fertilization molecules than species that evolved in isolation from other congeneric species. Neither SAC nor CFC predicts this pattern. This predicted biogeographic pattern occurs in sea urchins: the 3 genera with rapidly diverging bindin molecules contain species that live in sympatry with congeners; in contrast, species in 3 other genera that do not show rapid divergence in bindin do not have overlapping geographic ranges (Zigler and Lessios, 2003a). In addition, 1 species that overlaps with a congenor at some locations but not others shows the predicted accentuation of differences in areas of sympatry (Zigler, 2008). Species isolation is a feasible (although not proven) explanation for the rapid divergence of their fertilization molecules of the abalone species, which are generally sympatric with congeners.
Nevertheless, further details suggest that sexual selection may have played a role, at least in bindin evolution. In the first place, cause and effect are not clear in the biogeographic correlation: perhaps the divergence in bindin was originally caused by other selective factors and only incidentally allowed congeneric species to subsequently coexist in sympatry without fusing or becoming extinct (Zigler and Lessios, 2003b). In addition, there is rapid intraspecific divergence with signs of positive selection in 2 genera with sympatric congeners, contrary to species isolation predictions (Zigler and Lessios, 2003b), and there is no evidence of selection having produced the substantial differences in the bindin (relative to mDNA) of 2 sympatric species of 1 genus (Zigler and Lessios, 2004). Furthermore, a possible explanation for intraspecific divergence in some but not other parts of a species’ range that is based on the species isolation hypothesis is not supported in 1 group: the same bindin occurs in areas of overlap and nonoverlap with another congeneric species (H. Lessios, personal communication). Zigler (2008) concluded that different types of selection may have acted on bindin, and that final answers are not yet available.
Both species isolation and sexual selection hypotheses predict that once an egg has responded to 1 sperm by erecting a barrier that excludes other sperm proteins that are involved in subsequent interactions should fail to show the rapid divergence typical of fertilization molecules. The data appear to support this prediction in sea urchin bindin. In sea urchins the raising of the egg’s fertilization membrane, which prevents the entry of further sperm, is triggered by the sperm-egg membrane fusion that follows attachment of sperm to the vitelline membrane. The core of the bindin molecule, which is involved in the fusion of egg and sperm membranes is, as predicted, uniform even among quite distantly related sea urchins and other echinoids (Zigler, 2008). Thus, 2 patterns occur in the same molecule: the portion involved in an early stage of fertilization is highly diverse in some lineages, whereas another portion that is involved in a later stage (after paternity has been decided) is very conservative.