Two sister species of butterflies, Heliconius melpomene and Heliconius cydno, each mimic a different model species. While these species are largely sympatric, there are some areas in which H. melpomene occurs alone (i.e., allopatric to H. cydno). Mating studies show that H. melpomene females from these allopatric populations are more responsive to courtship by H. cydno males than are H. melpomene females from sympatry. What mechanism could explain this stronger behavioural isolation in sympatric compared to allopatric H. melpomene populations?
a) trait-based reproductive barriers
c) ecological speciation
d) prezygotic isolation
e) behavioural isolation