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

ABSTRACT

Divergence 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 morphology and either eastern or western haplotypes. Additionally, haplotypes from the western mitochondrial clade were found in A. m. raglessi. These patterns of morphology and mitochondrial diversity reveal discordance within the region of parapatry and to the east. We suggest that the subspecies have undergone asymmetric expansion from west to east, made secondary contact, and are currently hybridising.

KEYWORDS:

• Maluridae
• grasswren
• parapatry
• morphology
• mitochondria
• arid zone

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Corrigendum

Acknowledgements

This research was supported by Nature Foundation SA. This research was conducted with approval from the Department of the Environment, Water and Natural Resources (DEWNR), the Australian Bird and Bat Banding Scheme, and the Flinders University Animal Welfare Committee. The mapping vectors used in Figure 1 were provided by DEWNR. We thank the pastoral landowners and Arrium mining for access to their properties. We thank the Friends of Scott Creek, Valeria Zanollo, Lynn Pedler, and Reece Pedler for advice on fieldwork and field sites. We thank Andrew Black for insightful discussion about Thick-billed Grasswrens. Finally, we are grateful to the dedicated volunteers who helped with fieldwork.

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Funding

This work was supported by the Nature Foundation SA; Nature Conservation Society of South Australia; Birds SA Conservation Fund; Royal Society of South Australia; Birdlife Australia.