not correlated with behavioural isolation. They suggest that this is expected if premating isolation between closely related species is an ad hoc production of natural selection, while postmating isolation is an incidental result of genetic divergence.