THE SEEDY SIDE OF PLANT ECOLOGY: EVOLUTIONARY RATES, MORPHOLOGICAL DIVERSITY, AND THE EVOLUTION OF SEED SIZE

Teasing apart the relative impacts of phylogenetic constraint and ecological adaptation is fundamental to understanding the evolution of morphological diversity. The evolution of seed size represents a unique system in which to test such hypotheses because the mean size of a species’ seed is correlated with an array of factors associated with that plant’s ecological niche. Modern seed mass ranges across 11 orders of magnitude and is correlated with the species’ reproductive strategy, growth form, dispersal mechanism and climate zone. Multiple angiosperm lineages have independently evolved extremely large and extremely small seeds, expanding beyond morphospace boundaries that appear to have limited seed plants since the Paleozoic. Using a robust phylogenetic hypothesis of seed plant relationships and seed mass data (mean dry mass in g) for 17,375 species, I calculated morphological rates of seed size evolution as change in variance per Ma and assessed phylogenetic signal based on independent contrasts. Morphological rates in angiosperm clades range from 0.001 (Garryales) to 0.207 (Malvales). Closely related clades exhibit a slight (though not significant) tendency toward similar rates. Although there is no significant correlation between morphological rates and the clade’s mean seed size, taxonomic diversification rates are significantly correlated with the clade’s morphological rate and mean seed size, suggesting that the accumulation of species richness is at least partially driven by a clade’s ability to generate ecologically distinct lineages. Analyses of branch length-based measures of community phylogenetic structure indicate that lineages falling in the tails of the global seed size distribution (i.e., <0.001 g or >1000 g) are significantly more dispersed across the phylogeny than expected under a stochastic model. In contrast, lineages with “medium-sized” seeds (i.e., 0.007 to 7.389 g) were significantly clustered, suggesting that lineages evolving into unoccupied morphospace are undergoing adaptive radiations, while lineages evolving in previously occupied morphospace reflect varying degrees of phylogenetic niche conservatism.