Founder effect: what it is and how it affects biological evolution

From the date of publication of "The Origin of Species" by the famous Charles Darwin in 1859, humans no longer conceive of living beings as immovable and static entities in their history evolutionary. According to the postulation of the theory of natural selection, living beings undergo random mutations throughout the generations, and some characters are fixed due to their usefulness, while others are discriminated and disappear with the weather.

For example, an individual of a particular bank colored moth species may undergo a mutation in a melanin-producing gene during its development and, therefore, present a completely colored black. If this trait is heritable and helps the specimen to remain hidden in the bark of trees for longer, it will reproduce more times, as its biological aptitude will have increased. Thus, this trait will spread throughout the population, as black moths will reproduce more than white moths. As simple as that.

On the other hand, the black color of the moth may attract the attention of predators more easily and the mutated individual is preyed upon as soon as it is born. In this case, it would die directly and the deleterious gene would disappear from the population's gene pool. With these foundations in place, we immerse ourselves in

the founder effect, or what is the same, the consequences derived from the existence of a very small population of a species in a given ecosystem.

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The Basics of Genetic Drift

As we have already said, Darwin postulated in "The Origin of Species" natural selection as the evolutionary engine of populations, but it is interesting to know that this is not the only mechanism in nature that varies the allelic frequencies of beings alive. Also we have genetic drift, a completely stochastic process that is a consequence of random sampling in reproduction and that, in general, tends to reduce genetic diversity organisms (homozygosity). Let's look at the same example cited above from another perspective.

Let's say we have a mini-population of 5 moths, 4 white and one black. It turns out that the color black is truly beneficial to the species because it allows excellent mimicry in the tree bark, but unfortunately, the mutated black specimen dies when it collides with the glass of a car. His color has had nothing to do with his death and, despite presenting a beneficial character, it is completely erased from the population.

Due to this "sampling error", completely viable alleles for a specific population can sometimes disappear, without attending to logical reasons or the mechanisms of natural selection. Anyway, it should be noted that genetic drift works much more strongly in small populations: If we had 5,000 moths in the cited population and 1,000 of them were black, the chances that all the black ones would disappear at random are much lower.

In explaining genetic drift, many more concepts play essential roles. Some of them are allele frequencies, effective population size, potential bottlenecks, etc. In any case, in the remaining lines we are going to focus on one of the most well-known causes of genetic drift in the world of zoology: the founder effect.

What is the founder effect?

The founder effect is one of the clearest causes of genetic drift mechanisms, along with the resource constraints in a given environment and the evolutionary bottleneck. In this specific case, we are talking about the loss of genetic information when a small section of a large population becomes independent from it on a different terrain.

Let's look for a new example, because the color of the moths does not give for more. Now, suppose we have a population of 200 birds, which make a transatlantic migration every year from continent to continent to reproduce. For whatever reason, on one of these demanding trips, 10 of these birds are separated from the flock initial in search of new territories and, exhausted, seek refuge on a small island in the middle of the nothing.

If this island has the necessary resources and there is a clear lack of predators, these 10 birds may settle on the island land and decide not to migrate. Thus, a new population of 10 specimens has been established from another that consisted of 200. The selection sampling has been completely random and, therefore, the allele frequency of the new individuals may be very different from that expected in the general population.

For example, 1 in 100 birds may have a larger bill than the rest and 1 in 50 are green instead of yellow. If it turns out that, as a result of randomness, 3 of these founding birds present these traits in a total population of 10, it is more than possible that these alleles will be fixed in future generations despite not being "the rule". So that, the founder effect can cause traits to be fixed in a species that, if it were a larger population, would never do so.

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The effects of the founder effect

As you can imagine, depending on the characteristics of the "founders", a deep rift can occur over time between the members of the original population and the new one. The thing becomes much more interesting if we consider that, in addition, natural selection is also likely to act on the alleles of the founders differently than those that are present in the large population.

If we continue with the previous example, it is clear that being 10 specimens in an exotic environment is nothing like living in a group of 200 in a continental terrain. Therefore, the selected atypical traits (large bill and green color) may be beneficial in the long term for those who carry them. For example, it occurs to us that a green hue could mimic the bird on the tops of palm trees, and a large beak would be very useful to break the coconuts and access food.

Thus, in addition to the "sampling error" of the selection itself, it is possible that the selection favors atypical genotypes (and phenotypes) over time due to the new impositions of the environment. Thus, the offspring of the founders would be greener and greener and with the highest peak statistically speaking, until reaching a point of maximum adaptation to the exploitation of their new niche. Remember that evolution does not create perfect beings, because said colloquially and sinning reductionists, "you do what you can with what you have".

In this concrete and perfect scenario, it could be expected that the island colonizers would end up being a subspecies and, later, their own species throughout the centuries. When a member of the island population is unable to reproduce with another of the original (either by anatomy, behavior, precigotic barriers and more) it can be said that both specimens belong, ultimately, to a different species. This is a clear example of how the founder effect can induce speciation in an island environment.

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Resume

We have presented you with an idyllic setting so that you understand what the founder effect is, but unfortunately nature does not usually work like that. One of the great weaknesses of small populations is that they tend to homozygosity and inbreeding, it is In other words, that genetic variability is lost throughout the generations due to the lack of reproductive individuals not familiar. Thus, it is most likely that a population of 10 specimens will never start and, if it does, the offspring 3-4 generations later will end up not being viable.

It is also possible that, for whatever reason, a character that previously increased evolutionary viability ceases to do so over time.

If there is no genetic diversity (if the same alleles are always fixed), all individuals in a small population will be more or less equally prepared for environmental changes, so the risk of extinction is multiplied considerably. The founder effect can promote speciation, but also the total disappearance of a population due to lack of genetic diversity.

Bibliographic references:

• Greenbaum, G., Templeton, A. R., Zarmi, Y., & Bar-David, S. (2014). Allelic richness following population founding events – a stochastic modeling framework incorporating gene flow and genetic drift. PloS one, 9 (12), e115203.
• King, T. E., & Jobling, M. TO. (2009). Founders, drift, and infidelity: the relationship between Y chromosome diversity and patrilineal surnames. Molecular Biology and Evolution, 26 (5), 1093-1102.
• Pardo, L. M., MacKay, I., Oostra, B., van Duijn, C. M., & Aulchenko, Y. S. (2005). The effect of genetic drift in a young genetically isolated population. Annals of human genetics, 69 (3), 288-295.
• Slatkin, M., & Excoffier, L. (2012). Serial founder effects during range expansion: a spatial analog of genetic drift. Genetics, 191 (1), 171-181.
• Whitlock, M. C. (1997). Founder effects and peak shifts without genetic drift: adaptive peak shifts occur easily when environments fluctuate slightly. Evolution, 51 (4), 1044-1048.