A MOLECULAR VIEW OF PINNIPED RELATIONSHIPS WITH PARTICULAR EMPHASIS ON THE TRUE SEALS

Phylogenetic analysis of conservative nucleotide substitutions in 18 c omplete sequences of the mitochondrial cytochrome b gene of Phocidae ( true seals), Odobenidae (walruses), and Otariidae (sea lions and fur s eals), plus three ursid and three felid sequences, identified the pinn ipeds as monophyletic with Otariidae and Odobenidae on a common evolut ionary branch. Analysis of total nucleotide differences separated the evolutionary lineages of northern and southern phocids. Both lineages are distinct from the most ancestral phocid genus, Monachus (monk seal s), represented by the Hawaiian monk seal. The inclusion of the Hawaii an monk seal in the subfamily Monachinae makes the subfamily paraphyle tic. Among the northern phocids, the hooded seal (genus Cystophora, ch romosome number 2n = 34) is sister taxon to the Phoca complex. The Pho ca complex, which is characterized by the chromosome number 2n = 32, i ncludes genus Phoca and the monotypic genus Halichoerus (grey seal). T he comparison does not support a generic distinction of Halichoerus wi thin the Phoca complex. The present data suggest that Cystophora and P hoca separated greater than or equal to 6 million years ago. Among the southern phocids the close molecular relationship of the Weddell and leopard seals relative to their morphological distinction exemplifies rapid adaptation to different ecological niches. This result stands in contrast to the limited morphological differentiation relative to the pronounced molecular distinctions that may occur within the Phoca com plex.