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Evolution__3rd_Edition

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104 PART 2 / <strong>Evolution</strong>ary Genetics<br />

Generation n (1) Genotype frequencies among adults<br />

Generation n + 1<br />

(2)<br />

(3)<br />

Population genetic models specify<br />

the fitness of all genotypes<br />

Hardy–Weinberg<br />

theorem<br />

Genotype freqencies at birth<br />

Differential<br />

survival<br />

Genotype frequencies among adults<br />

frequencies: the genotype frequencies at birth in the next generation (stage 2) must<br />

then have Hardy–Weinberg frequencies, because the gene frequencies do not change<br />

between the adults of one generation and the newborn members of the next generation.<br />

A simple model of selection can concentrate on how the genotype frequencies are<br />

modified between birth and the adult reproductive stage (from stage 2 to stage 3 of<br />

Figure 5.3).<br />

5.6 The simplest model of selection is for one favored<br />

allele at one locus<br />

We shall start with the simplest case. It is the case of natural selection operating on only<br />

one genetic locus, at which there are two alleles, one dominant to the other. Suppose<br />

that individuals with the three genotypes have the following relative chances of survival<br />

from birth to the adult stage:<br />

Genotype Chance of survival<br />

AA, Aa 1<br />

aa 1 − s<br />

Figure 5.3<br />

The general model of population genetics simplified by the<br />

Hardy–Weinberg theorem.<br />

s is a number between 0 and 1, and is called the selection coefficient. Selection<br />

coefficients are expressed as reductions in fitness relative to the best genotype. If s is 0.1<br />

then aa individuals have a 90% chance of survival, relative to 100% for AA and Aa individuals.<br />

These are relative values: in a real case the chance of survival from birth to<br />

reproduction of an individual with the best genotype might be 50%, much less than<br />

100%. If it was 50%, then an s of 0.1 would mean that aa individuals really had a 45%<br />

chance of survival. (The convention of giving the best genotype a relative 100% chance<br />

of survival simplifies the algebra. If you are suspicious, check whether it makes any difference<br />

in what follows if the chances of survival are 50%, 50%, and 45% for AA, Aa,<br />

and aa, respectively, rather than 100%, 100%, and 90%.) The chance of survival is the<br />

fitness of the genotype (we are assuming that all surviving individuals produce the same<br />

number of offspring). Fitnesses are, like the chances of survival, expressed relative to a<br />

figure of 1 for the best genotype. This can be spelled out more by referring to fitnesses as<br />

..

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