Natural Selection vs. Genetic Drift

When a mutation occurs at a site that plays a vital role in life, the mutation can lead to death, but in some cases it may not bring about any change, and there may be only a few changes that are hard to notice. In such a situation, unexpected side effects may occur over generations. In terms of information and intelligence, it is important to know about this phenomenon because it is an important issue related to the diversity of information.


Neutral Mutation and Genetic Drift

A neutral mutation is a mutation that does not affect the fitness of each individual organism. These mutations can accumulate over time through the phenomenon of 'genetic drift', and eventually mutations that do not significantly affect the fitness of the organism can make up the majority. This concept is very important because it shows that there are conditions that play an important role in addition to natural selection. Neutral theory of molecular evolution has been argued that the genetic drift of this neutral mutation is even more important to evolution than natural selection. There have been many debates called "neutralist-selectionist" debates, but in recent years, scholars have acknowledged that both natural selection and genetic drift have had considerable influence.

Genetic drift is frequently caused by random sampling of alleles, which are mutations in genes in relatively restricted populations. So it is called 'allelic drift' or 'Sewall Wright effect' named after Sewall Wright, who first introduced this phenomenon.

The mechanisms of genetic drift are simple. Suppose there is a colony of genetically identical bacteria except one gene with two alleles, A and B. Since A and B are neutral alleles, they do not affect the viability or replicability of this bacterium, so it is assumed that all the bacteria of this bacterium colony can survive and reproduce with the same probability. If half of the bacteria have allele A and the other half has allele B, the allele frequencies of A and B are 0.5. If only four bacteria in this colony are capable of surviving enough nutrients, the four surviving bacteria will consist of the following sixteen combinations of A and B alleles.

(A-A-A-A), (B-A-A-A), (A-B-A-A), (B-B-A-A),
(A-A-B-A), (B-A-B-A), (A-B-B-A), (B-B-B-A),
(A-A-A-B), (B-A-A-B), (A-B-A-B), (B-B-A-B),
(A-A-B-B), (B-A-B-B), (A-B-B-B), (B-B-B-B).

Since all bacteria will survive the same probability, the probability that each of the four survivors will have a given allele is 1/2, so the probability of occurrence of any particular allele combination is will be (1/2)^4 = 1/16. Considering the combination of A and B above, the following probability table appears.

A	B	Combinations	Probability
4	0	1	1/16
3	1	4	4/16
2	2	6	6/16
1	3	4	4/16
0	4	1	1/16

As can be seen from the table, the probability that A and B alleles will have the same number of possible combinations is 6/16, and the probability that the number of A and B alleles will be different is 10/16. Therefore, even if the original colony started with the same number of A and B alleles, the number of combinations of A and B alleles is different with high probabilities. In this case, we can say 'genetic drift' is started by random sampling because population allele frequencies are different. If this progresses as the generation repeats, eventually the specific allele is greatly reduced.

The figure below shows that the red beads gradually disappear from generation to generation, eventually disappearing from the whole population, which is said to be fixed by the blue one. If the population is small, this fixation may appear after few generations.

Fixation in the blue "allele" within five generations (from Wikipedia).

Population is Matter! 

In finite populations, new alleles are unlikely to cross the next generation through random sampling. However, the sampling process can eliminate the existing allele. Therefore, genetic drift leads to a population to genetic uniformity. At this time, if the probability of existence of a certain allele reaches 100%, it is said to be 'fixed' in the population. The fixation occurs quickly as the population is small, and fixation does not occur when the population reaches infinity. Once an allele is fixed, the genetic drift stops and the change is removed before the new allele is introduced. New alleles are rarely seen by mutation and gene flow.

The following graph simulates how the first given distribution for a given allele is 0.5 and the genetic drift appears when 50 generations are in progress. According to different populations, 'fixation' can be seen as a different aspect.

from Wikipedia

Lessons Learned

Under normal circumstances, genetic drift and natural selection coexist together with mutations. Neutral evolution is the result of both this mutation and drift being affected. If the natural selection determines the direction in which the evolution will be favorable to the current environment, the genetic drift will only result in a change in mathematical coincidence, regardless of direction. Therefore, drift results in changes in genotypic frequencies in the population without phenotypic effects. Conversely, selection leads to the spread of phenotypic effects in alleles of individuals favorable for survival or reproduction, reduces traits that are unsuitable for the environment, and neglects alleles associated with neutral features. In particular, evolutionary changes in small and isolated populations are driven by products of mutation and genetic drift.

The way in which genes are involved in evolution is very diverse. It is not simply that living things evolve in relation to the environment through natural selection. Changes in distribution patterns in the population by random sampling of neutral mutations that have no effect are also involved here and the relationship between them depends on several factors such as population size, allele redundancy, genetic linkage, selection coefficients get affected. In other words, artificial intelligence technologies such as genetic algorithms inspired by evolution have still a lot to be developed. Research on genetics could also have a positive impact on the development of future artificial intelligence technologies as well as research on neuroscience.


References

Natural Selection: How Evolution Works