Evolutionarily Stable Seed Polymorphism and Small-Scale Spatial Variation in Seedling Density

Using the evolutionarily stable strategy (ESS) approach in a model for the evolution of seed size, I show that small-scale spatial variation in seedling density favors the evolution of variation in seed size within individual plants if competition among seedlings is sufficiently asymmetric in favor of larger seeds. A single seed size is found to be never evolutionarily stable. I always find at least some continuous adaptive variation in seed size. The model generates the following predictions. At least some continuous variation in seed size is adaptive. Plants with many resources have more variable seeds than plants with few resources. Plants with low juvenile mortality have more variable seeds than plants with high juvenile mortality. Plants with aggregated seed dispersal have seed size distributions that are more biased toward small seeds compared to plants with more even seed dispersal. An explicit formula for the evolutionarily stable seed size distribution was obtained for Poisson-distributed seed numbers and for negative binomial distributed seed numbers per safe site. The application of the model to seed size distributions of entire populations or plant communities is discussed.