TIME OF ORIGIN AND BIOGEOGRAPHICAL HISTORY OF SPECIALIZED RELATIONSHIPS BETWEEN NORTHERN MARINE PLANTS AND HERBIVOROUS MOLLUSCS.

A survey of cool-temperate North Pacific and North Atlantic marine plants and herbivorous molluscs and their descendants was undertaken to determine the time of origin and the biogeographical history of specialized host-guest relationships between plants and herbivores. Of 12 such associations, 6 (50%) were established no earlier than the Pliocene. These six associations resulted from the invasion of either the plant (three cases) or the herbivore (three cases) from geographical regions other than the one in which the association was forged. These results are contrary to the widely held view that specialized ("coevolved") relationships tend to be ancient, and highlight the importance of biotic interchange as a biogeographical process influencing the opportunity for trophic specialization. © 1992 The Society for the Study of Evolution.