Complex sentence: The specialized Pleistocene wolves did not contribute to the genetic diversity of modern wolves, and the modern wolf populations across the Holarctic are likely be the descendants of wolves from populations that came from more southern refuges.
Split into simpler sentences: The specialized Pleistocene wolves, thus, did not contribute to the genetic diversity of modern wolves. Rather, modern wolf populations across the Holarctic are likely be the descendants of wolves from populations that came from more southern refuges.