The Australian Ringneck (Barnardius zonarius) shows clear geographical replacement of populations across its range. These populations have been given taxonomic epithets barnardi, zonarius, semitorquatus and macgillivrayi. We investigated whether historical or non-historical processes explain the origin of their phenotypic differentiation from each other. We used complete ND2 gene sequences from mitochondrial DNA (mtDNA) to test whether there is geographical coincidence of breaks in phenotypic and neutral molecular diversity. Simply, geographical coincidence is expected by historical processes but not by non-historical ones. Phylogenetic analysis identified one clade with most barnardi samples and another with zonarius, semitorquatus and macgillivrayi. The latter included some phenotypically typical barnardi but they were from localities where it approaches zonarius and macgillivrayi. Differentiation between the two clades, and thus of barnardi from all other populations, likely occurred first by a historical process such as vicariance. Later gene flow appears to have eroded the mtDNA monophyly of barnardi. Phenotypic and mtDNA diversity among semitorquatus, zonarius and macgillivrayi are not correlated. Non-historical processes are clearly suggested in the origin of their phenotypic differentiation. Their low nucleotide diversity, however, leaves ambiguity as to whether very recent historical processes could have been involved. Ramifications to issues of Barnardius systematics are discussed. Isolated north-western Queensland populations (macgillivrayi) are not closely related to barnardi. Alternative taxonomic treatments of our findings, recognising no more than three taxa (barnardi, zonarius and macgillivrayi) under different species concepts are cautiously discussed while urging more study.
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