Host shifts and host-associated differentiation in herbivorous insects: ecological causes and consequences

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One of the most obvious features of life on Earth is its astonishing diversity - at least a million species are known to science, and perhaps as many as 30 million more remain to be discovered. Some lineages have diversified much more abundantly than others, though, and accounting for this differential success is one of the most fundamental questions in evolutionary ecology. Among the most spectacular examples of "hyperdiversity" are plant-feeding insects and the parasitoids that attack them - as many as half of all animal species may belong to one or the other of these groups. One reason for the evolutionary success of plant-feeding insects and parasitoids may be their tendency to evolve specialized diets. Diets evolve via host-shifting (adding new host plants to diet), and then new diversity evolves by host-associated differentiation (HAD), in which a single diet generalist diverges to form a new pair of host-specialist species.       While host-shifting and HAD are common, they are not universal, and different lineages can undergo these processes at different times and rates and by different mechanisms. My research program seeks to understand the ecological factors that control the occurrence of host-shifting and HAD, by studying the diverse set of insects feeding on two common Canadian goldenrods. By contrasting the ecology and population genetics of species that are evolving pairs of host specialists, vs. those that continue to feed as generalists, we can test hypotheses about forces that favour or retard diversification. Our current focus is on the way insect preferences for trait variation among host plant individuals can drive their evolutionary response to variation between host plant species. A predictive understanding of host-shifting and HAD will help us understand the origin of Earth's biodiversity, as well as to anticipate and manage attack by insects on novel hosts in agricultural and agroforestry systems. We will also build on our HAD work by testing the hypothesis that the impact of herbivores on their host plants lessens over evolutionary time, as the plants evolve greater tolerance and the insects reduced virulence. If this is true, novel insect-herbivore associations generated by host-shifting and HAD should show higher impact than ancient ones. This is important, because such novel insect-herbivore associations also result from the spread of invasive species and have enormous and increasing ecological and economic costs worldwide.