The convergence and divergence of floral traits are driven by the heterogeneity of pollinator and plant communities
Thesis (PhD)--Stellenbosch University, 2017.
ENGLISH ABSTRACT: Substantial evidence suggests that pollinators are responsible for generating floral variation when they select upon floral traits involved in attraction as well the efficiency of pollen transfer. Another source of floral variation that is often neglected, is the role of community structure which drives floral divergence of traits involved in competition for the same pollinator resource. When these traits are selected upon across a heterogeneous landscape, it is thought to give rise to the formation of local floral forms which are often referred to as ecotypes. This thesis uses the long-proboscid fly (Prosoeca longipennis) study system as a platform for asking questions regarding the generation of floral variation. In my first data chapter (chapter two), I show how this variation can arise when tube length converges upon fly proboscis length across several localities, leading to a pattern of geographic trait matching. In addition, I also identify populations of species which appear to be morphologically divergent, because in populations where P. longipennis is absent, they are often pollinated by morphologically different pollinators. In chapter three, I investigate pollinator driven floral divergence in Nerine humilis, a species pollinated by long-proboscid flies in some populations and short proboscid insects in others. In this chapter, I demonstrate local adaptation of different floral forms associated with different pollinators. In addition, I take an extra step and demonstrate that the mechanical fit between flower and pollinator morphology is the mechanism behind local pollinator adaptation. In my fourth chapter, I show that floral adaptation may not always be as clearly illustrated as in chapter three, because most plants are visited by a multitude of functionally different pollinators. Here, I explore pollinator-mediated selection in Tritoniopsis revoluta and Nerine humilis with multiple functional pollinator types. Using single visitations, I demonstrate that flowers adapt to the optima and slopes of the additive fitness functions from all functional pollinator types. In my last data chapter, chapter five, I demonstrate how floral divergence may occur through the context of the floral guild community, where floral divergence does not occur through selection exerted by pollinators, but occurs as a result of competition for the placement of pollen on the bodies of long-proboscid fly’s across different localities. This resultant process of ecological character displacement gives rise to a pattern where mean style lengths of Pelargoniums are more different when they co-occur compared to when they occur on their own, where they may have style lengths that are similar or different to those in sympatry. This thesis contributes to the existing literature by providing much needed evidence on how selection exerted by pollinators as well as the structure of the floral guild community may drive adaptive divergence of floral morphology across a heterogeneous landscape.
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