Floral Evolution Beyond Phenology: Adaptive Dynamics in Plant-Pollinator Interactions Under Global Change

Abstract

Global change generates rapid shifts in multiple environmental variables simultaneously, forming a multifactorial suite of pressures to which organisms must respond. To date, research regarding adaptive responses in plants has focused largely on flowering phenology; little is known about other floral traits that underlie adaptive responses or how they interact with phenology to direct the evolutionary rate and trajectory of plant populations. Thus, my research addresses the overarching question: Is there adaptive potential in floral traits beyond phenology to respond to selection from global change, and what are the implications for plant-pollinator interactions?

Using an annual, mixed mating plant, Ipomoea purpurea, I conducted resurrection experiments with seeds collected from populations in 2003 and 2012 to explore adaptation in floral traits to global change. Chapter 1 introduces the broad goals of the dissertation. In the first data chapter, Chapter 2, I investigate phenotypic evolution in a suite of pollination or mating system related traits and identify phenotypes underlying spatial and temporal adaptive responses. The findings from this study reveal that, beyond the well-documented shift to earlier flowering, corolla width, nectar sucrose content, and pollen count also play a role in adaptation to global change. I also find that changes in flowering phenology and corolla width are driven primarily by populations at more northern latitudes, and show that these changes are most likely adaptive.

In Chapters 3 and 4, I assess the potential for constraints on continued trait evolution and whether phenotypic evolution can be attributed to pollinators as an agent of selection in line with a hypothesis of an adaptive path that favors increased investment in pollinator attraction. The relationship between plant traits and fitness in ancestral (2003) and descendant (2012) populations revealed that descendant populations are more limited in their response to selection than ancestral populations, and correlative selection is present in descendant populations between flowering phenology and three other floral traits: corolla width, corolla length, and nectar sucrose content. I also show that the overall rate of evolution in these traits is constrained due to trait-trait covariances, and that the rate of evolution slows between 2003 and 2012, likely due to lowered variation. In Chapter 4, I find that pollinators select for large corolla width, high nectar sucrose content, and large ASD, but additional direct effects of corolla width and corolla length on fitness in descendant populations indicate that the constraint on the evolvability of flowering phenology I report in Chapter 3 is only partially explained by a mechanism of increased investment in pollinator attraction.

In brief, my dissertation provides novel evidence of floral traits beyond flowering phenology responding in real-time as a result of global change and highlights an adaptive pathway of increased investment in pollinator attraction. Furthermore, it represents the first study to use a resurrection framework to 1) assess differences in response to selection over time and show that adaptive potential in a plant-pollinator mutualism is declining, and 2) trace changes in adaptive potential in time to the selective mechanism and show that constraints on evolvability of flowering time are partially attributable to increased investment in pollinator attraction. While responses to global change are expected to be species and region specific, collectively, this work reveals that consideration of multivariate trait evolution provides important information regarding expected rate and direction of adaptive response to global change.