Increased differentiation and reduced gene flow in sex chromosomes relative to autosomes between lineages of the brown creeper Certhia americana

The properties of sex chromosomes, including patterns of inheritance, reduced levels of recombination, and hemizygosity in one of the sexes may result in the faster fixation of new mutations via drift and natural selection. Due to these patterns and processes, the two rules of speciation to describe the genetics of postzygotic isolation, Haldane's rule and the large‐X effect, both explicitly include quicker evolution on sex chromosomes relative to autosomes. Because sex‐linked mutations may be the first to become fixed in the speciation process, and appear to be due to stronger genetic drift (in birds), we may identify pronounced genetic differentiation in sex chromosomes in taxa experiencing recent speciation and diverging mainly via genetic drift. Here, we use nine sex‐linked and 21 autosomal genetic markers to investigate differential divergence and introgression between marker types in Certhia americana . We identified increased levels of genetic differentiation and reduced levels of gene flow on sex chromosomes relative to autosomes. This pattern is similar to those observed in other recently‐divergent avian species, providing another case study of the earlier role of sex chromosomes in divergence, relative to autosomes. Additionally, we identify three markers that may be under selection between Certhia americana lineages.