badshah

1. Mutation pressure
Mutation pressure is the change in allele frequencies due to the repeated occurrence of the same mutations.
There are not many biologically realistic situations where mutation pressure is the most important
evolutionary process while random drift will usually be more important. However, sometimes the mutation
rate is high enough that mutation pressure need to be considered; in addition, it provides a simple illustration
of a population genetic equilibrium.
Mutation pressure theory
A quantitative theory of directional mutation pressure proposed in 1962 explained the wide variation of
DNA base composition observed among different bacteria and its small heterogeneity within individual
bacterial species. The theory was based on the assumption that the effect of mutation on a genome is not
random but has a directionality toward higher or lower guanine-plus-cytosine content of DNA, and this
pressure generates directional changes more in neutral parts of the genome than in functionally significant
parts. Now that DNA sequence data are available, the theory allows the estimation of the extent of neutrality
of directional mutation pressure against selection. Newly defined parameters were used in the analysis, and
two apparently universal constants were discovered. Analysis of DNA sequence has revealed that practically
all organisms are subject to directional mutation pressure. The theory also offers plausible explanations for
the large heterogeneity in guanine-plus-cytosine content among different parts of the vertebrate genome.
Explaination
Imagine a population in which all individuals have the same allele ("red"), but there is a high rate of
mutation to a second allele ("blue"). At each generation, some red alleles will mutate and will become blue
alleles. The frequency of the blue alleles will therefore increase over time.
This process, under which allele frequencies change solely due to the same mutations occurring over and
over, is known as mutation pressure. For most kinds of genetic variation in most populations, random drift is
more important than mutation pressure; the changes in allele frequency from one generation to the next due
to random drift will be much larger than the changes due to mutation pressure.
In order for mutation pressure to play an important role in changing allele frequencies, the mutation rate
has to be relatively high. Some organisms, such as RNA viruses (including HIV), have extremely high
mutation rates. In other organisms, some categories of mutations have mutation rates that are high enough
that mutation pressure becomes important. For example, microsatellites are stretches of short sequence
repeated over and over, such as GAGAGAGAGAGAGA. Through a process known as strand slippage,
mutations that increase or decrease the number of repeats in a microsatellite occur often enough that you
would have to take mutation pressure into account when modeling the evolution of a microsatellite.
This simulation models mutation in a population of 20 haploid individuals. Each individual has exactly
one offspring, so there is no random drift or selection; this is unlikely in the real world, but possible for
some organisms in laboratory experiments. The empty red squares represent individuals with one allele, and
the filled blue squares are a different allele. The population starts out with all red alleles. Set the red-to-blue
mutation rate greater than 0 and less than 1, and the blue-to-red mutation rate from 0 to less than 1. If the
blue-to-red mutation rate is 0, you should see that the population will eventually consist of all blue alleles,
because sooner or later, each lineage will have a red-to-blue mutation. If the blue-to-red mutation rate is
greater than 0, the population should reach an equilibrium, with a mixture of red and blue alleles.
It is possible to calculate what the equilibrium allele frequency should be. At equilibrium, you would
expect the allele frequency to remain the same from one generation to the next, on average. In other words,
the average change in allele frequency from one generation to the next should be 0. Another way of stating

2. this is that at equilibrium, the proportion of alleles that mutate from red to blue is equal to the proportion that
mutate from blue to red.
The average proportion of red-to-blue mutations in the population is given by the proportion of alleles
that are red, pr, times the proportion of red alleles that mutate to blue (the red-to-blue mutation rate), μrb. The
proportion of alleles that are blue is 1−pr, so the average proportion of blue-to-red mutations is (1−pr)×μbr.
At equilibrium,
In other words, the expected proportion of red alleles is equal to the proportion of all mutations that are blue-
to-red mutations. For example, if the blue-to-red mutation rate is 0.002 and the red-to-blue rate is 0.005, the
expected proportion of red alleles is