The uncertain evolution of the sexes

What forces gave rise to the evolution of the size difference between sperm and eggs? For many years, it has been all but accepted wisdom that the answer was laid out by Parker et al. However, their model requires an unusual and unverified assumption regarding the relationship between zygote size and fitness. Although the first phylogenetically controlled test of the comparative predictions of the model is consistent, the results have a simple alternative interpretation. Furthermore, recent work has formalized different theoretical frameworks that require less unusual assumptions. These postulate, for example, that, under sperm limitation, a larger egg will have an increased chance of being fertilized, either because its own mass offers a larger target for sperm or because larger eggs can produce a greater quantity of attraction pheromone. Other frameworks either point to small sperm preventing transmission of cytoplasmic symbionts and/or organelles or having a motility advantage. At present, however, no model is capable of offering a universal explanation.