Evolution__3rd_Edition

..
Continuous characters are
influenced by many factors, each
of small effect
Frequency
25
CHAPTER 9 / Quantitative Genetics 227
number of alleles. This may well be the genetic system underlying many continuously
varying characters. However, the theory applies equally well when there are a few (even
one) loci and many alleles at each.
Mendel had noticed in his original paper in 1865 that multifactorial inheritance
(i.e., the character is influenced by many genes) can generate a continuous frequency
distribution; but it was not well confirmed until later work, particularly by East,
Nilsson-Ehle, and others, in about 1910. Quantitative genetics is concerned with
characters influenced by many genes, called polygenic characters. For a quantitative
geneticist, 5–20 genes is a small number of genes to be influencing a character; many
quantitative characters may be influenced by more than a hundred, or even several
hundred, genes. For characters influenced by a large number of loci, it ceases to be
useful to follow the transmission of individual genes or haplotypes (even if they have
been identified) from one generation to the next. The pattern of inheritance, at the
genetic level, is too complex.
There is an additional complication. So far we have only considered the effect of
genes. The value of a character, like beak size, will usually also be influenced by the environment
in which the individual grows up. Beak size is probably related to general body
size and all characters to do with bodily stature will be influenced by the amount of food
an organism happens to find during its life. If we take a set of organisms with identical
genotypes and allow some to grow up with abundant food and others with limited food,
the former will end up larger on average. In nature, each character will be influenced by
many environmental variables, some tending to increase it, others to decrease it. Thus
if we take a class of genotypes with the same value of a character before the influence of
the environment and add the effect of the environment, some of the individuals of each
genotype will be made larger and others smaller in various degrees. This produces a further
“spreading out” of the frequency distribution. Any pattern of discrete variation in
the genotype frequency distribution is likely to be obscured by environmental effects
and the discrete categories converted into a smooth curve (Figure 9.4).
(a) No environmental effect (b) One environmental variable (c) Two environmental variables
Frequency
5
20 10 15 20 25 30
0 5 10 15 20 25 30 35 40
Character
Character Character
Figure 9.4
Environmental effects can produce continuous variation.
(a) Twenty-five individuals in the absence of environmental
variation all have the same phenotype, with a value for a
character of 20. (b) Influence of one environmental variable.
The variable has five states, and according to which state an
organism grows up in its character becomes larger or smaller
or is not changed. The five states change the character by +10,
+5, 0, −5, and −10, and an organism has an equal chance of
Frequency
5
4
3
2
1
experiencing any one of them. (c) Influence of a second
environmental variable. This variable also has five
equiprobable states, and they change the character by +10, +5,
0, −5, and −10. Of the five individuals in (b) with character
value 10, one will get another −10, giving a value of 0, a second
will get −5, giving 5, etc. After the influence of both variables,
the frequency distribution ranges from 0 to 40 and is beginning
to look bell curved. With many environmental influences, each
of small effect, a normal distribution will result.