Rapid evolution of pollinator-mediated plant reproductive isolation

When considering the tree of life, the fact that some lineages are much more taxonomically rich than others suggests that rates of species diversification are highly variable. Explaining patterns of species diversity according to changes in diversification rate is limited by our power to reconstruct patterns of speciation and extinction through time, but this has not deterred speculation on the rate of evolution in mega-diverse groups such as flowering plants and insects (Sanderson and Donoghue 1994; Farrell 1998). The role of specialized interactions between insect herbivores and their host plants has been especially popular in explaining insect diversity by coevolutionary processes (Ehrlich and Raven 1964; Farrell et al. 1992). That reproductive isolation of herbivore populations may arise due to specialization on novel plant hosts is illustrated by the apple maggot fly, Rhagoletis pomonella, which broadened its host range over the past 300 years from native hawthorn (Crataegus) to include the introduced apple (Malus) in North America (Feder et al. 1994). Yet examples of herbivore speciation as a consequence of adaptation to different host plants, such as soapberry bugs (Carroll and Boyd 1992) and pea aphids (Peccoud et al. 2010), rarely consider the rate of host plant diversification. The role of herbivores in affecting plant diversification may be intensified when herbivores also provide pollination services, and directly affect reproduction of the host plant. In this chapter we focus on conditions in which insect pollinators acting as agents of reproductive isolation could influence the rate of speciation in flowering plants.