Contact zones reveal restricted introgression despite frequent hybridization across a recent lizard radiation

Introgression - the exchange of genetic material through hybridization - is now recognized as common among animal species. The extent of introgression, however, can vary considerably even when it occurs: for example, introgression can be geographically restricted or so pervasive that populations merge. Such variation highlights the importance of understanding the factors mediating introgression. Here we used genome-wide SNP data to assess hybridization and introgression at 32 contact zones, comprising 21 phylogenetic independent contrasts across a recent lizard radiation (Heteronotia). We then tested the relationship between the extent of introgression (average admixture at contact zones) and genomic divergence across independent contrasts. Early generation hybrids were detected at contact zones spanning the range of genomic divergence included here. Despite this, we found that introgression is remarkably rare and, when observed, geographically restricted. Only the two most genomically similar population pairs showed introgression beyond 5 km of the contact zone. Introgression dropped precipitously at only modest levels of genomic divergence, beyond which it was absent or extremely low. Our results contrast with the growing number of studies indicating that introgression is prevalent among animals, suggesting that animal groups will vary considerably in their propensity for introgression.