Evolutionary dynamics of seed size and seedling competitive ability

We present a model for the evolutionary dynamics of seed size when there is a trade-off between seed size and seed number, and seedlings from large se eds are better competitors and have a higher precompetitive survival than s eedlings from small seeds. We find that strong competitive asymmetry, high resource levels, and intermediate harshness of the precompetitive environme nt favor coexistence of plants with different seed sizes. If the evolution of seed size is mutation-limited and single mutations have only a small phe notypic effect, then an initially monomorphic population reaches the final evolutionarily stable polymorphic state through one or more discrete evolut ionary branching events. At each such branching event, a given lineage alre ady present in the population divides into two phenotypically diverging dau ghter lines, each with its own seed size. If the precompetitive survival of seeds and seedlings is high for small and large seeds alike, however, evol utionary branching may be followed by the extinction of one or more lineage s. Various results presented here are model-independent and point the way t o a more general evolutionary bifurcation theory describing how the number and stability properties of evolutionary equilibria may change as a consequ ence of changes in model parameters. (C) 1999 Academic Press.