populations in geographic isolation sometimes evolve marked differences in morphology and genetics -- yet do not evolve reproductive isolation -- during secondary contact interbreeding occurs in hybrid zones -- in recent decades hybrid zones have become a focus for evolutionary biologists interested in speciation -- if you can understand what happens in hybrid zones, you can understand geographic variation and speciation . . .

there are four possibilities when genetically different populations come into secondary contact and hybridize . . .

1. if genetic differences between the two populations do not affect survival or reproduction (neutral alleles), then alleles gradually diffuse from each population into the other -- the hybrid zone grows steadily wider (symmetric introgression) -- the rate of widening depends on generation time, natal dispersal, and population densities (hybrid zones shift toward lower densities because fewer immigrants arrive from areas of low density)

2. if alleles of one population have higher survival or reproduction, then they spread through the other population (asymmetric introgression) -- the rate of spread depends on the differences in survival or reproduction, generation time, and natal dispersal

3. if secondary contact occurs in an area of intermediate habitats and hybrid genotypes have higher fitness there in comparison to parental genotypes, the hybrid zone remains stable and coincides with the intermediate habitats (bounded hybrid superiority)

4. if hybrid genotypes have lower fitness, a stable hybrid zone persists by continual immigration from populations on either side (genetic sink) -- as in case (1), hybrid zones tend to move toward and to get trapped in areas of low population density

in some areas many hybrid zones between different pairs of species almost coincide -- on the Great Plains, for instance, there are hybrid zones between eastern and western populations of forest birds that extend across the plains in strips of trees along rivers

13 pairs of closely related eastern/western birds make secondary contact in Great Plains . . . some of these hybrid zones have been studied enough to analyze . . .

• the hybrid zone between Yellow-shafted and Red-shafted Flickers has remained stable for 150 years -- hybrid populations do not have lower reproductive success or survival than populations farther east or west -- absence of private alleles in local populations indicates high gene flow -- evidence thus suggests bounded hybrid superiority

• the hybrid zone between Baltimore and Bullock's Orioles has shifted but has not widened (for orioles in Nebraska-Colorado, zone has moved westward at 10 km/year; in southern Kansas, it has moved eastward at the same rate!) -- apparently in each area natural selection favors genes of one population -- asymmetric introgression

• in hybrid zones of Indigo/Lazuli Buntings and Rose-breasted/Black-headed Grosbeaks, mixed matings produce fewer offspring -- so selection operates against hybridization -- genetic sink

• in some cases hybrids are more common where parental species are rare -- indication that hybridization results from desperation in finding mates and/or that hybrid zone has been trapped by a region of low density