GENETIC DRIFT: CAUSES, EFFECTS, EXAMPLES - BIOLOGY - 2025

The genetic drift or gene is a stochastic evolutionary mechanism that causes fluctuations or variations purely random allelic frequencies in a population.

Charles Darwin's natural selection and gene drift are the two most important processes involved in evolutionary change in populations. Contrary to natural selection, which is considered a deterministic and non-random process, gene drift is a process evidenced as the random fluctuations of allele frequencies in the population or of haplotypes.

Source: Anjile, from Wikimedia Commons

Gene drift leads to non-adaptive evolution. In fact, natural selection - and not gene drift - is the only mechanism used to explain all the adaptations of organisms at different levels (anatomical, physiological or ethological).

This does not mean that gene drift is not important. One of the most striking consequences of this phenomenon is observed at the molecular level, between the differences in the DNA and protein sequences.

History

The theory of gene drift was developed in the early 1930s by a leading biologist and geneticist named Sewal Wright.

Similarly, Motoo Kimura's contributions were exceptional in this field. This researcher led the neutral theory of molecular evolution, where he exposes that the effects of gene drift have an important contribution to evolution at the level of DNA sequences.

These authors devised mathematical models to understand how gene drift works in biological populations.

Causes

The causes of gene drift are stochastic phenomena - that is, random. In the light of population genetics, evolution is defined as a variation over time in the allele frequencies of the population. Drift results in a change in these frequencies by random events called “sampling errors”.

Gene drift is considered a sampling error. The genes that are included in each generation is a sample of the genes carried by the previous generation.

Any sample is subject to error in sampling. That is, the proportion of the different items that we find in a sample are subject to change by pure chance.

Let's imagine we have a bag with 50 white chips and 50 black chips. If we take ten of these, maybe by pure chance we get 4 white and 6 black; or 7 white and 3 black. There is a discrepancy between the theoretically expected values ​​(5 and 5 of each color) and those obtained experimentally.

Effects edit

The effects of gene drift are evidenced as random changes in the allele frequencies of a population. As we mentioned, this occurs when there is no relationship between the characteristic that is varying and fitness. With the passage of time, the alleles will end up being fixed or lost from the population.

In evolutionary biology, the term fitness is widely used, referring to the ability of an organism to reproduce and survive. The parameter varies between 0 and 1.

Thus, the characteristic that varies by drift is not related to the reproduction and survival of the individual.

The loss of alleles leads to the second effect of gene drift: the loss of heterozygosity in the population. Variation at a certain locus will decrease, and eventually it will be lost.

How do we calculate the probability that an allele is lost or fixed?

The probability of an allele being fixed in the population is equal to its frequency at the time it is studied. The frequency of attachment of the alternative allele will be 1 - p. Where p is equal to the allelic frequency.

This frequency is not affected by the previous history of the change in allele frequencies, so predictions based on the past cannot be made either.

If, on the contrary, the allele has arisen by mutation, its probability of fixation is p = 1/2 N. Where N is the number of the population. This is the reason why new alleles that appear by mutation are easier to fix in small populations.

The reader must reason how the value of p would affect when the denominator is smaller. Logically, the probability would increase.

Thus, the effects of gene drift proceed more rapidly in small populations. In a diploid population (two sets of chromosomes, like us humans), the attachment of new alleles occurs, on average, every 4 N generations. Time increases proportionally with increasing N in the population.

Source: Professor marginalia, from Wikimedia Commons

Effective population number

The N that appears in the previous equations does not refer to a value identical to the number of individuals that make up the population. In other words, it is not equivalent to the census of organisms.

In population genetics, the parameter of “effective population number” (Ne) is used, which is normally less than all individuals.

For example, in some populations with a social structure dominated by only a few males the effective population number is very low, since the genes of these dominant males contribute disproportionately - if we compare them with the rest of the males.

For this reason, the speed with which gene drift acts (and the speed with which heterozygosity is lost) will be greater than expected if we carry out a census, since the population is smaller than it appears to be.

If in a hypothetical population we count 20,000 individuals, but only 2,000 reproduce, the effective number of the population decreases. And this phenomenon in which not all organisms occur in the population, is widely distributed in natural populations.

Bottlenecks and founder effect

As we mentioned (and demonstrated mathematically), drift occurs in small populations. Where alleles that are not so frequent have a greater probability of being lost.

This phenomenon is common after the population experiences an event called a “bottleneck”. This occurs when a significant number of members of the population are eliminated by some type of unforeseen or catastrophic event (for example, a storm or an avalanche).

The immediate effect could be a decrease in the genetic diversity of the population, reducing the size of the gene pool or gene pool.

A particular case of bottlenecks is the founder effect, where a small number of individuals separate from the initial population and develop in isolation. In the examples that we will present later, we will see what the consequences of this phenomenon are.

Source: Anjile, from Wikimedia Commons

Effect at the DNA level: neutral theory of molecular evolution

The neutral theory of molecular evolution was proposed by Motoo Kimura. Prior to the ideas of this researcher, Lewontin & Hubby had already found that the high proportion of variations at the enzyme level could not actively maintain all these polymorphisms (variations).

Kimura concluded that these amino acid changes could be explained by gene drift and mutations. He concludes that at the DNA and protein level, gene drift mechanisms play a fundamental role.

The term neutral refers to the fact that the majority of base substitutions that manage to fix (reach a frequency of 1) are neutral with respect to fitness. For this reason, these variations that occur by drift have no adaptive meaning.

Why are there neutral mutations?

There are mutations that do not have an effect on the phenotype of the individual. All information to build and develop a new organism is encrypted in DNA. This code is deciphered by ribosomes in the process of translation.

The genetic code is read in "triplets" (set of three letters) and every three letters code for an amino acid. However, the genetic code is degenerate, indicating that there is more than one codon that codes for the same amino acid. For example, the codons CCU, CCC, CCA, and CCG all code for the amino acid proline.

Therefore, if the CCU sequence changes to CCG, the translation product will be a proline, and there will be no modifications in the protein sequence.

Similarly, the mutation may change to an amino acid whose chemical properties do not vary much. For example, if an alanine changes to valine, the effect on the functionality of the protein may be imperceptible.

Note that this is not valid in all cases, if the change occurs in a portion of the protein that is essential for its functionality - such as the active site of enzymes - the effect on fitness can be very significant.

Examples

Hypothetical example: snails and cows

Imagine a meadow where snails and cows coexist. In the snail population we can distinguish two colorations: a black shell and a yellow shell. A decisive factor in the mortality of snails is the footprints of cows.

However, note that if a snail is stepped on, it does not depend on the color of its shell, as it is a random event. In this hypothetical example, the snail population starts with an equal proportion of colors (50 black snails and 50 yellow snails). In the case of cows, eliminate 6 blacks and only 2 yellows, the proportion of colors would change.

In the same way, it may be that in a following event, the yellow ones die in greater proportion, since there is no relationship between the color and the probability of being crushed (however, there is no type of “compensating” effect).

How will the proportion of snails change over time?

During this random process, the proportions of black and yellow shells will be fluctuating over time. Eventually, one of the shells will reach one of the two limits: 0 or 1.

When the frequency reached is 1 - suppose that for the yellow shell allele - all snails will be this color. And, as we can guess, the allele for the black shell will have been lost.

The only way to have that allele again is the population is that it enters by migration or by mutation.

Gene drift in action: the Cheetahs

The phenomenon of gene drift can be observed in natural populations, and the most extreme example is Cheetahs. These fast and stylish felines belong to the species Acinonyx jubatus.

About 10,000 years ago, Cheetahs - and other large mammal populations - experienced an extreme extinction event. This event caused a "bottleneck" in the Cheetah population, with only a few individuals surviving.

The survivors of the Pleistocene catastrophic phenomenon gave rise to all the Cheetahs of today. The effects of drift, coupled with inbreeding, have almost completely homogenized the population.

In fact, the immune system of these animals is practically identical in all individuals. If for any reason, one of the members needed an organ donation, any of their colleagues could do so without leading to any probability of rejection.

Donations are procedures that are carefully carried out and it is necessary to suppress the recipient's immune system so that it does not attack the “external agent”, even if it comes from a very close family member - call it brothers or children.

Example in human populations: the Amish

Bottlenecks and the founder effect also occur in current human populations, and have very relevant consequences in the medical field.

The Amish are a religious group. They are characterized by a simple lifestyle, free from technology and other modern conveniences - in addition to carrying an extremely high frequency of genetic diseases and pathologies.

About 200 colonizers arrived in Pennsylvania (USA), from Europe, and began to reproduce among the same members.

It is speculated that among the colonizers there were carriers of autosomal recessive genetic diseases, including Ellis-van Creveld syndrome. This syndrome is characterized by features of dwarfism and polydactyly (high number of fingers, greater than five digits).

The disease was in the initial population with a frequency of 0.001 and increased significantly to 0.07.

References

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