“The most effective pollinator principle implies that floral characteristics often reflect adaptation to the pollinator that transfers the most pollen, through a combination of high rate of visitation to flowers and effective deposition of pollen during each visit.” – Mayfield, et al., Annals of Botany (2001) 88 (4): 591-596
In part one, I reviewed a chapter by Jose M. Gomez and Regino Zamora in the book Plant-Pollinator Interactions: From Specialization to Generalization that argues that the most effective pollinator principle (MEPP) “represents just one of multiple evolutionary solutions.” In part two, I summarize a chapter by Paul A. Aigner in the same book that further explains how floral characteristics can evolve without strictly adhering to the MEPP.
Aigner is interested in how specialization develops in different environments and whether or not flowering plants, having adapted to interact with a limited number of pollinators, experience trade-offs. A trade-off occurs when a species or population adapts to a specific environmental state and, in the process, loses adaptation to another state. Or in other words, a beneficial change in one trait results in the deterioration of another. Trade-offs and specialization are often seen as going hand in hand, but Aigner argues that trade-offs are not always necessary for an organism to evolve towards specialization. Plant-pollinator interactions provide an excellent opportunity to test this.
“Flowers demand study of specialization and diversification,” Aigner writes, not only due to their ubiquity, “but because much of the remarkable diversity seen in these organisms is thought to have evolved in response to a single and conspicuous element of the environment – pollination by animals.” If pollinators have such a strong influence on shaping the appearance of flowers, pollination studies should be rife with evidence for trade-offs, but they are not. Apart from not being well-studied, Aigner has other ideas about why trade-offs are not often observed in this scenario.
Aigner is particularly interested in specialization occuring in fine-grained environments. A course-grained environment is “one in which an organism experiences a single environmental state for all of its life.” Specialization is well understood in this type of environment. A fine-grained environment is “one in which an organism experiences all environmental states within its lifetime,” such as “a flowering plant [being] visited by a succession of animal pollinators.” For specialization to develop in a fine-grained environment, a flowering plant must “evolve adaptations to a particular type of pollinator while other types of pollinators are also present.”
It’s important to note that the specialization that Aigner mainly refers to is phenotypic specialization. That is, a flower’s phenotype [observable features derived from genes + environment] appears to be adapted for pollination by a specific type of pollinator, but in fact may be pollinated by various types of pollinators. In other words, it is phenotypically specialized but ecologically generalized. Aigner uses a theoretical model to show that specialization can develop in a fine-grained environment with and without trade-offs. He also uses his model to demonstrates that a flower’s phenotype does not necessarily result from its most effective pollinator acting as the most important selection agent. Instead, specialization can evolve in response to a less-effective pollinator “when performance gains from adapting to the less-effective pollinator can be had with little loss in the performance contribution of the more effective pollinator.”
Essentially, Aigner’s argument is that the agents that are the most influential in shaping a particular organism are not necessarily the same agents that offer the greatest contribution to that organism’s overall fitness. This statement flies in the face of the MEPP, and Aigner backs up his argument with (among other examples) his studies involving the genus Dudleya.
Dudleya is ecologically generalized. Pollinators include hummingbirds, bumblebees, solitary bees, bee flies, hover flies, and butterflies. “Some Dudleya species and populations are visited by all of these taxa, whereas others seem to be visited by only a subset.” Aigner was curious to see if certain species or populations were experiencing trade-offs by adapting to a particular category of pollinators. Aigner found variations in flower characteristics among species and populations as well as differences in pollinator assemblages that visited the various groups of flowers over time but could not conclude that there were trade-offs “in pollination performance.”
In one study, he looked at pollination services provided by hummingbirds vs. bumblebees as corolla flare changed in size. In male flowers, bumblebees were efficient at removing pollen regardless of corolla flare size, while hummingbirds removed pollen more effectively as corolla flare decreased. Both groups deposited pollen more effectively as corolla flare decreased, but hummingbirds more strongly so. Ultimately, Aigner concluded that “the interactions did not take the form of trade-offs,” or, as stated in the abstract of the study, ” phenotypic specialization [for pollination by hummingbirds] might evolve without trading-off the effectiveness of bumblebees.”
Aigner goes on to explain why floral adaptations may occur without obvious trade-offs. One reason is that different groups of pollinators are acting as selective agents for different floral traits, “so that few functional trade-offs exist with respect to individual traits.” Pollinators have different reasons for visiting flowers and flowers use the pollination services of visitors differently. Another reason involves the “genetic architecture” of the traits being selected for. Results can differ depending on whether or not the genes being influenced are linked to other genes, and genetically based fitness trade-offs may not be observable phenotypically. Further studies involving the genetic architecure of specialized phenotypes are necessary.
And finally, as indicated in part one, pollinators are not the only floral visitors. In the words of Aigner, “if floral larcenists and herbivores select for floral traits in different directions than do pollinators, plants may face direct trade-offs in improving pollination service versus defending against enemies.” These “floral enemies” can have an effect on the visitation rates and per-visit effectiveness of pollinators, which can drastically alter their influence as selective agents.
Like pollination syndromes, the most effective pollinator principle seems to have encouraged and directed a huge amount of research in the field of pollination biology, despite not holding entirely true in the real world. As research continues, a more complete picture will develop. It doesn’t appear that it will conform to an easily digestible principle, but there is no question that, even in its complexity, it will be fascinating.
I will end as I began, with an excerpt from Thor Hanson’s book, The Triumph of Seeds: “The notion of coevolution implies that change in one organism can lead to change in another – if antelope run faster, then cheetahs must run faster still to catch them. Traditional definitions describe the process as a tango between familiar partners, where each step is met by an equal and elegant counter-step. In reality, the dance floor of evolution is usually a lot more crowded. Relationships like those between rodents and seeds [or pollinators and flowers] develop in the midst of something more like a square dance, with couples constantly switching partners in a whir of spins, promenades, and do-si-dos. The end result may appear like quid pro quo, but chances are a lot of other dancers influenced the outcome – leading, following, and stepping on toes along the way.”