The evolutionary improbability of 'generalism' in nature, with special reference to insects

Most organisms represent specialized forms that arose as a result of natural selection and genetic drift to occupy distinct ecological niches. In animals, this process of specialization includes the behaviour of the organisms concerned, honed by locally-induced adaptations to specific host food plants (in herbivores) or prey items (in predators and parasitoids), and possibly reinforced by kairomones, including sex pheromones. The major thrust of evolution is towards ecological specialization as a result of the direct effects of intra- and interspecific competition. Adaptation to new resources lowers such competition and allows survival in new habitats/niches. Other benefits of food resource/habitat switching include 'enemy free space'. If specialism is the norm for the vast majority of species, what of so-called generalists and generalism, which are widely used terms, but perhaps wrongly so? Does generalism exist or is it a mirage that disappears the closer that it is inspected? We review some of the aspects of specialism and generalism and argue that even apparent generalists are filling distinct ecological niches. Often, generalists are rather specific in terms of food preferences, although they may nevertheless remain opportunistic with an overall broad niche/resource width. When apparent 'good' species are examined using molecular (DNA) markers, they are often found to comprise cryptic species. Many generalists may be of this kind. If so, generalism warrants additional investigation to establish its scope and credentials. (C) 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 103, 1-18.