The film defines species as populations whose members don't interbreed. what keeps different galápagos finch species from mating?

Galapagos species are among those populations whose members do not interbreed. The reasons that keep them from mating is that they are separated due to differences in songs and beaks, individuals only recognize and respond to songs of their own species. Therefore, geographic isolation and different environments led to changes in traits that affected mating.

the answer to your question is b