Problem: The evidence indicates that Drosophila P elements first entered the D. melanogaster genome in the Americas in the early 1900s and then spread worldwide through- out all D. melanogaster populations by the 1980s such that today there are no longer wild flies that lack P elements. How can a fundamentally parasitic DNA element like P spread so rapidly and pervasively when its effect is to cause deleterious mutations by inserting into genes?