If a seal has increased blubber (for insulation), is that a structural or a physiological adaptation?

Physiological - a structural adaptation would indicate a change in the skeletal structure of the seal such as a lengthening of the spine etc. Seals with increased blubber share the same structure (morphologically alike) as seals with a lesser amount of blubber, and this physiological adaptation (in colder waters, of course) bestows a survival advantage.