Evidence for Bergmann's Rule and Not Allopatric Subspeciation in the Threatened Kaka (Nestor meridionalis)

Species of conservation concern characterized by small and declining populations greatly benefit from proactive management approaches such as population translocations. Because they often show intra-specific genetic and phenotypic variation, which can result from drift or differential selective pressures between habitats, understanding the distribution of such variation and its underlying processes is a prerequisite to develop effective management guidelines. Indeed, translocations among genetically differentiated populations potentially locally adapted are discouraged in order to avoid outbreeding depression, while translocations among populations characterized by high gene flow with no evidence for local adaptation are encouraged. Here, we first test whether 2 recognized subspecies, the North Island kaka (Nestor meridionalis septentrionalis) and South Island kaka (Nestor meridionalis meridionalis) of New Zealand fit a scenario of allopatric subspeciation following the separation of the North and South Islands at the end of the Pleistocene using 1 mtDNA (n = 96) and 9 microsatellite markers (n = 126). We then test whether morphological differences among the 2 subspecies support a pattern of local adaptation, comparing phenotypic divergence (P ST) and the level of divergence by drift alone (F ST) among populations. We find little population structure between islands, ruling out allopatric subspeciation in kaka. Further, P ST exceeds F ST, supporting an adaptive latitudinal size cline consistent with Bergmann's rule. These results therefore suggest that using neutral genetic diversity alone can be misleading when identifying management units and that the nature of phenotypic variation should be considered in translocations efforts. We finally discuss North and South Island management units but suggest that cross-island translocation be allowed.