Question

the barrier between the fruit fly populations _______. was an effective barrier to mating and gene flow between the two populations produced distinct species as soon as the barrier became impassible caused completely different mutations to occur in the two populations did not greatly interfere with both populations accessing both mountaintops was the first part of an extinction event for both populations

Explanation:

A barrier between populations often restricts mating and gene - flow. This is a key concept in population genetics and speciation. If a barrier is effective, it prevents the exchange of genetic material between the two populations. The other options are incorrect as speciation doesn't occur immediately when a barrier becomes impassable, mutations are not necessarily completely different just due to a barrier, a barrier would interfere with access if it's between populations, and it's not necessarily the start of an extinction event.

Answer:

was an effective barrier to mating and gene flow between the two populations