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Evolutionary consequences of dioecy in angiosperms : the effects of breeding system on speciation and extinction rates Heilbuth, Jana C.

Abstract

Dioecy, the breeding system with male and female function on separate individuals, may affect the ability of a lineage to avoid extinction or speciate. Dioecy is a rare breeding system among the angiosperms (approximately 6% of all flowering plants) while hermaphroditism (having male and female function present within each flower) is predominant. Dioecious angiosperms may be rare because the transitions to dioecy have been recent or because dioecious angiosperms experience decreased diversification rates (speciation minus extinction) compared to plants with other breeding systems. Many traits that might affect diversification rates are different between dioecious and hermaphroditic plants, namely seed dispersal, pollination, geographical distribution, and growth habit (woody versus herbaceous). This thesis is composed of four separate projects that attempt to describe and explain the patchy distribution of dioecy among the angiosperms. In all the chapters, dioecy is examined from a fresh perspective that considers the evolutionary consequences of dioecy rather than from the traditional angle that focuses on the forces involved in the evolution towards dioecy. The first project compares the species richness of dioecious groups and their most closely related sister-groups. Dioecious groups are, on average, smaller than their sister-groups, indicating that dioecious angiosperms experience higher extinction rates or lower speciation rates. Following from this observation, the next two projects use computer simulations and mathematical theory to explore how (a) the limited seed dispersal of dioecious angiosperms (in which only females can disperse seeds) and (b) the differing pollination dynamics of dioecious angiosperms (where males may become more attractive to pollinators than females) may affect extinction rates, finding that both processes are potential causes of the low representation of dioecy among the angiosperms. Finally, using a comparative phylogenetic framework, I explore the reported ecological correlations of dioecy with small, white flowers, fleshy fruits, tropical distribution, and woody growth form. I confirm the presence of these correlations but, contrary to previous theories, find evidence that the cause of the correlations is due to the ability of these traits to alleviate some of the aforementioned disadvantages (in terms of dispersal and pollination) of dioecy.

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