What historical event do researchers propose might explain the fragmentation of the isolated Utah *I. pacificus* population?

Range fragmentation caused by Pleistocene glaciation or a more recent, passive introduction via host migration.

The distinct and isolated nature of the *Ixodes pacificus* population in Utah has prompted speculation regarding its origin. Researchers hypothesize two main scenarios that could account for its current fragmented state and lower genetic diversity. The first possibility is an ancient event, specifically range fragmentation directly resulting from the impacts of Pleistocene glaciation, which would have divided populations inhabiting the region. Alternatively, a more recent, passive introduction event, perhaps facilitated by the migration patterns of its host animals, could also explain how this population became separated from the main distribution located along the Pacific Coast.