VANCOUVER, CANADA--Plants that team up with particular types of pollinators--hummingbirds, say, or bees--evolve to heighten their appeal. Sometimes, the theory goes, the plants get stuck with that look. But new research, presented this week at a major botanical meeting, bolsters the idea that specialist plants have more evolutionary flexibility than previously thought.
Many biologists believe that this specialization to particular pollinators like hummingbirds, bats, or hawkmoths can be an evolutionary dead end. For example, hummingbird-pollinated plants are usually red and have flower parts that arch away from the opening to allow access to a hovering bird. These physical structures are supposedly so specialized that the plant cannot return to the more generalized structures that work for insects, such as petals that serve as landing pads.
At the meeting, Erin Tripp, a graduate student at Duke University in Durham North Carolina, presented evidence of the complex evolutionary relationship between plants and flowers. She found that the flowers of the Ruellia genus frequently evolve from relying on hummingbird pollination to a more general strategy. The study was published in the July issue of Evolution, in addition to being presented at the joint meeting of the Botanical Society of America and three other associations.
Ruellia is a tropical genus that spans both the Old World and the New World. Most of the species are herbaceous, and all are animal-pollinated. Tripp and her adviser, Paul Manos of Duke University, studied pollination mechanisms in 115 species of Ruellia. Using DNA, they assembled a family tree and then applied statistical methods to reconstruct the physical characteristics and likely pollinator associations of ancestral species.
As expected, Tripp found that flower shape and color generally correlated with the types of pollinators. There was little evidence that Ruellia species pollinated by hawkmoths or bats have ever reverted to the more general strategy of catering to bees or other insects. But hummingbird pollination was a different story. Of the Ruellia species that Tripp studied, there appeared to be between eight and 11 transitions from hummingbirds in favor of insects, as evidenced by data from Tripp and others.
The transition was not a complete reversal, Tripp notes. Many of the descendents of hummingbird-pollinated ancestors share physical characteristics with modern flowers that are pollinated by hummingbirds. She suggests that the transitions may occur because some Ruellia species that colonized areas rich in hummingbird species later dispersed into habitats lacking hummingbirds. The idea could be tested by overlaying Ruellia habitats and hummingbird distribution maps.
The work suggests more complex evolutionary relationships between plants and pollinators, says Caroline Gross, a conservation biologist at the University of New England in Armidale, Australia: "I think it's a really good study."
[Editor's note: This replaces a previous version of this story.]