Summaries of PhD-theses

Summary of the PhD-thesis by Stefan Geritz, EEW Leiden University,
Email: geritz@zool.umd.edu

The evolutionary significance of variation in seed size

Variation in seed size between and within individual plants of the same species is common and widespread. The central question of this thesis is how this variation can be explained as a consequence of natural selection under specific circumstances.

• In Chapter 1 I review some of the empirical data on how seed size affects plant fitness, and I summarize a number of models for the evolution of seeds size variation each of which emphasizes a different aspect of a plant's life-history or its environment.
• In Chapter 2 I show that small-scale spatial variation in seed density due to random seed dispersal may favor the evolution of variation in seed size both within and between individual plants if competition among seedlings is sufficiently asymmetric in favor of larger seeds.
• In Chapter 3 I show that coevolution with a seed predator may favor the evolution of variation in seed size both between and within individual plants if there is a trade-off in the predator's attack rate for different seed sizes.
• In Chapter 4 I develop a general framework and a mathematical toolbox for modeling the dynamics of mutation-limited evolution. Special emphasis is put on the generation of phenotypic variation by branching of the evolutionary tree during which a given lineage already present in a population divides into two distinct and subsequently diverging daughter lineages.
• In Chapter 5 I apply the above mentioned modeling framework to show that strong competitive asymmetry and high resource levels favor evolutionary branching and coexistence of plants with different seed sizes when large seeds have a substantial precompetitive advantage over smaller seeds. If precompetitive seed and seedling survival is high for small and large seeds alike, evolutionary branching may be followed by extinction of one or more lineages (including mass-extinction). Various results presented in this chapter are model-independent and point the way to a more general evolutionary bifurcation theory describing how the number and stability properties of evolutionary equilibria changes as a consequence of changing model parameters.