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. Each point shows the relationship between the bill depth of a particular bird and 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): the mean beak size increased (blue line displaced upward ~0.5 mm) and 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 beween years).