Testing the adaptive nature of radiation: growth form and life history divergence in the African grass genus Ehrharta (Poaceae: Ehrhartoideae).

In most documented examples of adaptive radiation, the processes underlying divergence in form and function are poorly explored and remain speculative. Here, data from a comparative seedling growth experiment are used to explore growth form divergence in Ehrharta, a group of grasses that radiated in seasonally arid environments of the Cape region of South Africa. Seedlings of eight Ehrharta species of variable growth form were grown in liquid culture under conditions of high resource availabilty for 56 d, during which time changes in dry mass, allocation, and leaf parameters were measured. The results of this experiment reveal the existence of distinct seedling growth patterns that are associated with differences in adult plant form and seasonal drought survival strategy. Specifically, species that utilize a reseeding strategy have higher seedling growth rates and flower earlier than species that persist by vegetative means. A correlation between species' growth rates and their native substrates suggests that edaphic heterogeneity has been central in directing the evolution of alternative persistence strategies and growth forms. Parsimony reconstruction identifies slow growth and an association with nutrient-deficient sandstone-derived soils as ancestral in Ehrharta, with fast growth evolving after a transition to richer shale- and granite-derived soils. The emergence of annual species in two fast-growth lineages suggests that the latter is a key step in the evolution of an ephemeral strategy. An association between plant function and habitat identifies the radiation of Ehrharta as adaptive.