Neutral Theory of Evolution

Since its beginnings, the neutral theory of evolution has been crucial in the study of evolution at the molecular level because it promotes the formulation of plausible hypotheses that can be validated by credible evidence. According to the neutral theory of evolution, the bulk of molecular variation has no influence on fitness, so random processes are the most likely cause of genetic variation. This strategy may be beneficial in two future studies: biased gene conversion and the influence of effective population size on the neutrality of genetic variants.

Natural selection or Neutral Selection 

Two processes result in the evolution of living creatures. Second, mutations, which commonly occur within populations, offer the genetic variety essential for evolution. It is also impacted by variations in population allele frequencies across time.

Theory of Natural  selection 

Natural selection is capable of selecting for mutations that modify the carrier’s fitness. On the one hand, more beneficial new alleles have the predisposition to spread over time until they correct the original gene and take its place in the population. Positive selection or directed selection are names used to characterize this evolutionary process. On the other hand, mutations that boost the carrier’s fitness often leave groups as a consequence of negative or purifying selection. Naturally, heterozygotes may benefit from a mutation more than homozygotes. Owing to the mechanism known as balancing selection, such alleles frequently persist in populations at a medium frequency.

Increases in allele frequency are therefore not necessarily the outcome of natural selection. Imagine a population in which each person, or genotype, is equally fit. Natural selection is not at work here, even if every genotype has an equal probability of impacting the generation that follows it. Due to the fact that each individual creates numerous gametes and that populations can only increase so much, only some of these gametes will successfully develop into adults. This random gamete sampling strategy may cause allelic frequencies to vary over subsequent generations. Genetic drift is the name for this process. Allele frequency modifications that are purposeful are promoted by natural selection, whereas random oscillations are induced by genetic drift. In the end, genetic drift culminates in the fixation of some alleles and the extinction of others.

What about mutations that don’t affect a person’s fitness, though? Genetic drift is the element that has the biggest impact on these so-called neutral mutations, as natural selection has limited effect on them. It is vital to note that Darwin held the belief that some features might originate without the impact of natural selection[1].

Natural selection would not modify features that are neither desirable nor harmful; rather, they would either continue to fluctuate, as we may witness in some polymorphic species, or they would finally become permanent thanks to the attributes of the organism and its environment. Darwin stayed alive until his death in 1859.

Although they are not the major cause of genetic drift, neutral mutations have a substantial influence. Genetic drift allows the majority of good mutations to dissipate quickly, but some slightly unfavorable alterations have the potential to linger forever.

The allele frequency in sexual populations may be modified by three processes: biased gene conversion (BGC), drift, and selection. BGC and meiotic crossing-over are connected. When two homologous chromosomes cross over, heteroduplex DNA, in which one DNA strand is from one homologue and the other strand is from the other homologue, is generated. Base-pairing mismatches in the heteroduplex region must be addressed regardless of how the crossover intermediate is finally resolved. This includes whether or not the areas on either side of the crossover junction recombine (i.e., whether or not the regions on either side of the crossover junction recombine). As a consequence, during gene conversion, one allele may be “copied and pasted” onto the other one when a given locus is situated in the heteroduplex zone.[2]

If one allele has a larger risk of conversion than the other, BGC is said to be biased. In this instance, the donor allele will be favored by the gamete pool over the altered allele. Through BGC, these donor alleles may spread more broadly among populations. Numerous pieces of evidence show that the primary etiology of BGC may be a bias in the repair of DNA mismatches in the heteroduplex DNA created during recombination. According to research, BGC has been identified in various eukaryotic species. (Marais). It is critical to stress that BGC affects both the fixation of donor alleles and the creation of neutral mutations, even if both consequences are only moderately harmful.[3]

References

[1] Darwin, C. On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life (London, John Murray, 1859)

[2] Duret, L., & Arndt, P. F. The impact of recombination on nucleotide substitutions in the human genome. PLoS Genetics 4, e1000071 (2008)

[3] Graur, D., & Li, W. Fundamentals of Molecular Evolution (Sunderland, MA, Sinauer Associates, 2000)