ventricosus has considerable population structure, indicating a lack of gene flow between the American and African coasts. T. gratilla and T. ventricosus are assumed to have diverged at the Panamanian closure 3 million years ago.
The last genus in this cluster is more complex. Lytechinus has two sets of polytypic species: Lytechinus anamesus and Lytechinus pictus along the west coast of north America are indistinguishable genetically, as are the eastern Pacific Lytechinus semituberculatus and Lytechinus panamensis (Zigler and Lessios, 2004). These two Pacific species clusters diverged from each other 3–5 million years ago. In addition, there is a set of Atlantic species with questionable species status. Data from COI show that the subspecies L. variegatus variegatus and Lytechinus variegatus atlanticus cluster indistinguishably from one another but that L. williamsi, partially sympatric with L. variegatus variegatus, diverged at ≈500,000 years ago. An outgroup clade to this cluster is L. variegatus carolinus, which diverged 2–3 million years ago. The genealogy of bindin shows one discrepancy from that of COI: The phylogenetic positions of L. williamsi and L. variegatus carolinus are switched (Zigler and Lessios, 2004). In addition, there is evidence for acceleration of bindin evolution in the two sympatric species compared to COI, although maximum likelihood analysis fails to show positive selection, possibly because of low statistical power in these closely related sequences.
The summary of these studies of groups 1 and 2 genera is that molecular phylogenies support Mayr’s conclusions that widely distributed polytypic species are commonplace and that allopatric splitting events within ocean basins are sometimes very recent. However, some allopatric neighbors have been in existence for 2–8 million years without evidence that their ranges have begun to overlap.
Molecular phylogenies also support Mayr’s classification of genera into group 3 because their species are in the initial stages of sympatry; but they also show that they are comprised of species groups with very different evolutionary patterns. In Diadema, the widely distributed species Diadema setosum and Diadema savignyi overlap throughout the western Pacific and Indian oceans (Lessios et al., 2003b). Based on ATPase and COI sequence differences, these two species are highly divergent, having split 7–14 million years ago (Lessios et al., 2003b). The widespread Diadema savignyi is also sympatric in Japan and the Marshal islands with an undescribed species, from which it diverged 6.5–13.5 million years ago. Isozymes and mitochondrial DNA have recently uncovered unsuspected cases of sympatry between Diadema paucispinum, a species originally