Two models for
the evolution of ice-breeding behaviour in phocid seals
According to the traditional classification based on
morphology (left), seals in the family Phocidae ("True"
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
occurred only once. The molecular-based model is more
parsimonious. Harp Seals,
often considered more closely related (as Phoca
groenlandica) to Harbor
(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)
