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 (random drift will usually be more important), so you can skip this page if you want. 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.
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. The graph below models a population in which each individual has exactly one offspring (this is an extremely unrealistic model of a natural population, but it does represent a laboratory experiment in mutation accumulation). If you set the red-to-blue mutation rate to some number, such as 0.05, and the blue-to-red mutation rate to 0, you'll see that eventually, the frequency of blue alleles goes from 0 to 1. The higher the mutation rate, the faster the allele frequency will increase.
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; if you were studying the changes in allele frequency in the population of HIV in a single patient, you would have to consider mutation pressure along with drift and natural selection. 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 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,
A little algebra, and this becomes
pr = μbr/(μrb + μbr).
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 0.002/(0.002+0.005)=0.29.
This simulation is the same as the previous one, except it has 100 haploid individuals and runs for 1000 generations. The graph shows the frequency of the red allele through time. The dotted horizontal line shows the equilibrium frequency of the red allele, as calculated using the equation above. Because mutation is a random process, the allele frequency will change in an irregular fashion, but eventually it will approach the equilibrium value.
With high mutation rates, the allele frequency will reach equilibrium quickly. With lower mutation rates, the allele frequency will take much longer to reach equilibrium; it may take much longer than the 1000 generations shown on this graph.
This page was last revised March 6, 2009. Its address is http://udel.edu/~mcdonald/evolmutpress.html.
©2009 by John H. McDonald. You can probably do what you want with this content; see the permissions page for details.