A phylogeographic analysis of southern and eastern populations of the Australian magpie: evidence for selection in maintenance of the distribution of two plumage morphs

Abstract

The Australian magpie (Gymnorhina tibicen) is polymorphic for back colour with three distinct morphs recognized: the black-backed form (BB) which occurs in northern and north-eastern Australia; the white-backed form (WB) which occurs in south-eastern Australia and Tasmania and the Western form which occurs in the far south-west corner of the continent. Male and female WBs and BBs are both monomorphic for back colour while Westerns are sexually dimorphic, with males white-backed and females black-backed. In the south-east the WB and BB distributions overlap with individuals of intermediate phenotype interspersed with pure WB and BB phenotypes. This study used mtDNA control-region sequences to test the predictions of two alternative hypotheses to explain the distribution of WB and BB populations in eastern Australia and Tasmania. Our data support the hypothesis that the variation has evolved in situ, as no population genetic structuring was evident in eastern Australia related to back colour and Tasmanian WBs were no more closely related to mainland WBs than to mainland BBs (primary contact hypothesis). Back colour patterns may be maintained by different forms of natural selection favouring BB genes in the north-east and WB genes in the south-east.