Figure WN16.1. (A) The periwinkle Littorina saxatilis has evolved two forms, called H and M, which are adapted to upper shore and lower shore, respectively. (B) The degree of divergence between populations relative to variation within them is measured by F_ST. This is plotted against allele frequency for 290 genetic markers. (Top two panels) Examples of divergence between the two different forms, separated by 300 meters or less. (Bottom panels) Divergence between different populations of the same form, 45 km apart. (Solid line) Statistical limit calculated by simulation. (Red) 15 loci showing a significant excess of divergence. (B, Redrawn from Wilding et al. 2001, Fig. 1.)

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