Heritability of beak size in Darwin's Finch (Geospiza)

    Darwin's Finches in the Galapagos islands include the Nutcracker Finch (Geospiza fortis), which has evolved a deep bill with the mechanical force to crack seeds. The graph shows the relationship between the bill depth of offspring birds versus their mid-parent value (the average of the two parents). The correlation between these measures in 1976 (red circles)  was 0.90 in (slope of red line), which indicates a strong measure of heritability.

    A drought in 1978 produced tougher seeds with lower water content: only those finches with larger beaks that produced greater cracking strength were able to survive. Heritability remains constant in 1978 (the slope of blue line is parallel to red line). However, the mean beak size of offspring increased (blue line displaced upward ~0.5 mm). The graph also shows directional natural selection, as no parental birds with beaks <9 mm survived (blue circles). Although beak size has constant high heritability, beak size in any one year is highly variable (note ranges of axes), and varies between years when the environment changes (upward displacement of slope).


Text material ©2016 by Steven M. Carr