Speciation genomics and a role for the Z chromosome in the early stages of divergence between Mexican ducks and mallards

Speciation is a continuous and dynamic process, and studying organisms during the early stages of this process can aid in identifying speciation mechanisms. The mallard (Anas platyrhynchos) and Mexican duck (A. [p.] diazi) are two recently diverged taxa with a history of hybridization and controversial taxonomy. To understand their evolutionary history, we conducted genomic scans to characterize patterns of genetic diversity and divergence across the mitochondrial DNA (mtDNA) control region, 3523 autosomal loci and 172 Z‐linked sex chromosome loci. Between the two taxa, Z‐linked loci (ΦST = 0.088) were 5.2 times more differentiated than autosomal DNA (ΦST = 0.017) but comparable to mtDNA (ΦST = 0.092). This elevated Z differentiation deviated from neutral expectations inferred from simulated data that incorporated demographic history and differences in effective population sizes between marker types. Furthermore, 3% of Z‐linked loci, compared to