umbine genus, Aquilegia. We first outline the broad evolutionary trends pointing to specific traits as being important in this adaptive radiation and briefly review the studies explicitly testing the function of these traits. We then highlight how genomic studies are aiding in our understanding of the genetic basis of adaptation and speciation, emphasizing flower color as a primary model. We end with a look toward how the genetic dissection of more complex traits will be possible in the near future.
In a classic prediction of an evolutionary trend, Darwin (1862) hypothesized that the spurs of flowers and the tongues of pollinators could undergo a coevolutionary “race” and become increasingly long. However, an alternative hypothesis posited that spurs evolve to fit the already-established tongue length of pollinators, and that spur length increases only when a shift to a new and longer-tongued pollinator occurs (Wallace, 1867; Wasserthal, 1997). This second hypothesis also predicts a directional evolutionary trend: shifts to shorter spurs may be less likely because shorter-tongued pollinators will avoid visiting flowers whose nectar reward they cannot reach. Recently these alternative hypotheses were explicitly tested with a species-level phylogeny of the North American adaptive radiation of Aquilegia (Whittall and Hodges, 2007). Transitions between major classes of pollinators were found to be significantly directional, consisting of bee to hummingbird and hummingbird to hawkmoth but no reversals and no bee to hawkmoth transitions. Concomitant with these shifts were increases in spur length. In addition, models of the tempo of evolution strongly indicated the concentration of spur-length evolution at times of speciation. Thus, as with the broad correlation between the evolution of spurs themselves and species diversity, this study suggests that spur length is intimately linked with the speciation process, at least in North American species of Aquilegia (Whittall and Hodges, 2007; Hodges and Whittall, 2008). Functionally, matching of spur length to tongue length has been shown to be adaptive because Aquilegia flowers with artificially shortened spurs have less pollen removed from their anthers, and presumably deposited on their stigmas, because of the body of the pollinator being held further away from the flower (Fulton and Hodges, 1999).
The pollinator shifts found in Aquilegia entail transitions in other traits besides spur length. The orientation of flowers at anthesis can be either pendent or upright (Hodges et al., 2002). All hummingbird-pollinated species of Aquilegia have pendent flowers, which is common in other hummingbird-pollinated species as well (Cronk and Ojeda, 2008). All 5 inferred shifts from hummingbird to hawkmoth pollination entail a transition from fully pendent flowers to upright flowers. This shift is adaptive,