NEUTRAL GENE FLOW ACROSS SINGLE-LOCUS CLINES

We study the strength of the genetic barrier to neutral gene flow in a general one-locus, two-allele model that includes viability selection as well as fertility selection and premating isolation. We have separ ately considered adult migration and pollen migration. Our theoretical results suggest that selection against hybrid formation in the form o f fertility selection or assortative mating is more effective in preve nting introgression of neutral alleles than is reduction in hybrid via bility. We argue that in experimental studies of introgression testing of F-1 hybrids is as important as testing of parental forms. To illus trate the utility of this approach, we estimate the strength of the ba rrier to neutral gene flow between Piriqueta caroliniana and P. viridi s, between Iris hexagona and I. fulva, and between Chorthippus brunneu s and C. biguttulus.