RAPID HOST RANGE EVOLUTION IN A POPULATION OF THE PHYTOPHAGOUS MITE TETRANYCHUS URTICAE KOCH

Abstract

The question of what determines the number and specific identity of the host plants fed on by an herbivore species has been discussed frequently in the last twenty years. At this point, the proximal determinants of host range for many herbivore species are rather well understood (e.g., Waldbauer, 1962; Hsiao and Fraenkel, 1968; Beck, 1974), but there is little detailed information on the evolutionary factors which mold these proximal physiological and behavioral determinants. Ehrlich and Raven's (1964) general survey of butterfly host ranges led them to conclude that present herbivore host ranges were the outcome of a stepwise coevolutionary process in which plants evolved to resist the attack of herbivores and herbivores evolved to overcome this resistance. They felt that this process occurred over a geological time span and resulted in a pattern of herbivore host ranges that reflected the taxonomic affinity of plant species. As interesting and useful as this idea has been, it would be unjustified to conclude that most herbivore host ranges are the outcome of such coevolution. Indeed it is possible that the present diversity of plant defenses was generated in response to relatively few enemies (i.e., herbivores, pathogens, and competing plants) and that most herbivores subsequently evolved to survive within an already established matrix of defenses (see Jermy, 1976, for an extreme viewpoint). Moreover, it is possible that herbivore diets expand to include unrelated species of plants, especially if these plants are found in geographical and ecological association (Janzen, 1968). As these plant associations