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FIGURE 8.3 Genetic divergence of allopatric versus sympatric sister species of sea urchins in groups representing different stages of speciation. See the text for group definitions. Data are as in Figs. 8.1 and 8.2.

(Fig. 8.4). There is significant association between the frequency distribution of sympatric and allopatric species and the rate of their bindin evolution (Fisher’s exact test, P = 0.0033). Because the species are not phylogenetically independent, the statistical significance should not be interpreted to mean that the two quantities are related directly but rather that they tend to cooccur in the same genera. In the genera Echinometra and Strongylocentrotus, sympatric species are common, whereas in genera with slow rates of bindin evolution, sympatric species are rare. Among genera with rapid bindin evolution, 10 of 16 sister species comparisons are between sympatric species. In other genera, only two of 19 comparisons are between sympatric species (Fig. 8.4). The exception to this pattern is in the group 2 genus Lytechinus. Bindin evolution is generally slow in this ge-

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