Heritability of beak size in Darwin's Finch (Geospiza)
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
/ offspring measures. In 1976 (red
circles), the correlation was 0.90 (slope of red line), which
indicates a high degree of heritability.
drought 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 between 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). High heritability implies neither that
a trait is invariant, nor that it is constant.