the corresponding sperm and egg genes among closely related species (Clark NL, et al., 2006; Zigler, 2008), and changes in the genes coding for several of these divergent proteins indicates that many of these changes have resulted from positive selection. There are also informative exceptions to this rule (below). Strictly speaking, the term postcopulation is not appropriately applied to free-spawning species such as sea urchins, but they offer interesting comparisons. As in genitalia, species isolation hypotheses played a large role in early interpretations, but recent discoveries also suggest important roles for sexual selection.

Sperm proteins in free-spawning marine gastropods are among the most rapidly evolving proteins known (Clark NL, et al., 2006). The abalone sperm molecule lysin, which digests a hole in the vitelline egg membrane, has evolved up to 15 times faster than introns (Metz et al., 1998), and there is a link between sites of positive selection and functional changes (Clark NL, et al., 2006). Egg molecules with which lysin interacts have also undergone rapid divergence under selection (Swanson and Vacquier, 2002; Aagaard et al., 2006). Similarly, the bindin molecule of sea urchin sperm, which both attaches the acrosomal process of the sperm to the glycoprotein bindin receptor molecules in the egg’s vitelline layer and promotes fusion of egg and sperm membranes, also shows rapid divergence (Zigler, 2008). The section of the bindin molecule that is involved in attachment to the egg varies sharply among species in 3 genera, in each of which there is evidence that positive selection produced the changes; in 3 other genera, divergence in the attachment portion is low, and there is no evidence for positive selection (Zigler, 2008). Egg molecules involved in both induction of the acrosome reaction and the docking process on the egg have diverged in 1 genus that has rapidly diverging bindin (Kamei and Glabe, 2003).

A SAC explanation of this divergence would posit a coevolutionary race between males and females over control of sperm entry into the egg. When multiple sperm enter the same egg (polyspermy), usually the embryo dies, so sperm competition favoring sperm cells that are especially quick to enter eggs might also result in loss of some eggs because of polyspermy. Females could respond by making it more difficult for sperm to enter, and the resulting coevolutionary race could result in rapid divergent evolution of both sperm and egg molecules. A CFC explanation is that females are under selection to favor sperm cells with particularly effective designs, to obtain sons with these same designs; increased female selectivity could result in competition among males to evolve even more effective designs, resulting in rapid coevolution between male and female molecules.

A species isolation function for the rapid divergence of the fertilization molecules of sea urchins and abalone is also attractive because their gametes meet in open water. Species that have evolved in the presence of close relatives would be expected to show greater divergence in their