Hybrid zones are narrow regions where two closely related species interact, mate, and produce hybrids. Relative to either parent species, hybrids may possess distinct phenotypic or genetic differences that impact their survival and reproductive success. The genetic variances introduced by hybridization influence species diversity and speciation processes within the hybrid zone.

Gene flow and natural selection are evolutionary mechanisms that shape the outcome of a hybrid zone. Gene flow distributes, homogenizes, and preserves genetic variation between populations, while natural selection reduces genetic variation by favoring only the fittest individuals in a population. Thus, if a barrier to genetic exchange emerges, the isolated population becomes more distinct or diverges.

However, if that barrier breaks down, the population and its previously isolated counterpart may interbreed and produce hybrids. Depending upon hybrid fitness, populations may: (1) reduce hybrid gene flow by reinforcing selection against hybrids, (2) promote hybrid gene flow, causing parent and hybrid populations to fuse, or (3) preserve gene flow, allowing parent and hybrid populations to stably exist.

Hybrid zones follow either primary or secondary species contact. Most hybrid zones are the result of secondary contact, where two geographically separated populations reestablish gene flow. Primary contact, although less common, involves natural selection among neighboring populations within a shared geographic range. Since primary and secondary contact produce similar genetic and phenotypic outcomes, the two are difficult to distinguish.

Scientists can observe the frequency of a gene or phenotype, or cline, across a geographic area. Frequencies may change abruptly in the hybrid zone, creating a stepped cline. For example, the frequency of genes specific to fire-bellied toads decreases from nearly 100% in its geographic range to 50% in the hybrid zone to 0% within the yellow-bellied toad range. Clines reflect the gene flow or natural selection affecting interbreeding populations.

Hybrid zones are natural laboratories for studying the mechanisms and processes involved in divergence and speciation. Hybridization creates genetic variation which produces novel adaptations and thus, species diversity. Scientists can analyze multiple clines to characterize the gene flow and natural selection occurring within a hybrid zone. This knowledge allows scientists to better estimate how different factors impact species and populations.