question: What did Darwin consider that could explain why different regions could have such different species when they are close together ?
answer: barriers to migration played an important role in the differences between the species of different regions .
turker_answer: Darwin considered that barriers to migration played an important role in the differences between the species of different regions and that could explain why different regions could have such different species when they are close together .
rule-based: Darwin considered barriers to migration played an important role in the differences between the species of different regions that could explain why different regions could have such different species when they are close together .