The Evolutionary Ecology of Seed Heteromorphism

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Abstract
I develop a model of the ecology and evolution of seed heteromorphism. Heteromorphism is promoted over monomorphism by low correlation of the demographic success of the different morphs and high temporal variation in seed success. Available experimental data from two heteromorphic-seeded species satisfy the model conditions for heteromorphism and produce morph proportions similar to those predicted. Seed heteromorphism can arise when the evolution of morph behavior is constrained by a single concave fitness set or when different seeds on the same individual plant are constrained by different convex fitness sets. The fitness sets represent constraints on demographic success in different year types (trade-offs resulting from constraints in design, development, physiology, or genetics). Two kinds of heteromorphism may arise when the constraints differ for different seeds on the same plant. Whether a high-risk-low-risk heteromorphism or high-risk-high-risk heteromorphism evolves depends on the similarity of the evolutionary constraints. I evaluate the different evolutionary scenarios developed here in light of what is known about the natural history of seed heteromorphism.