A recent study has revealed that the flower Aeschynanthus acuminatus, a member of the lipstick vine group, evolved its distinctive shape before spreading to new areas. Unlike its relatives with bright red, tube-shaped flowers suited for sunbirds, A. acuminatus features shorter, wider, and yellowish-green flowers that attract birds with shorter beaks.
The research was led by Jing-Yi Lu, a research associate at the Field Museum and recent PhD graduate from the University of Chicago. “Compared to the rest of its genus, this species has weird, unique flowers,” said Lu. He became interested in how these differences arose after noticing that Taiwan, where some populations of A. acuminatus are found, lacks sunbirds—the typical pollinators for other lipstick vines.
To identify which birds were pollinating A. acuminatus in Taiwan, Lu used camera traps to observe bird visits to the flowers. He found that various birds with shorter beaks visited these blooms more frequently than sunbirds would have.
However, A. acuminatus is also present on mainland Southeast Asia—regions where sunbirds are common. This raised questions about whether the plant evolved its unique flower shape before or after arriving in Taiwan.
“At the heart of our study is a question of where species originate,” said Rick Ree, curator at the Field Museum’s Negaunee Integrative Research Center and senior author of the study. “There must have been a switch when this species evolved, when it went from having narrow flowers for sunbirds to wider flowers for more generalist birds. Where and when did the switch occur?”
Since 1970, botanists have often relied on the Grant-Stebbins model to explain such evolutionary changes in plants. According to Ree: “The Grant-Stebbins model says that when a plant species extends its range into an area with new pollinators or without its ancestral pollinator, that switch to a new pollinator will drive speciation.” This model would predict that A. acuminatus evolved in Taiwan after leaving behind its original pollinators.
To test this idea, researchers analyzed DNA from A. acuminatus plants across both Taiwan and mainland Asia as well as from related lipstick vine species. Their analysis indicated that Taiwanese populations descended from mainland ancestors already possessing their distinctive flower traits.
“The branching patterns on the family trees we made revealed that the A. acuminatus plants on Taiwan descended from other A. acuminatus plants from the mainland— the species originated on the mainland,” said Ree.
This finding contradicts predictions made by the Grant-Stebbins model in this case because some lipstick vines on the mainland diverged into a new form suitable for short-beaked birds even though sunbirds were still present there.
“It was really exciting to get these results, because they don’t follow the classic ideas of how we would have imagined the species evolved,” said Lu.
Ree noted that while this discovery answers one question about plant evolution, it raises others: “Why did this pollinator switch happen, when the original pollinator, the sunbird, is still there?” He suggested natural selection may have favored generalist passerine birds with shorter beaks under certain conditions.
Beyond insights into plant evolution mechanisms and exceptions to established models like Grant-Stebbins’, Ree emphasized fieldwork’s importance: “This study shows the importance of natural history, of actually going out into nature and observing ecological interactions,” he said.
The findings are detailed in New Phytologist—a peer-reviewed journal focused on original research across plant sciences and operated by a not-for-profit foundation dedicated to promoting plant science (https://www.newphytologist.org/).
