Law of the unspecialized: broken?

Abstract

Specialization has posed crucial problems for evolutionary biologists: What are the benefits of specialization? Is specialization an evolutionary dead end? Theory has long predicted that specialization is favored because of tradeoffs in fitness when organisms utilize different resources. These microevolutionary benefits are thought to be offset by a reduction in macroevolutionary diversification. Again, theory has suggested that the narrow ecology of specialists will result in fewer opportunities for speciation, and, therefore, lower levels of diversification than in less specialized clades. This suggestion dates back to E. Cope in 1896, who coined the idea ‘law of the unspecialized’. However, specialization as an evolutionary dead end appears inconsistent with the existence of diverse and highly specialized clades. For example, it is estimated that 80% of phytophagous insects, one of the most diverse groups of organisms, are specialized. Why is this group of specialists so diverse? There are now several phylogenies of groups with more and less specialized clades, and a new study by Termonia et al. 1xFeeding specialization and host-derived chemical defense in Chrysomeline leaf beetles did not lead to an evolutionary dead end. Termonia, A. et al. Proc. Natl. Acad. Sci. U. S. A. 2001; 98: 3909–3914Crossref | PubMed | Scopus (93)See all References1 ‘ups the ante’ by combining knowledge of the mechanistic basis of specialization in Chrysomelina beetles with a phylogenetic analysis of diversification.In response to harassment, Chrysomelina beetles produce defensive secretions that are repulsive to predators. The type of defensive secretion produced defines three different types of beetles: those that synthesize their own defense chemicals; those that are fully reliant on their host plant for such chemicals; and an intermediate group that are only partially dependent on the host plant for their chemical defense. Termonia et al.'s phylogenetic analysis reveals that this partially dependent strategy is the most derived of the three, and is descended from the more specialized group, which relies fully on the host plant for defense. In addition to specialization not being a dead end in this system, the authors show that the derived defense strategy has allowed for beetles to move to host plants that are both chemically and phylogenetically distinct.This paper 1xFeeding specialization and host-derived chemical defense in Chrysomeline leaf beetles did not lead to an evolutionary dead end. Termonia, A. et al. Proc. Natl. Acad. Sci. U. S. A. 2001; 98: 3909–3914Crossref | PubMed | Scopus (93)See all References1 is part of a new generation of studies that link chemical ecology to evolutionary biology. Conventional wisdom has been that specialization of herbivores is an evolutionary response to plant defenses. However, for the Chrysomelina beetles, specialization is apparently a response to avoid predation. Although it is not clear how frequently specialization in herbivorous insects is driven by predators, host-plant chemistry plays a prominent role in both plant defense and predator avoidance. This still begs the question: why are specialized clades of phytophagous insects so diverse? Does the type of resource used by such insects (i.e. hosts that are defended by chemicals or hosts that provide insect defense) and the spatial scale over which these resources occur delimit opportunities for reproductive isolation? If so, the law of the unspecialized will fail whenever novel specializations promote reproductive isolation. Well-described cases of incipient sympatric speciation (e.g. sticklebacks Gasterosteus spp. and Apple maggot fly Rhagoletis spp.) and the maintenance of species boundaries in sympatry (columbine Aquilegia spp. and monkey flower Mimulus spp.) occurring as a result of the evolution of novel specializations for resource use support this view.