Beyond clines: lineages and haplotype blocks in hybrid zones
Hybrid zones formed between recently diverged populations offer an opportunity to study the mechanisms underlying reproductive isolation and the process of speciation. Here, we use a combination of analytical theory and explicit forward simulations to show how selection against hybrid genotypes impacts patterns of introgression across genomic and geographic space. By understanding how lineages move across the hybrid zone, in a model without coalescence, we describe how clines form and how parental haplotypes are broken up during introgression. Clines form in about 1/s generations, where s is the strength of selection against hybrids, and linked clines persist over a physical scale of 1/T, where T is the age, in generations, of the hybrid zone. Locally disadvantageous alleles tend to exist as small families, whose lineages trace back to the side from which they originated at speed [Formula] dispersal distances per generation. The lengths of continuous tracts of ancestry provide an additional source of information: blocks of ancestry surrounding single-locus incompatibilities can be substantially longer than the genome-wide average block length at the same spatial location, an observation that might be used to characterize the age of hybrid zones and identify candidate targets of selection.