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

The Australian magpie (Gymnorhina tibicen) is polymorphic for back colour w ith three distinct morphs recognized: the black-backed form (BB) which occu rs in northern and north-eastern Australia; the white-backed form (WB) whic h 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 di morphic, with males white-backed and females black-backed. In the south-eas t the WB and BB distributions overlap with individuals of intermediate phen otype interspersed with pure WB and BB phenotypes. This study used mtDNA co ntrol-region sequences to test the predictions of two alternative hypothese s to explain the distribution of WB and BB populations in eastern Australia and Tasmania. Our data support the hypothesis that the variation has evolv ed in situ, as no population genetic structuring was evident in eastern Aus tralia related to back colour and Tasmanian WBs were no more closely relate d to mainland WBs than to mainland BBs (primary contact hypothesis). Back c olour patterns may be maintained by different forms of natural selection fa vouring BB genes in the north-east and WB genes in the south-east. (C) 2001 The Linnean Society of London.