Tabla de Contenidos
The founder effect is a particular type of genetic drift in which the frequency of appearance of alleles in a population changes due to the separation of a small group of individuals, who establish a new community reproductively isolated from the first. This small group of individuals is called founders , comparing it to a group of people who decide to leave the place where they live and establish (or found) a new community elsewhere.
Once established in a new site or “founded” the new community, the founder group reproduces and passes on to the offspring its particular combination of alleles, which, due to the small size of the group compared to the original population, is not representative. of the entire population.
How and why does the allele frequency change?
The founder effect is a phenomenon similar to sampling errors. In each population there is a certain distribution of alleles for each phenotype. Some will be more common than others, while others will be present in only a very small fraction of the population.
If we take a small random sample from a population and analyze the alleles that are present in the sample, that sample may not have exactly the same genetic makeup (on average) as the entire population. This difference between the sample and the population is the sampling error, and it occurs when the selected sample is not representative of the population. These types of errors become larger as the sample size gets smaller.
This is exactly what happens with the founder effect. When a small portion of the population separates from it and settles elsewhere, it is as if a sample of the population were taken. If the group is very small, it will not be representative of the population, so it may have a completely different genetic makeup from that population.
How is the allele frequency affected?
The founder effect can have two radically different effects on the frequency of the different alleles. In some cases it can increase the proportion of a certain allele, because that allele turned out to be much more frequent than normal in the group of founders. On the other hand, it can also have the opposite effect, reducing the appearance of an allele or even making it disappear completely.
To understand how this can happen, consider the example of the following figure in which the different colored dots represent individuals with different alleles. The population on the left is a large population living on a landmass. A small group of this population colonizes a nearby island at any given time. The arrow in the figure shows the group that migrated.
In the original population on the left, the ratio of red, green, yellow, and purple alleles is 36:12:3:3, so the frequencies of the four alleles are, respectively, 36/54, 12/54, 3/54 and 3/54. However, if we look at the group of founders, we will realize that the proportions are very different. The small group contains only 9 individuals with the red allele, 4 with the green allele, 3 with the yellow allele, and none with the purple. In other words, the allele frequencies in this case are 9/15, 4/15, 3/15, and 0/15, respectively.
As can be seen, for the group of founders of the island subpopulation, the frequency of the red allele decreased compared to that of the original population, while that of the green and yellow alleles increased (especially that of yellow). On the other hand, the purple allele did not even reach the island, so it will completely disappear from this population.
The particular alleles that the group of founders take with them are completely random, that is, they do not respond to patterns of any kind, so there is no way to know in advance what the final effect will be. Some populations in which the founder effect is manifest have a much higher proportion of some recessive alleles compared to the general population. In other cases, some traits are lost (like the purple allele in the example) because, by sheer coincidence, the founders did not have that allele.
Examples of genetic drift due to the founder effect
the amish community
A clear example of the founder effect can be seen in the Amish community in the United States. This community is located in the state of Pennsylvania, and was founded by European immigrants of Swiss-German origin. The Amish descend from a very small group of founders, and they do not accept new members to the community that do not come from among their own, so they marry each other, which in many cases results in inbreeding. These characteristics isolate them from the rest of the population, which is why generation after generation the Amish have been characterized by presenting a very marked influence of the alleles contributed by the original founders.
As a consequence, among the Amish there are several genetic conditions, such as polydactyly, which appear much more frequently than in the rest of the American population.
The red deer of Corsica
This deer is currently found on the Tyrrhenian islands, but many investigations have determined that this population is genetically derived from the Sardinian population, where the original founders are presumed to have come from.
The South African population
The increased incidence of Huntington’s disease and Fanconi anemia in South Africa, compared with the rest of the general population, is believed to be due to the founder effect. It is assumed that the founding colonists carried the recessive mutations that cause these diseases, so they passed them on to their offspring.
