This BiologyWise post tries to make an in-depth comparison of genetic drift vs. gene flow vs. natural selection – three of the four main mechanisms that have played a fundamental role in driving evolution forward.
Charles Darwin was the first to use the term “evolution” in his book, The Descent of Man, in 1871. In his book ‘On the Origin of Species‘, he put forth the scientific theory that populations evolve over generations by the process of natural selection. He based this theory on the different varieties of finches he saw on the different islands of Galapagos, and was an evidence to the fact that diversity of life arose from a common descent.
A complete set of an organism’s genes is called its genome. All physical characteristics of an individual are controlled by his/her genes as well as the environment in which he resides. According to modern evolutionary synthesis, evolution is the changes in the genome over a period of time. The frequency with which one gene becomes more or less prevalent in a population depends on the variants or the alleles of the gene that are present in the population.
Evolution is a process that is responsible for bringing about diversity in individuals in every level of a biological organization. The four main mechanisms that have played a fundamental role in driving evolution forward include: mutation, genetic drift, gene flow, and natural selection.
Let us take a look a closer look at the differences between genetic drift, gene flow, and natural selection.
Genetic Drift Vs. Gene Flow Vs. Natural Selection
It is a change in the allele frequency that is brought about by random sampling. Allele frequency is the proportion of individuals carrying a particular allele in a population. In other words, it is a change in the composition of the gene pool of a population that is brought about by random chance.
Example: In a population of ten green birds and ten yellow birds, all have equal chances of mating successfully to produce equal number of offspring. Due to some reason, yellow birds mated more successfully than the green ones. The new generation has say, 13 yellow birds and 9 green birds. As the number of yellow birds is greater than the number of green birds, there is a higher chance of the yellow birds interbreeding to give rise to more offspring as compared to that of green birds. Thus, an increase in the population of yellow birds is seen.
There are two mechanisms that may give rise to genetic drift.
Bottleneck effect: A drastic effect (such as a natural calamity) that may cause a drastic change in the allele frequencies due to reduced population size; this will lead to over representation of certain alleles in the population. This population size will reduce further as interbreeding between the survivors may increase the chances of homozygosity of an allele that may have deleterious effect.
Founder effect: When individuals of one population migrate to a new isolated geographical area (that is not inhabited by any other population) to constitute the gene pool of that area. These new individuals make up the gene pool and are a representation of the original population in the new habitat, but in a smaller proportion.
It is a change in the allele frequency brought about by the transfer of alleles or gametes from one population to another. When individuals migrate from one population to another, new alleles are introduced into the gene pool of that population, leading to a change in the allele frequency of that population.
Example: When American soldiers migrated to Vietnam during the Vietnam War, they had children with the Vietnamese women there, introducing their genes into the gene pool of the Vietnamese population.
It is a process through which a particular allele of a physical characteristic becomes more or less common in a population over a few generations. The physical characteristic that provide an adaptive advantage are selected and become more common in the population over generations.
Example: Before the Industrial Revolution, the dark peppered moth was quite rare in the United Kingdom. During the Industrial Revolution, the trees on which these moths rested were covered with soot. This resulted in a better chance of survival of these moths to reproduce efficiently, resulting in an increase in the population of these moths significantly.
Difference Between Genetic Drift and Natural Selection
► Natural selection accounts only for the positive changes in the genome that may give its possessor an adaptive advantage; genetic drift accounts for all changes in the genome that may be advantageous, deleterious, or may have no effect on its possessor.
► Natural selection is usually driven as response to an organism’s environmental challenges. Genetic drift, on the other hand, is completely random and is solely based on luck.
► Natural selection will always result in the selection of allele that give an advantage to its possessor, while genetic drift may cause advantageous genes to be removed in the following generations.
► Genetic drift is largely influenced by the population size, whereas natural selection is not.
► Genetic drift may sometimes lead to the reduction of genetic variations, or may sometimes be responsible for introducing genetic variation in a population. Natural selection will always result in introducing more genetic variations in a population.
Difference Between Genetic Drift and Gene Flow
► Although, the migration of alleles is observed in the founder effect, it must be noted that unlike in gene flow, where individuals migrate from one population to another, the individuals of a population migrate to a geographical region that is not inhabited by any other population.
► A constant genetic flow is usually required to reduce genetic variation in a population, i.e., it increases that homogeneity between the two populations in which a constant genetic flow is maintained. Genetic drift, on the other hand, may or may not reduce the genetic variation.
Difference Between Gene Flow and Natural Selection
► Natural selection strives to bring about speciation (give rise to a new species) by increasing the genetic variations in the population, whereas a constant gene flow between two populations is usually required to maintain homogeneity of the alleles in the population.
Despite the differences in their mechanisms, these processes are constantly and simultaneously occurring in nature to drive evolution in the forward direction.