Evolution__3rd_Edition

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Figure 5.1
The general model of
population genetics.
CHAPTER 5 / The Theory of Natural Selection 97
p = P + 1 /2Q (5.1)
q = R + 1 /2Q
(and p + q = 1). The calculation of the gene frequencies from genotype frequencies is
highly important. We shall make recurrent use of these two simple equations in the
chapter. Although the gene frequencies can be calculated from the genotype frequencies
(P, Q, R), the opposite is not true: the genotype frequencies cannot be calculated
from the gene frequencies (p, q).
Now that we have defined the key variables, we can see how population geneticists
analyze changes in those variables through time.
5.2 An elementary population genetic model has
four main steps
Population geneticists try to answer the following question: if we know the genotype
(or gene) frequencies in one generation, what will they be in the next generation? It
is worth looking at the general procedure before going into particular models. The procedure
is to break down the time from one generation to the next into a series of stages.
We then work out how genotype frequencies are affected at each stage. We can begin
at any arbitrarily chosen starting point in generation n and then follow the genotype
frequencies through to the same point in generation n + 1. Figure 5.1 shows the general
outline of a population genetics model.
We start with the frequencies of genotypes among the adults in generation n. The
first step is to specify how these genotypes combine to breed (called a mating rule); the
Generation n
Generation n + 1
(1) Genotype frequencies among adults
(2)
(3)
(4)
(5)
Mating rule
Frequencies of matings between genotypes
Mendelian ratios
Frequencies of genotypes in offspring of each type of mating
Add frequencies of each
genotype for all matings
Genotype frequencies at birth
Any selection by
differential survival
Genotype frequencies among adults