A comparative test of a theory for the evolution of anisogamy

Why are sperm small and eggs large? The dominant explanation for the evolut ion of gamete size dimorphism envisages two opposing selection pressures ac ting on gamete size: small gametes are favoured because many can be produce d, whereas large gametes contribute to a large zygote with consequently inc reased survival chances. This model predicts disruptive selection on gamete size (i.e. selection for anisogamy) if increases in zygote size confer dis proportional increases in fitness (at least over part of its size range. It therefore predicts that increases in adult size should be accompanied by s tronger selection for anisogamy Using data from the green algal order Volvo cales, we provide the first phylogenetically controlled test of the models predictions using a published phylogeny and a new phylogeny derived by a di fferent method. The predictions that larger organisms should iii have a gre ater degree of gamete dimorphism and (ii) have larger eggs are broadly uphe ld. However, the results are highly sensitive to the phylogeny and the mode of analysis used.