Testing Postulate 2: Is some of the variation among individuals heritable?
2.4.- Within a population, individual finches could vary in beak depth because the environments
they have experienced are different or because their genotypes are different, or both. For instance
variation in the amount of food that individual birds happened to have received as chicks can
lead to variation in beak depth among adults. Likewise injuries or abrasion against hard seeds
or rocks can also affect beak size and shape
2.5.- To estimate whether at least part of the variability among finch beaks is genetically based the
heritability of this trait was estimated. Heritability is the proportion of the variation observed
in a population that is due to variation in the effects of genes. It varies between 0 (all variation
in this population is due to the environment and therefore all the individuals of the population are
genetically identical for a particular trait) and 1 (the levels of genetic variation for a particular trait in this
popualtion are so high that each individual has a different genotype for this trait). Comparisons of
beak depth of G. fortis young after they had attained adult size to the average bill depth of their
mother and father show a very strong correlation (Fig. 3.4).
2.6.- Parents with deep beaks tend to have offspring with deep beaks, and parents with
shallow beaks tend to have chicks with shallow beaks. This is evidence that a large