The pollination niche and its role in the diversification and maintenance of the southern African flora

The flora of southern Africa has exceptional species richness and endemism, making it an ideal system for studying the patterns and processes of evolutionary diversification. Using a wealth of recent case studies, I examine the evidence for pollinator-driven diversification in this flora. Pollination systems, which represent available niches for ecological diversification, are characterized in southern Africa by a high level of ecological and evolutionary specialization on the part of plants, and, in some cases, by pollinators as well. These systems are asymmetric, with entire plant guilds commonly specialized for a particular pollinator species or functional type, resulting in obvious convergent floral evolution among guild members. Identified modes of plant lineage diversification involving adaptation to pollinators in these guilds include (i) shifts between pollination systems, (ii) divergent use of the same pollinator, (iii) coevolution, (iv) trait tracking, and (v) floral mimicry of different model species. Microevolutionary studies confirm that pollinator shifts can be precipitated when a plant species encounters a novel pollinator fauna on its range margin, and macroevolutionary studies confirm frequent pollinator shifts associated with lineage diversification. As Darwin first noted, evolutionary specialization for particular pollinators, when resulting in ecological dependency, may increase the risk of plant extinction. I thus also consider the evidence that disturbance provokes pollination failure in some southern African plants with specialized pollination systems.

The pollination niche and its role in the diversification and maintenance of the southern African flora

The flora of southern Africa has exceptional species richness and endemism, making it an ideal system for studying the patterns and processes of evolutionary diversification. Using a wealth of recent case studies, I examine the evidence for pollinator-driven diversification in this flora. Pollination systems, which represent available niches for ecological diversification, are characterized in southern Africa by a high level of ecological and evolutionary specialization on the part of plants, and, in some cases, by pollinators as well. These systems are asymmetric, with entire plant guilds commonly specialized for a particular pollinator species or functional type, resulting in obvious convergent floral evolution among guild members. Identified modes of plant lineage diversification involving adaptation to pollinators in these guilds include (i) shifts between pollination systems, (ii) divergent use of the same pollinator, (iii) coevolution, (iv) trait tracking, and (v) floral mimicry of different model species. Microevolutionary studies confirm that pollinator shifts can be precipitated when a plant species encounters a novel pollinator fauna on its range margin, and macroevolutionary studies confirm frequent pollinator shifts associated with lineage diversification. As Darwin first noted, evolutionary specialization for particular pollinators, when resulting in ecological dependency, may increase the risk of plant extinction. I thus also consider the evidence that disturbance provokes pollination failure in some southern African plants with specialized pollination systems.

Local adaptation and maladaptation to pollinators in a generalist geographic mosaic

The Geographic Mosaic Theory of Coevolution predicts the occurrence of mosaics of interaction-mediated local adaptations and maladaptations. Empirical support to this prediction has come mostly from specialist interactions. In contrast, local adaptation is considered highly unlikely in generalist interactions. In this study, we experimentally test local adaptation in a generalist plant-pollinator geographic mosaic, by means of a transplant experiment in which plants coming from two evolutionary hotspots and two coldspots were offered to pollinators at the same four localities. Plants produced in the hotspots attracted more pollinators in all populations, whereas coldspot plants attracted fewer pollinators in all populations. Differences in adaptation were not related to genetic similarity between populations, suggesting that it was mainly due to spatial variation in previous selective regimes. Our experiment provides the first strong support for a spatially structured pattern of adaptation and maladaptation generated by a generalist free-living mutualism.