Evolution__3rd_Edition

104 PART 2 / Evolutionary Genetics
Generation n (1) Genotype frequencies among adults
Generation n + 1
(2)
(3)
Population genetic models specify
the fitness of all genotypes
Hardy–Weinberg
theorem
Genotype freqencies at birth
Differential
survival
Genotype frequencies among adults
frequencies: the genotype frequencies at birth in the next generation (stage 2) must
then have Hardy–Weinberg frequencies, because the gene frequencies do not change
between the adults of one generation and the newborn members of the next generation.
A simple model of selection can concentrate on how the genotype frequencies are
modified between birth and the adult reproductive stage (from stage 2 to stage 3 of
Figure 5.3).
5.6 The simplest model of selection is for one favored
allele at one locus
We shall start with the simplest case. It is the case of natural selection operating on only
one genetic locus, at which there are two alleles, one dominant to the other. Suppose
that individuals with the three genotypes have the following relative chances of survival
from birth to the adult stage:
Genotype Chance of survival
AA, Aa 1
aa 1 − s
Figure 5.3
The general model of population genetics simplified by the
Hardy–Weinberg theorem.
s is a number between 0 and 1, and is called the selection coefficient. Selection
coefficients are expressed as reductions in fitness relative to the best genotype. If s is 0.1
then aa individuals have a 90% chance of survival, relative to 100% for AA and Aa individuals.
These are relative values: in a real case the chance of survival from birth to
reproduction of an individual with the best genotype might be 50%, much less than
100%. If it was 50%, then an s of 0.1 would mean that aa individuals really had a 45%
chance of survival. (The convention of giving the best genotype a relative 100% chance
of survival simplifies the algebra. If you are suspicious, check whether it makes any difference
in what follows if the chances of survival are 50%, 50%, and 45% for AA, Aa,
and aa, respectively, rather than 100%, 100%, and 90%.) The chance of survival is the
fitness of the genotype (we are assuming that all surviving individuals produce the same
number of offspring). Fitnesses are, like the chances of survival, expressed relative to a
figure of 1 for the best genotype. This can be spelled out more by referring to fitnesses as
..