Question

what do we call a physical barrier or obstacle that prevents two species from mating?
prezygotic barrier
postzygotic barrier

Explanation:

Prezygotic barriers prevent mating or fertilization. Physical barriers like different habitats or mating times are prezygotic. Postzygotic barriers act after fertilization (e.g., hybrid infertility). The question asks about a barrier preventing mating (pre - mating), so it's a prezygotic barrier.

Answer:

prezygotic barrier