When polymorphism and monomorphism meet: discordant genomic and phenotypic clines across a lizard contact zone

Colour polymorphism can promote rapid evolution and speciation, particularly when populations differ in the number or composition of morphs. The contact zone between Ctenophorus modestus (swift dragon) and C. decresii (tawny dragon) is a compelling study system in which to examine evolutionary processes and outcomes when polymorphic and monomorphic populations meet. Ctenophorus modestus is polymorphic for male throat coloration and lacks ultraviolet (UV) reflectance while C. decresii is monomorphic 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 C. modestus but not C. decresii, accompanied by the prevalence of the C. modestus mtDNA haplotype in hybrids. The clines for throat phenotype are abrupt and displaced towards the range of C. decresii, relative to the genetic and dorsolateral phenotype clines. By contrast, both throat and dorsolateral phenotypes in captive-bred F1 hybrids are intermediate. Our results are consistent with the hypothesis that throat coloration, a polymorphic sexual signal in C. modestus, is the target of selection during incipient speciation and provide insight into the microevolutionary processes that may link polymorphism and speciation.