Finch beak depth (see Figure 1) is an inheritable trait (it can be passed from parents to offspring).
Researchers studied the beak depth of 2 species of finches, Geospiza fortis and Geospiza fuliginosa. Both species live on Island A. G. fortis alone lives on Island B, and G. fuliginosa alone lives on Island C. For both species, the primary food is seeds. Birds with shallower beaks can efficiently crush and eat only small seeds. Birds with deeper beaks can crush and eat both large and small seeds, but they prefer small seeds.
Researchers captured 100 G. fortis finches and 100 G. fuliginosa finches on Island A. They tagged each bird, measured its beak depth, and released it. Then they calculated the percent of birds having each of the beak depths that had been measured. The researchers followed the same procedures with 100 G. fortis finches from Island B and 100 G. fuliginosa finches from Island C. The results of this study are shown in Figure 2.
After completing Study 1, the researchers returned to Island B each of the next 10 years, from 1976 to 1985. During each visit, the researchers captured at least 50 G. fortis finches and measured their beak depths. Then they calculated the average G. fortis beak depth for each of the 10 years. The researchers noted that, during the 10-year period, 3 years were exceptionally dry, and 1 year was very wet (see Figure 3). Small seeds are abundant during wet years. During dry years, all seeds are less abundant, and the average size of the available seeds is larger.
Figures adapted from Neil A. Campbell, Jane B. Reece, and Lawrence G. Mitchell, Biology, 5th ed. ©1999 by Benjamin/ Cummings.