Pelargonium (Geraniaceae) and the long-tongued fly pollination guild: coevolution and polymorphism in a specialized pollination system

In South Africa, a group of long-tongued flies and the flowering plants they pollinate form a unique and diverse pollination guild system almost entirely endemic to the Greater Cape Floristic Region. This system makes up only a small portion of the biodiversity of the Region, but it is likely a key to understanding the evolutionary processes that have lead to the observed biodiversity. Aside from its unusual pollinators, this guild system is of particular interest due to the extremely labile morphology of both the fly species and the associated flowering plant species, which geographically vary significantly in proboscis lengths and floral traits, depending on the species composition of the local community. This leads to remarkable polymorphic diversity within an otherwise obligately mutualistic system. In this paper, information about long-tongued flies and their pollination system is collected, reviewed and synthesized. The floral guild system, especially the focal genus Pelargonium, is also reviewed, given the high percentage of Pelargonium species that are obligately or facultatively pollinated by long-tongued flies. A revision to the pollination guild system is suggested by introducing the sub-guild concept. Additionally, the patterns of adaptive shifts and morphological changes within the guild system are analyzed as they relate to reciprocal adaptation, coevolution, and the process of ecological speciation within this highly-endemic, biodiverse region of South Africa. This synthesis indicates that this specialized pollination mutualism may maximize pollination efficiency in a continually shifting biotic and abiotic environment. More intense specialization and perhaps eventual speciation likely occurs only where both the pollinators and the associated floral guild species exert continuous selection pressure on each other over time. Additionally, new venues of study are suggested in order to better understand the genetic and environmental bases for adaptive shifts within the pollination guild system system