Variation in seed size is structured by dispersal syndrome and cone morphology in conifers and other nonflowering seed plants.

Seed size varies tremendously in plants and its evolution is influenced by multiple ecological and biological factors that are difficult to disentangle. In this study, we focus on understanding the role of seed dispersal by animals in the evolution of seed size in conifers, the most diverse extant nonflowering seed plant group. Relationships among seed size, dispersal syndrome, climate and cone morphology were analyzed across conifers using quantitative models of character evolution and phylogenetic regression techniques. Dispersal syndrome is a more consistent predictor of seed size within major extant conifer clades than climate. Seeds are generally larger in animal-dispersed than wind-dispersed species, and particular cone morphologies are consistently associated with specific ranges in seed size. Seed size and cone morphology evolve in a correlated manner in many animal-dispersed conifers, following a trade-off that minimizes the total size of the dispersal unit. These relationships are also present in other nonflowering seed plant groups, and have been important in the evolution of seeds and cones at least over the Cenozoic and perhaps over much of the later Mesozoic.