Hybrid zones as barriers to gene flow

The hybrid zone which forms when two partially incompatible populations meet acts as a barrier to gene flow. We discuss electrophoretic and theoretical evidence on the strength of such barriers. Hybrid zones generally involve considerable electrophoretic divergence. The enzyme clines are consistent in position and width; in some cases, they show consistently asymmetric patterns of introgression. This consistency suggests that the clines are maintained primarily by the indirect effects of selection at linked loci, rather than by the effect of each individual locus on fitness. A cline at a single locus will present some barrier, regardless of the selective mechanism which maintains it. However, unless the locus induces virtually complete assortment or hybrid unfitness, the barrier will be weak. Spreading the same selection over more clines gives a stronger barrier. If the clines are staggered, this barrier is still unlikely to be significant; if they coincide, and if selection is stronger than recombination, then the barrier will be very strong; its strength and asymmetry will be consistent over different loci. Thus, the taxonomic status of divergent populations cannot be inferred just from the total amount of pre- or post-mating isolation; the number of genetic differences, and the interactions between them are equally important in determining rates of gene flow.