Evolution of pagophilic breeding in phocid seals

          Traditional taxonomy of "true" or earless seals (Phocidae) has placed most species in the Linnean genus Phoca. Phylogenetic analysis of mtDNA sequences indicates that the six species of phocid or "true" seals found in the North Atlantic can be classified as three tribes of the subfamily Phocinae (Carr & Perry 1997). The bearded seal (Erignathus) is the sister group to all other phocine seals. Sometimes regarded as a close relative of harbour seals (Phoca vitulina), harp seals are a distinct genus, Pagophilus, whose closest relatives in the North Atlantic are hooded seals (Cystophora), which are phocine seals not closely related to elephant seals (Monachinae). Grey seals (Halicherous) and ringed seals (Pusa) are probably distinct genera from Phoca. The phylogeny suggests that the ice-breeding (pagophilic) habit and associated brief lactation in Pagophilus, Cystophora, and other species are ancestral characters for the Phocinae, and that fast-ice or terrestrial breeding is evolutionarily derived  and a convergence on similar conditions in other phocid subfamilies (Perry et al. 1995).

        Comparison of phocid DNA sequences with those from other carnivoran families suggest that the marine families ["true seals" (Phocidae), "eared seals" (Otariidae), and the walrus (Odobenidae)] form a monophyletic lineage with respect to terrestrial caniform carnivorans.