in geographic distribution, these species tend to live at different depths. A similar sympatric cluster in Japan is composed of several species that diverged slightly earlier. The two species clusters, one along North America and one along the coast of Japan, show no geographic overlap. A deep split in the genus at ≈10–15 million years ago gave rise to two allopatric sister species: Strongylocentrotus franciscanus in the east and Strongylocentrotus nudus in the west. The divergence time of this pair of species was ≈5 million years ago. Bindin evolution is rapid in this genus (Biermann, 1998).
An additional genus, also not included by Mayr because it does not occur in the West Indies, has received ample attention with regards to its molecular evolution, in part because of the remarkable divergence in mode of development between its two species. The genus Heliocidaris is restricted to Australia and consists of Heliocidaris tuberculata and Heliocidaris erythrogramma. These two species overlap in range along the southeast coast of Australia, and diverged at ≈5 million years ago (McMillan et al., 1992; Zigler et al., 2003). H. erythrogramma, which has an extremely large egg and has evolved direct development, has subpopulations in western and eastern Australia. There are no published sequence data from the western Australian subspecies, but populations from Sydney and Tasmania differ by ≈2% in COI. Bindin evolution has been rapid along the lineage leading to the direct developing H. erythrogramma.
The prediction that genetic divergence of sister species increases from stage to stage of speciation is borne out by comparison of phylogenies and genetic distances among the eight genera of sea urchins (Fig. 8.2). Median COI genetic distance among 10 comparisons in groups 1 and 2 genera (the first stage of allopatric speciation) is ≈1%, whereas divergence is 3% and >10% for groups 3 and 4, respectively. Sister species in groups 1 and 2 tend to have adjoining ranges. In addition, polytypic species are most common in groups 1 and 2 genera. Five of 10 species comparisons in group 1 genera show no discernable genetic divergence, indicating genetic exchange among allopatric populations rather than species-level differentiation. By contrast, all sister-species comparisons in groups 3 and 4 genera are >2%, and there are no species with zero genetic divergence. Instead, in these genera, widely distributed species often contain allopatric populations with genetic divergences as high as that of many species pairs.
Simple predictions begin to falter, however, when comparisons are divided into those between sympatric species and those between allopatric species. Sympatric species are not generally more divergent than allo-