Evolution__3rd_Edition

226 PART 2 / Evolutionary Genetics
Beak size is a continuous character
Figure 9.3
(a) The phenotypic character,
beak size for example, is
controlled by one locus with
two alleles (A and a); A is
dominant to a. There are two
discrete phenotypes in the
population. (b) The character is
controlled by two loci with two
alleles each (A and a, B and b);
A and B are dominant to a and
b. There are three discrete
phenotypes. (c) Control by six
loci, with two alleles each.
(d) Control by many loci with
two alleles each. As the number
of loci increase, the phenotypic
frequency distribution becomes
increasingly continuous.
9.2 Quantitative genetics is concerned with characters
controlled by large numbers of genes
The beak size of Galápagos finches is an example which illustrates a large class of
characters. It shows continuous variation. Simple Mendelian characters, like blood
groups or the mimetic variation of Papilio, often have discrete variation; but many of
the characters of species are like beak size in these finches a they vary continuously, and
every individual in the population differs slightly from every other individual. There
are no discrete categories of beak size in G. fortis or in most other species of birds.
The other important point about beak size is that we do not know the exact genotype
that produces any given beak size. We can, however, say something about the general
sort of genetic control it may have. Characters like beak size, which has an approximately
normal frequency distribution (that is, a bell curve), are probably controlled by
a large number of genes, each of small effect. The reason is as follows (Figure 9.3).
Imagine first that beak size was controlled by a single pair of Mendelian alleles at one
locus, with one dominant to the other, AA and Aa long and aa short. In this case, the
population would contain two categories of individuals (Figure 9.3a). Imagine now
that it was controlled by two loci with two alleles each. Beak size might now have a
background value (say, 0.4 in or 1 cm) plus the contribution of the two loci, with an
A or a B adding 0.04 in (0.1 cm). If A and B were dominant to a and b, then an aabb
individual would have a 0.39 in (1 cm) beak; AAbb, Aabb, aaBB, and aaBb 0.43 in
(1.1 cm); and AABB, AaBB, AABb, and AaBb 0.47 in (1.2 cm). Figure 9.3b is the
frequency distribution if all alleles had a frequency of one-half and the two loci were in
linkage equilibrium. The distribution now has three categories and has become more
spread out. It becomes still more spread out if it is influenced by six loci (Figure 9.3c)
and becomes normal when many loci are at work (Figure 9.3d).
When a large enough number of genes influence a character, it will have a continuous,
normal frequency distribution. The normal distribution can result either if there
are a large number of alleles at each of a small number of loci influencing the characters,
or if there are fewer alleles at a larger number of loci. In this chapter, we shall mainly
discuss the theory of quantitative genetics as if there were many loci, each with a small
Frequency
Frequency
(a) One locus, two alleles (b) Two loci, two alleles each
(c) Six loci, two alleles each (d) Many loci, two alleles each
Frequency
Beak size Beak size
Frequency
Beak size Beak size
..