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FIGURE 8.2 Genetic divergence among sister species in eight sea urchin genera separated into four groups, representing different stages of allopatric speciation. See the text for the definition of each group. Mitochondrial DNA divergence is based on Kimura two-parameter distances at the COI gene (COI plus ATPase8/6 for Diadema). Data are based on Fig. 8.1 and Lessios et al. (1999, 2003a,b), Zigler and Lessios (2004), McCartney et al. (2000), Landry et al. (2003), Biermann et al. (2003), Metz et al. (1998), Zigler et al. (2003). Medians are marked by arrows.

patric species. In fact, in groups 1 and 2 genera, allopatric species are more divergent than sympatric species (Fig. 8.3). This pattern is largely due to the existence of a large fraction of ancient allopatric neighbors, species that have diverged from sister taxa long ago and are separated by no insurmountable major geographic barrier, such as a land mass, even in early-stage genera. Dividing the species into allopatric and sympatric comparisons shows that divergence between sympatric species increases stage by stage but that divergence between allopatric species does not (Fig. 8.3).

Further comparisons show that most sympatric species are found in genera in which the sperm recognition protein bindin is evolving quickly

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