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Hawkmoths prefer columbines with long, slender spurs. (Courtesy of Scott A. Hodges/UCSB)

Adaptive radiation is a principle in evolutionary biology in which one species, in response to opportunities in its environment, quickly adapts and develops new traits and diversifies into many species. An example of adaptive radiation is found in columbine flowers (genus Aquilegia), a group of about 70 species that have nectar spurs extending from the base of the flower petals. What makes these spurs special is that each species has spurs of a different length, seemingly tailored to that species’ pollinator, whether it be a hummingbird, hawkmoth or bee.

Scientists since Charles Darwin have observed similar examples of adaptive radiation but have been unable to describe what happens on a cellular or genetic scale. “Darwin, observing orchids, recognized that the extraordinarily long nectar spur on the Angraecum must have evolved in concert with the equally long tongue of the moth that pollinated it, but the exact mechanism for this kind of adaptation has been a matter of speculation,” says Sharon Gerbode of Harvard University.

Gerbode and her colleagues at Harvard and the University of California at Santa Barbara investigated that mechanism in columbines and report their findings in the Proceedings of the Royal Society B. For decades, scientists had thought that the differences in nectar spur length were due to the number of cells in the nectar spur. But when the researchers counted the number of cells and calculated the area and degree of elongation of each cell–which required more than 13,000 measurements across several species–they found that the assumptions were wrong. Nearly all of the difference in spur length can be attributed to the length of the cells.

In each species, cell division in the nectar spur stops when the spur is about 5 millimeters long. Then the spurs begin to elongate, and how many days they spend growing determines the eventual length of the spur.

“Now that we understand the real developmental basis for the first appearance and diversification of spurs, we can make more informed guesses about what genes contributed to the process,” says study co-author Elana Kramer. Further research should give the scientists insight into the genetic basis behind the radiation of this genus.

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