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 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
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).