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 ans wer was laid out by Parker et al. However, their model requires an unusual and unverified assumption regarding the relationship between zygote size an d fitness. Although the first phylogenetically controlled test of the compa rative predictions of the model is consistent, the results have a simple al ternative interpretation. Furthermore, recent work has formalized different theoretical frameworks that require less unusual assumptions. These postul ate, for example, that, under sperm limitation, a larger egg will have an i ncreased chance of being fertilized, either because its own mass offers a l arger target for sperm or because larger eggs can produce a greater quantit y of attraction pheromone. Other frameworks either point to small sperm pre venting transmission of cytoplasmic symbionts and/or organelles or having a motility advantage. At present, however, no model is capable of offering a universal explanation.