Abstract
Colour polymorphism can promote rapid evolution and speciation, particularly when populations differ in the number or composition of morphs. The tawny dragon, Ctenophorus decresii, is a compelling study system in which to examine evolutionary processes and outcomes when polymorphic and monomorphic lineages meet. The species comprises a northern lineage polymorphic for male throat coloration which lacks ultraviolet (UV) reflectance and a monomorphic southern lineage with UV-blue throats. We characterised genomic and phenotypic clines across the contact zone based on single nucleotide polymorphisms, the mitochondrial ND4 gene, and male colour traits, and concurrently assessed the phenotype of captive-bred F1 hybrids. Our results indicate that genomic introgression is asymmetric, with high frequencies of backcrossing to the northern but not southern lineage, accompanied by the prevalence of the northern mtDNA haplotype in hybrids. The clines for throat phenotype are abrupt and displaced to the south, relative to the genetic and dorsolateral phenotype clines. This suggests strong selection for the northern throat phenotype within the contact zone, particularly for the absence of ultraviolet reflectance, given the intermediate throat phenotype in captive-bred F1 hybrids. Our results demonstrate that a polymorphic sexual signal is the target of selection during incipient speciation and provide insight into the microevolutionary processes linking polymorphism and speciation.