Hybrid Zones in Australian Birds

Abstract

Geographic patterns in positions of 79-87 avian hybrid zones and 12 parapatric contacts in Australia are analysed in the context of past climates, palaeo-refugia, mechanisms of origin, and temporal stability. No hybrid zone occurs in rain-forest species and differentiated vicariant forms of these are generally separated by unsuitable tracts of sclerophyll forest or savanna woodland. Most hybrid zones occur in semi-arid and sub-humid species. The zones appear to have originated by secondary contact of differentiated vicars rather than by parapatric divergence, because most lie between refugia postulated on the basis of late Pleistocene climates; their widths are generally in accord with those predicted from dispersion rates. The evidence suggests that most, if not all, hybrid zones were formed when expansion of isolates from refugia became possible following the marked improvement in climate at the close of the severe arid phase of about 17 000 y before present. The persistence of hybnd zones since then and their widths offer no support for the concept of reinforcement of previously acquired differences in recognition (signalling) systems. Where a hybrid zone coincides closely with the boundary of a presumed refuge, a semi-arid form has differentially expanded towards its subhumid counterpart.