Does strong selection promote host specialisation and ecological speciation in insect herbivores? Evidence from Neochlamisus leaf beetles

1. Outstanding issues in evolutionary ecology include: (1) the prevalence of host‐associated ecological specialisation among herbivorous insects, and (2) the speciation processes contributing to the tremendous species‐level diversity of herbivores. 2. Neochlamisus leaf beetle species are generally monophagous, but cumulatively employ a disparate array of host plants. These patterns occur in microcosm within the putatively oligophagous Neochlamisus bebbianae , which uses specific host species within each of six plant genera representing five families. I refer to all beetle populations associated with any one of these hosts as a particular ‘host form’. 3. This paper describes aspects of Neochlamisus biology and some experimental findings relevant to both host‐use evolution and host specialisation in Neochlamisus , and to ecological divergence, reproductive isolation, and genetic differentiation among N. bebbianae host forms. It thereby reviews a research programme focused on ecological speciation. 4. With respect to these host forms, our findings include: (1) ubiquitous host specialisation, (2) constraints on ‘multitasking’ that select for host specialisation, (3) ubiquitous ecological divergence, (4) pre‐ and post‐mating reproductive barriers, (5) genetic and environmental contributions to ecological divergence and reproductive isolation, (6) positive statistical associations between ecological divergence and reproductive isolation, (7) strongly differentiated loci despite gene flow, reflecting divergent host‐related selection, and (8) possible host race formation. We additionally describe a unique example of intimate host adaptation in another species, Neochlamisus platani . 5. The intimate and discrete nature of herbivore–host associations may generate unusually strong (host‐related) selection that could simultaneous explain pronounced rates of herbivore specialisation and speciation.