Hybrid Zones

Hybrid zones are areas where the geographic ranges of two closely related species meet and overlap. This allows individuals to interbreed. For example, fire-bellied toads and yellow-bellied toads sometimes mate in regions where their habitats overlap. The offspring from these mating pairs are called hybrids.

Over time, a hybrid zone can affect the species involved in three ways: reinforcement, stability, or fusion.

Reinforcement strengthens reproductive barriers and reduces how often hybrids are produced. For example, in geographically separated populations of pied and collared flycatchers, males have similar plumage. But in a hybrid zone between the two species, males evolved distinct plumage colors that reliably attract females of their own species.

When hybrids do form, the resulting females are sterile, and males struggle to attract mates. The reduced fitness of hybrid flycatchers and the reproductive barrier between pied and collared flycatchers reinforce divergence between the two species.

Fusion happens when reproductive barriers weaken until two species become one. In Lake Victoria, female cichlids prefer to mate with males of the same species that display a specific color. But