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 that provides sufficient 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 their two parents). One common measure of heritability is the correlation between parent / offspring measures. In 1976 (red circles), the correlation was 0.90 (slope of red line), which indicates a high degree of heritability.

    A prolonged drought from 1976 ~ 1978 produced tougher seeds with lower water content. Only finches with larger, deeper beaks that produced greater cracking strength were able to survive. Heritability remained constant in 1978 (the slope of blue line remains 0.90, parallel to red line). However, beak measurements show the result of directional natural selection: in 1978, no parental birds with beak depth < 9mm survive (truncation of the left end of the X-axis). Mean beak depth of offspring birds has increased ~1.0 mm, as measured by the difference between the midpoints of the two distributions (the + & + crosses).

    These data show several important aspects of on the concept of heritability. Beak size has constant, high heritability, but beak size in any one year is highly variable (wide range on X-axis), and beak sizes varies between years when the environment changes (displacement of slopes on the Y-axis). Thus, high heritability implies neither that a trait is invariant, nor that it is constant.