wasps, in turn, require the ovaries of the figs as oviposition sites (Wiebes, 1979). Strong reciprocal species specificity suggests that when individual fig species or individual wasp species undergo speciation, they do so in tandem with their mutualistic partner (cospeciation). But Machado et al.’s phylogenetic and population genetic study (Chapter 7) shows that the history has not been this straightforward and that host switches by wasps, and possibly species hybridization by figs, have created partly independent phylogenetic histories of figs and wasps.
In Mayr’s world view, new species arise under allopatry, and, after that, as divergence accrues, the geographic ranges of related species may later come to overlap. In this way, related but divergent species may be sympatric, in contrast to most closely related species, which are expected to have disjunct, allopatric distributions. This sequence of events was outlined explicitly by Mayr in a 1954 paper on the biogeography of sea urchins (Mayr, 1954). Stephen Palumbi and Harilaos Lessios, in “Evolutionary Animation: How Do Molecular Phylogenies Compare to Mayr’s Reconstruction of Speciation Patterns in the Sea?” (Chapter 8), have returned to this same Echinoid system and reconsidered Mayr’s synthesis using DNA sequence data. They find that although the pattern described by Mayr still largely applies, rapidly evolving gamete recognition proteins play a strong role in reproductive isolation. In contrast, Mayr had envisioned the evolution of reproductive isolation by a more genomewide steady accumulation of substitutions.
For many biologists, the question of whether geographic separation is strictly necessary for speciation (i.e., the question of whether sympatric or parapatric speciation occurs) comes into sharpest focus with the case of Rhagoletis pomonella. This is the apple maggot fly that has diverged into two host races (apple and hawthorne), apparently under geographic sympatry and aided by the different fruiting times of the two hosts (Filchak et al., 2000). Mayr’s former student Guy Bush discovered the history of sympatric divergence in Rhagoletis, and it has long been a standard component of the debates on the prevalence of sympatric speciation. Now we learn from Guy Bush’s former student Jeffrey Feder and his colleagues, in “Mayr, Dobzhansky, and Bush and the Complexities of Sympatric Speciation in Rhagoletis” (Chapter 9), that the sympatric divergence that occurred within U.S. populations may have been facilitated by genetic variation that came in by means of gene flow from largely separated populations in Mexico.
The question of sympatric speciation has also been much discussed in the context of the highly speciose cichlid fishes from the great African lakes: Victoria, Malawi, and Tanganyika (Mayr, 1984). Particularly in the cases of Lakes Malawi and Victoria, which are relatively young, it is a wonder how hundreds of species could form within confined bodies of water within <1 million years. Yong-Jin Won, Arjun Sivasundar, Yong