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Introduction to the Symposium: On the Evolution of Specialization

On July 10, 1995, at the 1995 meeting of the American Society of Naturalists, held on the campus of McGill University in Montreal, Quebec, speakers at the Vice Presidential Symposium addressed issues on the evolution of specialization. The proceedings of that symposium appear here in this special supplement issue of the American Naturalist. The topic is ideally one of broad interest to the readership of the journal if for no reason other than that specialization for particular diets or habitats is a remarkably widespread biological phenomenon. In a sense, considering the great species richness of certain specialist taxa, specialization of one sort or another is the predominant lifestyle choice on the planet. Thus, the phenomenon attracts attention simply by virtue of its remarkable ubiquity. It has also undoubtedly attracted attention because it is so foreign to human ecology. Homo sapiens can be considered the ultimate diet/habitat generalists (barring, of course, finicky 5-yr-olds). As Brues (1952, p. 38) aptly writes of the extreme oligophagy of many herbivorous insects, "It is as if a human would eat corn pone only, or cabbage, or onions, or cottage cheese, and never venture a baked potato, hot dog, or ice cream cone to vary the monotony." It was perhaps inevitable that, for many decades, discussions of the evolution of specialization were heavily colored by preconceptions based on human prejudices. Among the most basic problems in understanding how specialization evolves is simply defining the term. As Futuyma and Moreno (1988, p. 208) succinctly state, "Specialization must lie in the eye of the beholder." The meaning of the term is heavily context dependent. In terms of dietary specialization in insects, for example, a specialist has been defined as a species feeding on three or fewer plant families (Bernays and Graham 1988). Yet, according to this definition, a "specialist" associated with a locally species-rich plant family may utilize many more host species than does a sympatric generalist. Moreover, definitions of specialization well suited to describe certain types of ecological associations are woefully inappropriate for other types of ecological associations. For acarine parasites of vertebrates, for example, dietary specialization generally means restriction not to three or fewer families of hosts, as it does for herbivorous insects, but restriction to a single species (or handful of species), or even to a particular part of a single species (e.g., the actinedid follicle mites that infest eyebrow hairs