Figure 7.6 Evolution by genetic drift. (a) Mexican cavefish populations that live in surface streams have large eyes and dark pigmentation, whereas populations that live in caves have small eyes and reduced pigmentation. (b) When researchers compared how different regions of DNA that code for eye size changed between surface and cave populations, they found that all 12 regions coded for smaller eyes in the cave population than in the surface population. Because all 12 regions changed in the same direction, this suggests that natural selection selected for smaller eyes. (c) When researchers looked at changes in regions of DNA that code for pigmentation, they found 5 regions coded for increased pigmentation while 8 regions coded for decreased pigmentation. Because the 13 regions did not change in the same direction, this suggests that changes in pigmentation were due to genetic drift.

Data from M. Protas et al., Regressive evolution in the Mexican cave tetra, Astyanax mexicanus, Current Biology 17 (2007): 452–454. Photo courtesy of Dr. Richard Borowsky.