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

This is how genetic drift, one of the drivers of Biological evolution, works.

Biological evolution, conceived as the set of changes in hereditary characteristics in living beings over 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 the DNA that occur randomly, i.e., it does not respond to a conscious mechanism. Even so, the selection processes of these mutations can be random, or on the contrary, have a completely well-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 all about this fascinating process.

What is genetic drift?

First of all, we can define this complex term as "an evolutionary force that acts together with natural selection by changing the allele frequencies of species over time". As a preface, it should be noted that it is a stochastic process, i.e., it happens due to chance or uncorrelated sporadic 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 this in more detail in the following lines.

About alleles and population

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

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

Secondly, it is necessary to explore a little the term "population" in the field of biology, because genetic drift acts on populations and not on the species itself.. A species is a "closed" concept, since it cannot exchange genes with other distinct entities. On the other hand, a population is conceived as an "open" compartment, since different members of other populations but of the same species can enter and reproduce among themselves, an event that will be of vital importance in later lines. Once we have grounded both terms in a general way, we are ready to understand the basis of genetic drift.

Theoretical foundation of drift

It's time to hold on to your seat, as there are curves ahead and terms that are a bit complex to explain. Genetic drift is determined by the variance of the allele frequency, i.e. the variability of traits with respect to the mean.. Thus, we can calculate this evolutionary force by using the following formula:

• sp2 corresponds to the variance of the allele frequencies of the populations, i.e. genetic drift per se.
• p and q are the allele frequencies of two populations for one character.
• N is the number of individuals within each of the two populations.

Of course, each of these parameters is obtained by complex formulas, so we will not focus further on the mathematical basis of this evolutionary force. If one idea should 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..

Effective population size

We have introduced a key term in the previous paragraph: population size. The truth is that, when it comes to taking into account the magnitude of genetic drift, it is not enough for scientists to simply count the individuals in a population. In these cases we have to quantify, in a reliable way, the number of animals that reproduce within the population..

A very clear example of the difference between total population and effective population size is the demographic studies of amphibians. A common toad population, for example, may be composed of 120 members. If we resort to genetic analysis, we can observe that, surely, only about 40 total adults reproduce annually, leaving offspring at most. 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 populations of living beings, but we can divide them into two large blocks:

• It produces a change in allele frequencies within the population. This can mean that these increase or decrease, as it is a matter of pure chance.
• It reduces the genetic variation of populations in the long term.

This last point is of essential importance, because genetic drift decreases variability, which ultimately translates into increased vulnerability of the population to environmental change.. Let us take a practical example.

If we have in a fictitious population of 10 birds, 8 of red color and 2 of yellow color, it is natural to think that, by pure chance, it is more probable that in the following generation the red members will be more represented (because if only 3 of those 10 reproduce, there is the possibility that all 3 will be red). In the first generation, the allele 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 red males and females reproduce in an event, theoretically the q allele could disappear in the next generation, so that p=1 and q=0, with all the offspring being red (the p trait would have been fixed). This is the real effect of the genetic drift, which by chance, produces a fixation of the most distributed characters in the population and ends up discarding the most unique ones..

The salvation of populations

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

It should also be noted that the example cited above is rather lame because of self-imposed reductionism, since clearly many morphological characters are encoded by more than one gene (such as eye color, for example). Moreover, in a population of 1000 individuals and not 10, it is clear that the disappearance of an allele is much more complex than its "deletion" in a single generation.

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

Finally, it should be noted that mutations occur randomly in living beings.. Thus, variations can arise in the DNA that code for new alleles, which is why (at least theoretically) new characters can continue to appear sporadically in a closed population.

Summary

As we have seen, genetic drift is the main evolutionary engine of living beings together with natural selectionbut differs from the latter due to its random and haphazard nature. From a purely theoretical point of view, if there were no events such as gene flow, the appearance of mutations or natural selection itself, all populations would end up having only one allele of 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 environmental changes and inclemencies. Thus, genetic drift is counteracted by life itself, because of course, it presents a clear deleterious character.

Bibliographical references:

• Genetic drift, khanacademy.org. Retrieved October 23 from https://es.khanacademy.org/science/ap-biology/natural-selection/population-genetics/a/genetic-drift-founder-bottleneck#:~:text=The%20g%20g%C3%A9nical%20drift%20happens%20in,0%25%20%2C%20of%20other%20alleles.
• Eguiarte, L., Aguirre-Planter, E., Scheinvar, E., González, A., & Souza, V. (2010). Gene flow, differentiation and population genetic structure, 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). Biologia evolutiva (Vol. 2). 2. ed. Ribeirão Preto: SBG.