2003; Metz and Palumbi, 1996; Palumbi and Metz, 1991). Where E. sp. C is in sympatry with E. oblonga (Okinawa) bindin alleles are highly divergent; in contrast, where they are allopatric, E. sp. C and E. oblonga (Hawaii) have very similar alleles (Geyer and Palumbi, 2003). In addition, bindin sequences in E. insularis are distinct and monophyletic and do not suggest a close relationship with E. sp C (Landry et al., 2003). Thus, the genus Echinometra has species with extensive sympatry, whereas the genus Diadema shows sympatry of just a few species pairs. However, contrary to predictions of Mayr’s animation, overall genetic divergence between species of Echinometra is smaller than between those of Diadema (Fig. 1). In particular, sympatric species of Echinometra show much less genetic divergence than sympatric species of Diadema.
The genus Arbacia consists of purely allopatric species, but Mayr included it in group 4 because he doubted the validity of specific rank, even for species that were found in different oceans. He may actually have been right for the wrong reasons, because COI and bindin show that the original species designations correspond to divergent molecular clades and should not be considered as conspecific. However, neighboring allopatric clades are old and thus qualify the genus for inclusion in group 4. Based on COI sequences, Arbacia punctulata along the east coast of North America and Arbacia lixula from the eastern Atlantic and from Brazil are ≈3–5 million years old (Metz et al., 1998). The most recently derived species pair, Arbacia dufresnei and Arbacia incisa, are 2–4 million years old. Species also tend to be widespread: A. lixula occurs from the coast of Brazil to the Mediterranean, with an ≈500,000-year divergence between these genetically distinct populations. The western Atlantic species A. punctulata ranges from Cape Cod to Curacao, Trinidad and Tobago, and shows a 2% sequence divergence in COI between Florida and the northern end of its range.
Although Mayr mentioned studies of morphological variation in Strongylocentrotus droebachiensis and Strongylocentrotus pallidus, he did not place this genus in any of his groups, because it lacks tropical representatives. However, because molecular and morphological variation in Strongylocentrotus have been well studied, it can receive the same consideration as the other genera. There are many sympatric species in this genus. A cluster of species sympatric in the Northeast Pacific diverged from one another at ≈3–5 million years ago based on COI and fossil evidence (Biermann et al., 2003). Strongylocentrotus purpuratus, Strongylocentrotus droebachiensis, S. pallidus, and Allocentrotus fragilis all diverged from one another at about the same time. Although broadly overlapping