Evolution__3rd_Edition

228 PART 2 / Evolutionary Genetics
Continuous characters are studied
by quantitative genetics
The value of a character is
expressed as a deviation from
the mean
The small effects of many genes and environmental variables are two separate
influences that tend to convert the discrete phenotypic distribution of characters controlled
by single genes into continuous distributions. If a character shows a continuous
distribution, it in principle could be because of either process. Quantitative genetics is
mainly concerned with characters influenced by both. Quantitative genetics employs
higher level genetic concepts that are genetically less exact than those of one- or twolocus
population genetics, but which are more useful for understanding evolution in
polygenic characters. Instead of following changes in the frequency of genes or haplotypes,
we now follow changes in the frequency distribution of a phenotypic character.
Quantitative genetics is important because so many characters have continuous variation
and multilocus control.
9.3 Variation is first divided into genetic and
environmental effects
Quantitative genetics contains an unavoidable minimum of formal concepts that we
need to understand before we can put it to use: those formalities are the topic of this
section and the next. To understand how a quantitative character like beak size will
evolve, we have to “dissect” its variation. We tease apart the different factors that cause
some birds to have larger beaks than others. Suppose, for example, that all the variation
in beak size was caused by environmental factors a that is, all birds have the same
genotype and they differ in their beak sizes only because of the different environments
in which they grew up. Beak size could not then change during evolution (except for
non-genetic evolution due to environmental change). For the character to evolve, it has
to be at least partly genetically controlled. We need to know how much beak size varies
for environmental, and how much for genetic, reasons. However, even if different
finches vary in their beak size for genetic reasons, that does not necessarily mean it can
evolve by natural selection. As we shall see, we have to divide genetic influence into
components that allow evolutionary change and those that do not.
In quantitative genetics, the value of a character in an individual is always expressed
as a deviation from the population mean. Beak size will have a certain mean value in a
population, and we talk about environmental and genetic influences on an individual
as deviations from that mean. The procedure is easy to understand if we think of the
population mean as a “background” value, and then the influences leading to a particular
individual phenotype are expressed as increases or decreases from that value. Let us
see how it is done. Suppose there is one locus with two alleles influencing beak depth.
AA and Aa individuals’ beaks are 1 cm from top to bottom, and aa individuals’ beaks
0.5 cm; the environment has no effect. If the population average was 0.875 cm (as it
would be for a gene frequency of a = 1 /2), then we should write the beak phenotype
of AA and Aa individuals as +0.125 cm and that of aa as −0.375 cm. In general, we
symbolize the phenotype by P. In this case, P =+0.125 for AA and Aa individuals
and P =−0.375 for aa.
Clearly, the value of P for a genotype depends on the gene frequencies. When the
frequency of A is one-half, the genotypic effects are those just given. But when the
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