overall genetic divergence or adaptive differentiation (Mayr, 1942, 1963). The critical importance of a few gamete recognition loci to reproductive isolation can disrupt the steady pace of evolution of isolation (Dobzhansky, 1937, 1970; Orr, 1991; Wyckoff et al., 2001). In these cases, the way a few loci evolve may be more important than overall genetic divergence or adaptive differentiation (Dobzhansky, 1937, 1970).
Our phylogenetic summary shows that the distinction between sea urchin genera at different speciation stages is related to bindin evolutionary rate. Groups 1 and 2 genera have slow bindin evolution, have largely allopatric species, and are therefore earlier frames in the evolutionary animation. Groups 3 and 4 genera, with many more sympatric species, are classified later in the evolutionary series, but their species are not necessarily older. Instead, these can be the genera in which rapid bindin change generates reproductive isolation among even closely related species.
A similar hierarchy of speciation rate might be present in Mayr’s early work on tropical Pacific birds. Genera with bright male plumage and strong sexual selection were textbook cases of geographic variation within sister-species complexes (Mayr, 1942). Such genera might be akin to the sea urchin genera with fast bindin evolutionary rates. A key difference, however, is that evolution of plumage generates morphological diversity across a species that allows it to be divided into taxonomic units based on morphology (Mayr, 1942). By contrast, rapid evolution of bindin does not in and of itself generate strong morphological variation, and in genera with rapid bindin evolution, species designations had to await the ability of molecular tools to assay genetic differences. The most diverse set of closely related sea urchin species known, the five or six Indo-West Pacific species of Echinometra, had been classified as a single, large polymorphic species by Mortensen (1928–1951). Only after reproductive barriers and genetic differences became clear were subtle morphological and ecological distinctions discovered (Matsuoka and Hatanaka, 1991; Palumbi and Metz, 1991; Uehara and Shingaki, 1985).
Another potential difference between bindin and plumage evolution is the driver producing different evolutionary rates. Sexual selection is thought to drive the divergence of male coloration in birds through a runaway process based on female preference (Kirkpatrick, 1982; Lande, 1981). Because female preference and male traits coevolve differently in separate isolated areas, different populations can attain novel trait and preference combinations (Ryan and Wilczynski, 1988). For bindin and its receptor genes, an excess of amino acid replacement substitutions is a signal that evolution is driven by selection, but the source of this selection remains unclear. It is possible that selection is for increasingly better fertilization systems, with the bindin genes evolving to produce more optimal fertilization phenotypes. Positive selection in this case would be