Dictionary of Human Evolution and Biology

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1. as used in biology, any obstacle to reproduction between populations or species, e.g. geographical barriers such as bodies of water, deserts, or mountains that separate populations and prevent breeding. Other barriers to reproduction include structural barriers (e.g. male intromittent organs that do not fit the reproductive organs of females of other species), behavioral barriers (e.g. differences in courtship rituals or activity cycles) and, in humans, social and cultural barriers.

2. In physiology, a barrier that limits movement between entities; e.g. the blood-brain barrier.

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