Genetic drift: what is it and how does it affect biological evolution?

Biological evolution, conceived as the set of changes in hereditary characteristics in living beings throughout generations, is the engine of life itself and adaptation to new environments.

Variation within a population of living beings is due to a series of mutations in DNA that happen randomly, that is, it does not respond to a conscious mechanism. Even so, the selection processes for these mutations can be random, or on the contrary, have a fully founded explanation.

Thus, evolution is a force that is maintained by random mutations, genetic recombination during sexual reproduction and gene flow (entry of new members into a population), among many other factors. One of these factors of essential importance that often escapes general understanding is the term that concerns us here: genetic drift. Here we explain everything about this fascinating process.

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What is genetic drift?

First, we can define this complex term as "an evolutionary force that acts in conjunction with natural selection by changing the allelic frequencies of species over time." As a preface, it should be noted that

it is a stochastic process, that is, it happens due to chance or sporadic uncorrelated effects.

In addition to this initial characteristic, another of the terms that define genetic drift is loss, since this selection force promotes the variation of alleles in the population, fixing some and promoting the disappearance of others. We will see it in more detail in the following lines.

About alleles and population

Simplifying genetic terms, we can affirm that an allele is each of the variations that the same gene can present. A classic example of this is the pea seeds used by Mendel to explain genetic segregation across generations. A dominant "A" allele can code for a green seed color, while a recessive "a" allele codes for a yellow color.

As most animals are diploid beings (which have two sets of homologous chromosomes in their nucleus), each of the two coding alleles of a character will come from the father and mother respectively, which is why the possible variations in this case would be the following: AA, Aa and aa. Thus, if we understand that an individual inherits two alleles for each gene, his phenotype (external characteristics) will come directly encoded by their genotype (allelic combinations in their genome), which is inherited as a combination of those of His parents.

Second, the term “population” needs to be explored a bit in the realm of biology, since genetic drift acts on populations and not on the species itself. A species is a "closed" concept, since it cannot exchange genes with other different entities. On the other hand, a population is conceived as an "open" compartment, since different members of other members can enter. populations but of the same species and reproduce among them, an event that will be of vital importance in lines later. Once we have established both terms in a general way, we are ready to understand the foundation of genetic drift.

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Theoretical foundation of drift

It is time to hold on to the seat, because curves and terms come a little complex to explain. Genetic drift is determined by the variance of the allelic frequency, that is, the variability of characters with respect to the mean. Thus, we can calculate this evolutionary force using the following formula:

• sp2 corresponds to the variance of the allele frequencies of the populations, that is, the genetic drift itself.
• p and q are the allele frequencies of two populations for a character.
• N is the number of individuals within each of the two populations.

Of course, each of these parameters is obtained through complex formulas, so we are not going to focus more on the mathematical foundation of this evolutionary force. If an idea has to be clear after reading these lines, it is the following: the smaller the population size, the more power genetic drift will have over its members.

The effective population size

We have introduced a key term in the previous paragraph: population size. The truth is that, when taking into account the magnitude of genetic drift, scientists are not only worth counting the individuals in a population. In these cases the number of animals that reproduce within it must be reliably quantified.

A very clear example of the difference between total population and effective population size is amphibian demographic studies. A common toad population, for example, can be made up of 120 members. If we resort to genetic analysis, we can observe that, surely, only about 40 total adults reproduce annually, leaving a maximum of offspring. Thus, the effective population size (Ne) that would suffer the effects of drift would be 40, not 120.

The effects of genetic drift

Genetic drift has several effects on the populations of living beings, but we can divide them into two large blocks:

• Produces a change in allele frequencies within the population. This may mean that these increase or decrease, as it is a matter of pure chance.
• Reduces long-term genetic variation in populations.

This last point is of essential importance, since genetic drift decreases variability, which ultimately translates into greater vulnerability of the population to environmental changes. Let's take a practical example.

If we have in a fictitious population of 10 birds, 8 red and 2 yellow, it is natural to think that, by pure chance, it is more likely than in the next generation, the red members appear more represented (because if of those 10 only 3 are reproduced, there is the possibility that all 3 are colored Red). In the first generation, the allelic frequency of the red character "p" would be 0.8, while the yellow character "q" would have a frequency of 0.2.

If only 3 males and females of red color reproduce in an event, theoretically the allele q could disappear in the next generation, so p = 1 and q = 0, all descendants being red (the character p would have set). This is the real effect of genetic drift, which by chance, it produces a fixation of the most distributed characters in the population and ends up discarding the most unique.

The salvation of the populations

Fortunately, we have a force that largely avoids this random selection: natural selection. In this case, we are facing an evolutionary engine that does not correspond at all to random and stochastic processes, since the characteristics of each individual can determine their survival, reproduction and consequent representation in future generations.

It should also be noted that the example cited above is quite limping due to self-imposed reductionism, since clearly many morphological characters are encoded by more than one gene (such as the color of the eyes, for example). Furthermore, in a population of 1000 individuals and not 10, it is clear that the disappearance of an allele is much more complex than its “erasure” in a single generation.

On the other hand, gene flow is another key concept that avoids the effects of genetic drift. An allele could be fixed in a population over time, but if new members appear with different alleles and reproduce with the individuals of the initial population, a renewed genetic variability is introduced in the following generations.

Finally, it is necessary to limit that mutations occur randomly in living things. Thus, variations may arise in the DNA that code for new alleles, which is why (at less theoretically) in a closed population, new characters may continue to appear in a sporadic.

Resume

As we have seen, genetic drift it is the main evolutionary engine of living beings along with natural selection, but it differs from the latter due to its haphazard and random nature. From a purely theoretical point of view, if there were no events such as gene flow, the appearance of mutations or the natural selection, all populations would end up having only one allele for each gene, even if it took many generations.

This, naturally, translates into less genetic variability, which means a worse response at the population and individual level to changes and environmental inclemency. Thus, genetic drift is counteracted by life itself, since of course, it has a clear deleterious character.

Bibliographic references:

• Genetic Drift, khanacademy.org. Picked up on October 23 at https://es.khanacademy.org/science/ap-biology/natural-selection/population-genetics/a/genetic-drift-founder-bottleneck#:~:text=La%20deriva%20g%C3%A9nica%20sucede%20en, 0% 25% 20% 2C% 20de% 20other% 20alelos.
• Eguiarte, L., Aguirre-Planter, E., Scheinvar, E., González, A., & Souza, V. (2010). Gene flow, differentiation and genetic structure of populations, with examples in Mexican plant species. Laboratory of Molecular and Experimental Evolution, Department of Evolutionary Ecology, Institute of Ecology, National Autonomous University of Mexico, 1-30.
• Futuyma, D. J. (1992). Evolutionary Biology (Vol. 2). 2. ed. Ribeirão Preto: SBG.