Several lineages have independently evolved from isogamy (all sexes producing similar gametes) through anisogamy (dissimilar gametes) to the familiar male (producing sperm) and female (producing eggs) condition of most large, multicellular organisms (oogamy). A variety of hypotheses explaining the selective mechanisms causing such evolution and the success of these lineages have been proposed, but little evidence and some confusion persists. Here, a few simplifying assumptions are used to extract and compare the essential features of the various ecological hypotheses. The comparisons reveal that the critical need is to identify a selective advantage of large, immobile gametes (eggs). Assumptions about the effect of sperm size on swimming speed are not important. The classic assumption of increasing zygote success with large size requires a relationship even stronger than survival proportional to volume, which seems unlikely and lacks empirical support. An assumption that eggs produce a pheromone sperm attractant leads, by established physical principles, to a more than sufficient advantage of large egg size. Without pheromones, combinations of increased target size and weaker increased zygote fitness or increased gamete longevity also provide sufficient selection.