Two models for the evolution
of ice-breeding behaviour in phocid seals
According to the traditional classification
based on morphology (left), seals were originally land-breeding
(red lines): pagophilic
(ice-breeding) behaviour evolved in
parallel three times, in response to separate adaptive pressures.
A molecular systematic classification (right)
shows instead that pagophilic breeding is ancestral: a shift to terrestrial
breeding has occured only once. The molecular-based model is more parsimonious.
Harp Seals, often considered more closely related (as Phoca groenlandica) to Harbour (Phoca vitulina), are in fact more closely related (as Pagophilus groenlandicus) to Hooded Seals (Cystophora). Grey Seals (Halichoerus), despite their distinctive morphology, are the sister species to Ringed Seals (Pusa) (after Perry et al. 1995, Carr & Perry 1997)
