Morphological and genetic variability of Snow Petrels Pagodroma nivea

Snow Petrels vary greatly in size, the largest birds being over twice as heavy as the smallest. Two morphological types have been distinguished, but the Adelieland population alone shows continuous variation spanning the complete range of the species. There is no evidence of size-related assortative mating. Size variation cannot be attributed to nest site location nor to individual age, nor is the large sexual dimorphism sufficient to account for the situation. The great individual variation in size is accompanied by an important genetic variability as revealed by a starch gel electrophoretic study.
Comparisons of wing length data from many localities around the Antarctic Continent suggest a cline, with a peak at the Balleny Islands. Taking into account the extent of size variation at breeding places only, it appears that morphological variability is low at Balleny Islands, where only large form birds are known, and at Antarctic localities inhabited exclusively by small form birds. In contrast, several localities on the continent in the vicinity of Balleny Islands, and especially Adelieland, exhibit a high degree of variability.
We propose that during the quaternary glaciations, the Antarctic continent provided no refugia for this species, but, when glaciation ended, Snow Petrels re-established colonies, coming from two refugia, a low latitude one inhabited by small birds Pagodroma nivea nivea , and one in higher latitudes characterized by large birds P. n. major. Reproductive isolation broke down, and a hybridization zone was created. The high proportion of polymorphic loci exhibited by the Adelieland population is then attributable to the mixing of two previously isolated genetic pools.