GENETIC VARIABILITY: CAUSES, SOURCES AND EXAMPLES - BIOLOGY - 2024

The genetic variability comprises all differences, in terms of genetic material, which exist in populations. This variation arises from new mutations that modify genes, from rearrangements resulting from recombination, and from gene flow between populations of species.

In evolutionary biology, variation in populations is a sine qua non for the mechanisms that give rise to evolutionary change to act. In population genetics, the term "evolution" is defined as the change in allele frequencies over time, and if there are no multiple alleles, the population cannot evolve.

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Variation exists at all levels of organization and as we go down the scale, variation increases. We find variations in behavior, in morphology, in physiology, in cells, in the sequence of proteins and in the sequence of DNA bases.

In human populations, for example, we can observe variability through phenotypes. Not all people are physically the same, everyone has characteristics that characterize them (for example, eye color, height, skin color), and this variability is also found at the level of genes.

Today, there are massive DNA sequencing methods that allow evidence of such variation in a very short time. In fact, for some years now, the entire human genome has been known. In addition, there are powerful statistical tools that can be incorporated into the analysis.

Genetic material

Before delving into the concepts of genetic variability, it is necessary to be clear about various aspects of the genetic material. With the exception of a few viruses that use RNA, all organic beings that inhabit the earth use the DNA molecule as their material.

This is a long chain made up of nucleotides grouped in pairs and has all the information to create and maintain an organism. There are approximately 3.2 x 10 ⁹ base pairs in the human genome.

However, not all the genetic material of all organisms is the same, even if they belong to the same species or even if they are closely related.

Chromosomes are structures made up of a long strand of DNA, compacted at various levels. Genes are located along the chromosome, in specific places (called locus, plural loci), and are translated into a phenotype that can be a protein or a regulatory characteristic.

In eukaryotes, only a small percentage of the DNA contained in the cell codes for proteins and another portion of the non-coding DNA has important biological functions, mainly regulatory.

Causes and sources of variability

In populations of organic beings, there are several forces that result in variation at the genetic level. These are: mutation, recombination, and gene flow. We will describe each source in detail below:

Mutation

The term dates from 1901, where Hugo de Vries defines mutation as “hereditary changes in the material that cannot be explained by segregation or recombination processes”.

Mutations are permanent and inheritable changes in the genetic material. There is a wide classification for them that we will deal with in the next section.

Types of mutations

- Point mutations: errors in DNA synthesis or during the repair of damage to the material, can originate point mutations. These are base pair substitutions in the DNA sequence and contribute to the generation of new alleles.

- Transitions and transversions: depending on the type of base that changes, we can speak of a transition or a transversion. The transition refers to the change of base of the same type - purines for purines and pyrimidines for pyrimidines. Transversions involve changes of different kinds.

- Synonymous and non-synonymous mutations: they are two types of point mutations. In the first case, the change in the DNA does not lead to a change in the type of amino acid (thanks to the degeneracy of the genetic code), while the non-synonymous ones do translate into a change in the amino acid residue in the protein.

- Chromosome inversion: mutations can also involve long segments of DNA. In this type, the main consequence is the alteration of the order of the genes, caused by breaks in the strand.

- Gene duplication: genes can be duplicated and create an extra copy when an uneven crossover occurs in the process of cell division. This process is essential in the evolution of genomes, since this extra gene is free to mutate and can acquire a new function.

- Polyploidy: in plants, it is common for errors to occur in the mitotic or meiotic cell division processes and complete sets of chromosomes are added. This event is relevant in the speciation processes in plants, since it quickly leads to the formation of new species due to incompatibility.

- Mutations that run the open reading frame. DNA is read three at a time, if the mutation adds or subtracts a number that is not a multiple of three, the reading frame is affected.

Do all mutations have negative effects?

According to the neutral theory of molecular evolution, most mutations that are fixed in the genome are neutral.

Although the word is usually immediately associated with negative consequences - and indeed, many mutations have large deleterious effects on their carriers - a significant number of mutations are neutral, and a small number are beneficial.

How do mutations occur?

Mutations can have a spontaneous origin or be induced by the environment. The components of DNA, purines and pyrimides, have a certain chemical instability, which results in spontaneous mutations.

A common cause of spontaneous point mutations is deamination of cytosines, which pass to uracil, in the DNA double helix. Thus, after several replications in a cell, whose DNA had an AT pair in one position, it is replaced by a CG pair.

Also, errors occur when DNA is replicating. While it is true that the process is proceeding with great fidelity, it is not without its errors.

On the other hand, there are substances that increase the rates of mutations in organisms, and therefore are called mutagens. These include a number of chemicals, such as EMS, and also ionizing radiation.

Generally, chemicals give rise to point mutations, while radiation results in significant defects at the chromosome level.

The mutation is random

Mutations occur randomly or randomly. This statement means that changes in DNA do not occur in response to a need.

For example, if a certain population of rabbits is subjected to increasingly lower temperatures, the selective pressures will not cause the mutations. If the arrival of a mutation related to the thickness of the fur occurs in rabbits, it would occur in the same way in warmer climates.

In other words, needs are not the cause of the mutation. Mutations that arise at random and provide the individual who carries it with a better reproductive capacity, this will increase its frequency in the population. This is how natural selection works.

Examples of mutations

Sickle cell anemia is a hereditary condition that distorts the shape of the red blood cell or erythrocyte, having fatal consequences on the oxygen transport of the individual carrying the mutation. In populations of African descent, the condition affects 1 in 500 individuals.

When looking at diseased red blood cells, you don't have to be an expert to conclude that, compared to a healthy one, the change is extremely significant. The erythrocytes become rigid structures, blocking their passage through the blood capillaries and damaging the vessels and other tissues as they pass.

However, the mutation that causes this disease is a point mutation in the DNA that changes the amino acid glutamic acid for a valine at position six of the beta-globin chain.

Recombination

Recombination is defined as the exchange of DNA from the paternal and maternal chromosomes during meiotic division. This process is present in virtually all living organisms, being a fundamental phenomenon of DNA repair and cell division.

Recombination is a crucial event in evolutionary biology, since it facilitates the adaptive process, thanks to the creation of novel genetic combinations. However, it has a downside: it breaks up favorable allele combinations.

In addition, it is not a regulated process and is variable throughout the genome, in taxa, between sexes, individual populations, etc.

Recombination is a heritable trait, several populations have additive variation for it, and it can respond to selection in experiments carried out in the laboratory.

The phenomenon is modified by a wide range of environmental variables, including temperature.

Furthermore, recombination is a process that greatly affects the fitness of individuals. In humans, for example, when recombination rates are altered, chromosome abnormalities occur, reducing the fertility of the carrier.

Gene flow

In populations, individuals from other populations may arrive, altering the allelic frequencies of the arrival population. For this reason, migrations are considered evolutionary forces.

Suppose that a population has fixed allele A, indicating that all organisms that are part of the population carry the allele in the homozygous condition. If certain migrant individuals who carry the a allele arrive and reproduce with the natives, the answer will be an increase in genetic variability.

In what part of the cell cycle does genetic variation occur?

Genetic variation occurs in metaphase and later in anaphase.

Is all the variability we see genetic?

No, not all the variability that we observe in populations of living organisms is genetically based. There is a term, widely used in evolutionary biology, called heritability. This parameter quantifies the proportion of phenotypic variance due to genetic variation.

Mathematically, it is expressed as follows: h ² = V _G / (V _G + V _E). Analyzing this equation, we see that it will have the value of 1 if all the variation that we see is clearly due to genetic factors.

However, the environment also has an effect on the phenotype. The "norm of reaction" describes how identical genotypes vary along an environmental gradient (temperature, pH, humidity, etc).

In the same way, different genotypes can appear under the same phenotype, by channeling processes. This phenomenon works as a developmental buffer that prevents the expression of genetic variations.

Examples of genetic variability

Variation in Evolution: The Moth

The typical example of evolution by natural selection is the case of the Biston betularia moth and the industrial revolution. This lepidopteran has two distinctive colorations, one light and one dark.

Thanks to the existence of this heritable variation - since it was related to the individual's fitness, the characteristic could evolve through natural selection. Before the revolution, the moth was easily hidden in the light bark of birch trees.

With increased pollution, the bark of the trees turned black. In this way, now the dark moths had an advantage compared to the light ones: these could be hidden much better and were consumed in a smaller proportion than the light ones. Thus, during the revolution, black moths increased in frequency.

Natural populations with little genetic variation

The cheetah (Acinonyx jubatus) is a feline known for its stylized morphology and for the incredible speeds it reaches. This lineage suffered a phenomenon known in evolution as "bottleneck", in the Pleistocene. This drastic population decline resulted in the loss of variability in the population.

Today, the genetic differences between the members of the species reach alarmingly low values. This fact represents a problem for the future of the species, since if it is attacked by a virus, for example, that eliminates some members, it is very likely that it will be able to eliminate them all.

In other words, they do not have the ability to adapt. For these reasons, it is so important that there is sufficient genetic variation within a population.

References

1. Alberts, B., Johnson, A., Lewis, J., et al. (2002). Molecular Biology of the Cell. 4th edition. New York: Garland Science.
2. Freeman, S., & Herron, JC (2002). Evolutionary analysis. Prentice Hall.
3. Graur, D., Zheng, Y., & Azevedo, RB (2015). An evolutionary classification of genomic function. Genome biology and evolution, 7 (3), 642-5.
4. Hickman, CP, Roberts, LS, Larson, A., Ober, WC, & Garrison, C. (2001). Integrated principles of zoology (Vol. 15). New York: McGraw-Hill.
5. Lodish, H., Berk, A., Zipursky, SL, et al. (2000). Molecular Cell Biology. 4th edition. New York: WH Freeman.
6. Palazzo, AF, & Gregory, TR (2014). The case for junk DNA. PLoS genetics, 10 (5), e1004351.
7. Soler, M. (2002). Evolution: the basis of Biology. South Project.
8. Stapley, J., Feulner, P., Johnston, SE, Santure, AW, & Smadja, CM (2017). Recombination: the good, the bad and the variable. Philosophical transactions of the Royal Society of London. Series B, Biological sciences, 372 (1736), 20170279.
9. Voet, D., Voet, JG, & Pratt, CW (1999). Fundamental of biochemistry. New York: John Willey and Sons.