Founder effect: what is it and how does it affect biological evolution?

The founder effect explains how many groups of organisms evolve by colonizing new areas

Since the 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 evolutionary history. According to the postulation of the theory of natural selection, living beings undergo random mutations over generations, and some characters become fixed due to their usefulness, while others are discriminated against and disappear over time.

For example, an individual of a particular species of bank-colored moth may undergo a mutation in a melanin-producing gene during its development and thus become completely black. If this trait is heritable and helps the specimen to remain hidden in the bark of trees longer, it will reproduce more times, as its Biological fitness will have increased. Thus, this trait will spread through the population, as black moths will reproduce more on average than white moths. It is 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 will be predated as soon as it is born. In this case, it would die directly and the deleterious gene would disappear from the population gene pool. With this foundation in place, we now dive into the founder effect, or, in other words, the consequences derived from the existence of a very small population of a species in a given ecosystem.

The basics of genetic drift

As we have already mentioned, in "The Origin of Species" Darwin postulated 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 allele frequencies of living beings. Also genetic drift, a completely stochastic process that is a consequence of random sampling in reproduction and that, in general, tends to reduce the genetic diversity of organisms (homozygosity) of 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 1 black. It turns out that the black color is really beneficial for the species because it allows an excellent mimicry in the bark of trees, but unfortunately, the mutated black specimen dies when it hits the glass of a car. Its color has had nothing to do with its demise and, despite having a beneficial character, it is completely erased from the population.

Because of this "sampling error", completely viable alleles can sometimes disappear for a given population, without any logical reasons or the mechanisms of natural selection. In any case, it should be noted that genetic drift works much more strongly in small populationsIf we had 5,000 moths in the above population and 1,000 of them were black, the chances that all the black moths would disappear at random are much lower.

When it comes to explaining genetic drift, many more concepts play essential roles. Some of them are allele frequencies, the effective population size, possible bottlenecks, etc. However, in the remaining lines we will 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, together with resource limitations in a given environment and the evolutionary bottleneck. In this particular case, we are talking about the loss of genetic information when a small section of a large population becomes independent of it in a different terrain.

Let's look for a new example, because the color of moths is not enough. 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 journeys, 10 of these birds break away from the initial flock in search of new territories and, exhausted, seek refuge on a small island in the middle of nowhere.

If this island has the necessary resources and there is a clear lack of predators, these 10 birds may settle on the island terrain and decide not to migrate. Thus, a new population of 10 birds has been established from a population 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 beak than the rest and 1 in 50 may be green instead of yellow. If it turns out that, as a result of randomness, 3 of these founder birds have these traits in a total population of 10, it is more than possible that these alleles will be fixed in future generations even though they are not "the norm". Thus, the founder effect can cause traits to become fixed in a species that, if it were a larger population, would never be fixed.

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The effects of the founder effect

As you can imagine, depending on the characteristics of the "founders", a deep split can occur over time between members of the primordial population and the new one. It becomes much more interesting if we take into account that, in addition, natural selection is also likely to be at work, natural selection is also likely to act on the alleles of the founders in a different way than those present in the large population.

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

Thus, in addition to the "sampling error" of selection itself, it is possible that selection favors atypical genotypes (and phenotypes) over time because of new environmental impositions. Thus, the offspring of the founders would be increasingly greener and with statistically larger peak, until they reach a point of maximum fitness to exploit their new niche. Remember that evolution does not create perfect beings, because, to put it colloquially and erring on the side of reductionism, "we do what we can with what we have".

In this specific and perfect scenario, we could expect that the island colonizers would end up being a subspecies and, later, a species of their own over the centuries. When one member of the island population is unable to reproduce with another member of the primordial population (whether due to anatomy, behavior, prezygotic barriers and more) it can be said that both specimens ultimately belong to a different species. This is a clear example of how the founder effect can induce speciation in an insular environment.

Summary

We have presented you with an idyllic scenario for you to understand what the founder effect is, but unfortunately nature doesn't usually work that way. One of the great weaknesses of small populations is that they tend to homozygosity and inbreeding, i.e., genetic variability is lost over generations due to the lack of unfamiliar breeding individuals. Thus, it is most likely that a population of 10 individuals will never start and, if it does, the offspring 3-4 generations ahead will end up not being viable.

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

If there is no genetic diversity (if the same alleles are always fixed), all the individuals of a small population will be more or less equally prepared to environmental changes, so that 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.

Referencias bibliográficas:

• 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. A. (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.