TRADE-OFFS. “Specialization on clades of hosts or mutualists is widespread and that the use of certain hosts is indeed lost over evolutionary timescales. Notably, this pattern is not only consistent with trade-offs, but also consistent with ecological models of the loss of selection for interacting with hosts or mutualists that are outside of a species’ range.”
http://rspb.royalsocietypublishing.org/content/281/1795/20142004.short