Ethological isolation among three species of the planitibia subgroup of Hawaiian Drosophila.

Drosophila heteroneura and D. silvestris are sympatric species living on the island of Hawaii, while D. planitibia is allopatric on the nearby island of Maui. A pronounced ethological isolation is found between the sympatric species, and none between allopatric ones, except that D. planitibia females discriminate against D. heteroneura males. Male hybrids are sterile in allopatric crosses but fertile in sympatric ones. The ethological isolation and the hybrid sterility are uncorrelated. This is expected if premating isolation between closely related species is an ad hoc product of natural selection, while postmating isolation is an incidental result of genetic divergence. Some exceptions to the rule are discussed.