Weak coupling among barrier loci and waves of neutral and adaptive introgression across an expanding hybrid zone

Hybridization can serve as an evolutionary stimulus, but we have little
understanding of introgression at early stages of hybrid zone formation.
We analyze reproductive isolation and introgression between a
range-limited and a widespread species. Reproductive barriers are
estimated based on differences in flowering time, ecogeographic
distributions, and seed set from crosses. We find an asymmetrical mating
barrier due to cytonuclear incompatibility that is consistent with
observed clusters of coincident and concordant tension zone clines
(barrier loci) for mtDNA haplotypes and nuclear SNPs. These groups of
concordant clines are spread across the hybrid zone, resulting in weak
coupling among barrier loci and extensive introgression. Neutral clines
had nearly equal introgression into both species’ ranges, while putative
cases of adaptive introgression had exceptionally wide clines with centers
shifted towards one species. Analyses of cline shape indicate that
secondary contact was initiated within the last 800 generations with the
per-generation dispersal between 200 and 400 meters, and provide some of
the first estimates of the strength of selection required to account for
observed levels of adaptive introgression. The weak species boundary
between these species appears to be in early stages of dissolution, and
ultimately will precipitate genetic swamping of the range-limited species.