most potent has been that between Polynesia and the American coast.” Molecular phylogenies have revealed this statement to hold true for Eucidaris (Lessios et al., 1999), Diadema (Lessios et al., 2003b), and Echinometra (McCartney et al., 2000), although in the latter two genera there are indications that larvae are occasionally able to breach the barrier, which may represent the early stages leading toward secondary sympatry. Tripneustes in the Indo-Pacific, on the other hand, shows no evidence that its gene flow is in any way impeded by the 5,000 km of deep open water between Clipperton and the Marquesas (Lessios et al., 2003a), and a similar situation exists for Echinothrix, an Indo-Pacific genus of sea urchins that appears to have recently colonized the eastern Pacific (Lessios et al., 1996, 1998). It is unclear why some species are able to traverse the barrier so easily when the majority cannot, because there are no pronounced differences in the length of the competent larval stage of the genera.

Lessios and coworkers (Lessios and Cunningham, 1990; McCartney et al., 2000) suggest that rare immigrants into the range of an allopatric neighbor will most likely fail to reproduce or will hybridize with the resident species. Maintenance of a rare species within the range of a more numerous one demands some mechanism of reproductive isolation or assortative mating. Both mechanisms operate at the surfaces of gametes during sea urchin spawning (Palumbi, 1992). Sperm attachment and fusion is facilitated by interaction of bindin with a large protein receptor on the egg surface (Kamei and Glabe, 2003; Palumbi, 1999). Rapid evolution of bindin in sea urchins generates assortative mating (Palumbi, 1999) and egg–sperm incompatibility (Biermann, 1998; McCartney and Lessios, 2004; Metz and Palumbi, 1996). Other than the timing of gamete release (Lessios, 1984), few behavioral mechanisms of mate choice operate in these free spawning invertebrates; therefore, interactions of gametes assume a greater role in reproductive isolation. Positive selection in the bindin gene is associated with functional divergence of gamete recognition within and between species (McCartney and Lessios, 2004; Palumbi, 1999). In turn, this divergence may allow sympatry of species at an earlier stage of divergence. Genera with rapid bindin evolution include many closely related sympatric species. By contrast, in genera without rapid change in bindin, closely related or moderately related species are nearly exclusively allopatric (Fig. 8.4).

This difference among genera enhances our understanding of species formation and helps us interpret Mayr’s animation in more mechanistic ways. In Mayr’s original formulation, groups 1 and 2 genera eventually evolve into groups 3 and 4 genera. The major difference between these categories is the amount of time that species have had to diverge genetically: Reproductive isolation was thought to evolve as a consequence of