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.
Hybrid zones are areas where two closely related species cohabit and interbreed. For instance, fire-bellied toads and yellow-bellied toads sometimes interbreed in regions where their habitats overlap. The offspring of such mating pairs are called hybrids.
Over time, a hybrid zone may impact the involved species in three different ways: reinforcement, stability, or fusion.
Reinforcement strengthens reproductive barriers, decreasing the incidence of hybrids. For example, in geographically separated populations of pied and collared flycatchers, the males have similar plumage. However, in a hybrid zone between the two species, the males evolved distinct plumage colorations that reliably attract females of their species.
When hybrids occasionally 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 thus reinforces divergence.
Fusion involves the weakening of reproductive barriers until two species become one. Lake Victoria cichlid females, for example, prefer to mate with males of the same species that display a particular color. However, the lake’s murky, polluted water conditions reduced female cichlids’ ability to differentiate between males of the same species and those of closely related species.
Hybrid cichlids’ fitness was not strongly affected, and the weak reproductive barrier between cichlid species ultimately yielded a single species through fusion.
Stability occurs when hybrids continue to survive and reproduce. For example, in the hybrid zone between yellow-bellied toads and fire-bellied toads, hybrid toads continue to exist despite being less fit than either parent species.
Since reproductive barriers are not strong enough to prevent hybrids or weak enough to allow fusion into a single species, hybrids continue to stably exist.
Hybrid zones influence the outcomes of speciation by testing the reproductive barriers between closely related species. Depending on the fitness of the hybrids, hybrid zones may either reinforce reproductive barriers between species, fuse species, or stabilize them.
