Figure 22.1: Marine populations of threespine sticklebacks possess pelvic spines (a) but the spines are reduced or missing in sticklebacks from many freshwater lakes (b).
[Adapted by permission from Macmillan Publishers Ltd: Neil H. Shubin and Randall D. Dahn, Evolutionary Biology: Lost and Found. Nature 428, 703-704 (15 April 2004), copyright 2004.]