If hybrid zones are transient, what are the likely outcomes? One possibility is that hybrid zones represent secondary contact and neutral diffusion, that the differentiated populations will fuse, yielding a single, possibly polymorphic, species. Alternatively, hybrid zones might represent the "wave of advance" of a superior competitor, resulting in the eventual extinction of one of the two hybridizing taxa. In fact, fusion and extinction are not mutually exclusive outcomes because the fusion of two taxa might involve either selective or random extinction of alleles from each of the parental types (Harrison, 1990).
In this sense, "extinction" means the disappearance of one of the parental types, not necessarily the disappearance of its alleles from the subsequent population.
(continued) If certain recombinate genotypes produced by hybridization and backcrossing persist as local "hybrid swarms" or "stabilized introgressants," the product of fusion may be considered a distinct species.
One of the most controversial issues surrounding hybrid zones is whether they are sites of "reinforcement" -- the evolution of prezygotic barriers to gene exchange in response to selection against hybrids. A mode of speciation originally championed by Dobzhansky (1940, 1941), the "reinforcement model" has met with considerable criticism in recent years (Patterson 1978, 1982; Butlin, 1987, 1989)....
This strain is picked up in the third chapter of the volume by Howard (1993).
(continued) Finally, selection within hybrid zones can lead to the weakening (rather than the strengthening) of barriers to gene exchange. Selection may favor those variants that show the least reduction in viability and fertility when crossed with either of the parental types. </p>