Two species of flies, Drosophila melanogaster and Drosophila simulans, can mate. They usually do not mate, however, even if they are kept together in a laboratory. Males court the females of both species, but the females prefer to mate with males of their own species. Which isolating mechanism does this describe?
behavioral

In the case of flies of different species, when males court the females of both species, but the females prefer to mate with males of their own species, the isolating mechanism described is behavioral isolation.

Explanation:

D. melanogaster and D. simulans are very similar fly species but, although the male may court the females of both species when in captivity, the females only respond to the courtship of the males of their own species.

Behavioral isolation, also called sexual isolation, prevents reproduction between different species -even with compatible genitalia- and courtship is one way of ensuring isolation. In this case, mating depends on the female's response to the male's courtship.

Learn more:

Behavioral isolation link

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b. read the phase at the intersection of any given temperature and pressure.