Research on other penguin species shows a correlation between speciation and what geological event, which also influenced Hoiho evolution?

The formation of isolated islands.

Studies concerning the evolution of various penguin species indicate a strong, direct correlation between the speciation of resident colonies and the physical formation of isolated landmasses, such as Gough Island or the Galápagos Islands. When birds colonize newly emergent land, geographic isolation acts as a powerful evolutionary barrier. Over extended geological timeframes, this isolation naturally leads to genetic drift and localized adaptation, which can eventually culminate in the establishment of new species. Although the specific drivers for the Hoiho splits involved more recent post-glacial environmental changes in the Southern Ocean, the fundamental principle that geographic isolation dictates unique evolutionary trajectories remains a central mechanism in their history.