Patterns of morphological and mitochondrial diversity in parapatric subspecies of the Thick-billed Grasswren (Amytornis modestus)

ABSTRACTDivergence is the first phase of speciation and is commonly thought to occur more readily in allopatric populations. Subspecies are populations that are divergent but generally retain the capacity to interbreed should they come into contact. Two subspecies of the Thick-billed Grasswren (Amytornis modestus) are divergent by 1.7% at the mitochondrial ND2 gene and were previously considered to be allopatric. In this study, we discovered that the subspecies were parapatric. We use a larger sample size than previous studies to examine variation in morphology and mitochondrial haplotype across the distribution of each subspecies and within the region of parapatry. The subspecies occurring to the west, Amytornis modestus indulkanna, had larger body size and longer and narrower bill than the subspecies occurring to the east, A. m. raglessi. Within the region of parapatry, females were morphologically similar to A. m. indulkanna but had eastern mitochondrial haplotypes while males had intermediate morpholo...