thought to be limited to Hawaii, and the other Indo-West Pacific species (Lessios and Pearse, 1996; Lessios et al., 2003b). The divergence time between the sympatric D. paucispinum and D. savignyi is <2 million years. By contrast, the eastern Pacific Diadema mexicanum has remained allopatric from the Indo-West Pacific species for 3 million years, with only a hint of range overlap with D. savignyi at the Clipperton Atoll, the closest point to the central Pacific (Lessios et al., 1996, 2003b). Within this genus, broadly distributed species tend to show divergence of supposedly conspecific allopatric populations. Diadema antillarum populations from the eastern and western Atlantic are as different from one another as are accepted species in this genus. Two clades of D. setosum in the Indian Ocean probably diverged 5 million years ago. Thus, the species in this genus show a generally higher degree of genetic divergence than genera at earlier stages, with moderately old allopatric populations within a morphospecies. Sympatry occurs between very old species pairs. Allopatric neighbors are old. One exception is the previously unsuspected sympatry of the relatively recently diverged species pair of D. savignyi and D. paucispinum.
By contrast, the genus Echinometra shows a large number of sympatric species with low divergence from one another. In the Pacific there is a cluster of very closely related sympatric species. Echinometra mathaei, Echinometra oblonga and a currently unnamed species “Echinometra sp. A” diverged 1–2 million years ago (Landry et al., 2003; Palumbi, 1996). COI data show that this cluster split from Echinometra sp. C and the Easter Island endemic E. insularis at about the same time. In the Caribbean, the sympatric species Echinometra lucunter and Echinometra viridis diverged ≈1.5 million years ago (McCartney et al., 2000). One ancient allopatric split persists in this genus: The eastern Pacific Echinometra vanbrunti differs from other Pacific species by ≈13% nucleotide differences in COI, corresponding to separation of ≈3.5 million years. However, the allopatry of E. vanbrunti from the other Pacific species may be in the process of being erased through infrequent larval influx from the central into the eastern Pacific: E. oblonga, although rare, is now present in the outer islands of the eastern Pacific (Lessios et al., 1996; McCartney et al., 2000). There is also one very recent allopatric split: E. oblonga appears to be at least two species (E. oblonga Okinawa and E. oblonga Hawaii). Distinguishable by sperm morphology and genetics, these species are allopatric and have diverged at most 250,000 to 500,000 years ago (Landry et al., 2003).
Data from the bindin locus show rapid evolution in this genus, and generally support the COI phylogeny (Landry et al., 2003; McCartney and Lessios, 2004; Metz and Palumbi, 1996). One major exception is that bindin alleles in E. sp. C differ dramatically from one region to another. In this genus, major differences in bindin gene sequence are associated with strong reproductive isolation among closely related species (Landry et al.,