Understanding Natural Selection On Floral Traits: Variation, Agents And Consequences
For animal pollinated plants, flowers are highly interactive organs expected to be under strong and potentially diverse selection pressures. Plants not only need to attract their pollinators but also deter their enemies, as well as keep the costs of these structures low. Thus natural selection on floral characters is an important area of inquiry. The following series of studies examines multiple aspects of natural selection on plants with a focus on floral characters. The first paper asks whether natural selection is more variable in space or time for both a focal species (Penstemon digitalis) and for in plants in general. Indeed, selection is variable, but not more so in either of these dimensions, suggesting that constraints to local adaptation and speciation are equally as likely as their occurrence. Knowing the agents of selection on traits can inform the direction and expectations for evolutionary change. The second paper discusses the finding that pollinators were driving selection for larger flowers and displays in P. digitalis. Furthermore, selection on floral traits was generally stronger when pollinators are present than when they were excluded for multiple species for which there is data. These results suggest that pollinators may indeed be major agents of natural selection on flowers. However, with so few studies directly testing agents of selection, this assumption should be applied with caution in contemporary populations. The third paper looks at an understudied trait in an evolutionary context. Although scents have been characterized for many plant species, very few studies examine the variation in this trait or how natural selection acts on it. Scents vary among populations of P. digitalis on a small geographic scale and there was significant natural selection to produce more scent in a common garden. Finally, the last paper examines the expected outcomes of selection for later flowering in Lobelia siphilitica by pre-dispersal seed predators. As predicted by optimal defence theory, flowering time and latex are correlated in this system suggesting that selection on flowering time could also effect selection on defence.
floral evolution; natural selection; plant-insect interactions
Geber, Monica Ann; Agrawal, Anurag
Ph.D. of Ecology
Doctor of Philosophy
dissertation or thesis