Convergent Evolution, Adaptive Radiation, and Species Diversification in Flowering Plants

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Convergent evolution among lineages, and divergent evolution within lineages leading to adaptive radiation, are key drivers of phenotypic and ecological diversity. Here we discuss the context specificity of selective pressures that lead to convergent evolution and adaptive radiation, including environmental diversity, the number of lineages present in an area, and how closely related local competitors are to each other. Adaptive radiation is sometimes thought to drive species diversification, but exceptions to that hypothesis are frequent. Across angiosperms, rates of net species diversification tend to be higher in herbs (especially annuals) vs. woody plants, in animal-vs. wind-pollinated species, in families with a greater diversity of growth forms, pollination mechanisms, and species distributions, in families at lower latitudes, in lineages with bisexual vs. unisexual flowers, in plants with bilateral vs. radial floral symmetry, in plants with hummingbird pollination or with spurred or tubular flowers, and in young lineages. Topographic complexity, the evolution of C4 photosynthesis, and the rise of highly specialized flowers or chemical defenses all appear to drive higher rates of species diversification. However, species richness and rates of species diversification are themselves uncoupled. The number of closely related species that can coexist locally should increase with both ecological and reproductive isolation among species. Drivers of convergence, divergence, and species richness and diversification in deserts and alpine vegetation at low and high latitudes are discussed, as is the phenomenon of gradient convergence, involving regular shifts in form or physiology along spatial environmental gradients.