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 driving force of life itself and adaptation to new environments.
The variation within a population of living beings occurs due to a series of mutations in DNA that occur randomly, that is, they do not respond to a conscious mechanism. Even so, the selection processes of these mutations can be random, or on the contrary, have a completely substantiated 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 usually escapes general understanding is the term that concerns us here: genetic drift Here we explain everything about this fascinating process.
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, 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.
Since most animals are diploid beings (which have two sets of homologous chromosomes in their nucleus), each of the two alleles coding for a character will come from the father and the 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 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 the term “population” a little in the field 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 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 established 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, because there are curves and terms that are a bit complex to explain. genetic drift It is determined by the variance of the allelic frequency, that is, the variability of characters with respect to the average Thus, we can calculate this evolutionary force by using the following formula:
Of course, each of these parameters is obtained through complex formulas, so we are not going to focus further on the mathematical foundation of this evolutionary force. If one idea is 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 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 The number of animals that reproduce within it must be quantified reliably
A very clear example of the difference between total population and effective population size are demographic studies of amphibians. 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 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 the populations of living beings, but we can divide them into two large blocks:
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 a fictitious population of 10 birds, 8 red and 2 yellow, it is natural to think that, by pure chance, it is more likely that in the next generation the red members will appear more represented (because if of those 10 only 3 are reproduced, there is a possibility that all 3 are 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 red males and females reproduce in an event, theoretically the allele q could disappear in the next generation, so p=1 and q=0, with all the offspring being red (the character p would have been fixed). 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 ones
The salvation of the populations
Luckily, we have a force that largely prevents 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 is also necessary to note that the example cited above suffers quite a bit from self-imposed reductionism, since clearly many morphological characters are coded by more than one gene (such as eye color, 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 with different alleles appear and reproduce with the individuals of the initial population, renewed genetic variability is introduced in subsequent generations.
Finally, it is necessary to note 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) in a closed population new characters can continue to appear sporadically.
Summary
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 natural selection itself, all populations would end up having a single 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 inclement conditions. Thus, genetic drift is counteracted by life itself, since it certainly has a clear deleterious character.
