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

    Darwin's Finches in the Galapagos islands included Geospiza fortis, the nutcracker finch, which has evolved a deep bill for cracking seeds. The graph shows the relationship between the bill depth of a bird versus its midparent value (the average of the two parents). The correlation between these measures in 1976 (red circles)  was 0.90 in (slope of red line). 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 (slope of blue line is parallel to red line), however, the mean beak size increased (blue line displaced upward ~0.5 mm). The graph also shows directional natural selection, as no birds with beaks <9 mm survived (blue circles). Although beak size has constant high heritability, this does not mean that the trait is constant: beak size in any one year is highly variable (note ranges of axes), and varies when the environment changes (upward displacement of slope between years).