Evolution__3rd_Edition

9 Quantitative
Genetics
T he Mendelian genetics of beak size is unknown, but the
character shows evolutionary changes as the food supply
changes through time. We start by looking at beak size in
Darwin’s finches as an example of the kind of character
studied by quantitative genetics. We then move on to the
theoretical apparatus used to analyze characters controlled
by large numbers of unidentified genes. The influences on
these characters are divided into environmental and genetic,
and the genetic influences are divided into those that are
inherited and influence the form of the offspring and those
that are not. A number called “heritability” expresses the
extent to which parental attributes are inherited by their
offspring. With the theoretical apparatus in place, we can
then apply it to a number of evolutionary questions:
directional selection, in both artificial and natural examples,
and stabilizing selection. We look at the effect of selection
on heritability, and at mutation–selection balance. We end
with some apparently puzzling observations, in which
populations that are predicted to undergo evolutionary
change in fact stay constant over time.
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