What is Genetic Drift?
Genetic drift is a mechanism of evolution characterized by random fluctuations in the frequency of a particular version of a gene (allele) in a population.
Though it primarily affects small, isolated populations, the effects of genetic drift can be strong, sometimes causing traits to become overwhelmingly frequent or to disappear from a population.
Genetic drift is one of the basic mechanisms of evolution.
In each generation, some individuals may, just by chance, leave behind a few more descendants (and genes, of course!) than other individuals.
The genes and other genetic elements of the next generation will be those of the “lucky” individuals, not necessarily the healthier or “better” individuals. That, in a nutshell, is genetic drift. It happens to ALL populations there’s no avoiding the vagaries of chance.
Earlier we used this hypothetical cartoon. Genetic drift affects the genetic makeup of the population, but unlike natural selection, through an entirely random process. So, although genetic drift is a mechanism of evolution, it doesn’t work to produce adaptations.
What causes Genetic Drift?
Genetic drift usually occurs in smaller populations. In a small population with many alleles, any of the alleles can become extinct. In a population with many organisms, there is less chance of losing an entire allele. This is because many organisms contain the alleles and all the alleles cannot be wiped away.
If the allele affects the organism such that it causes more reproduction of DNA, the allele frequency increases. If the allele harms the organism, the allele frequency decreases.
When the allele frequency increases or decreases because of its presence in some random organism that survived, it is known as genetic drift.
Types of Genetic Drift
There are two main types of genetic drift: population bottlenecks and the founder effect. They differ by the mechanisms and events that cause them to occur
1. Bottleneck Effect
The bottleneck effect is an extreme example of genetic drift that happens when the size of a population is severely reduced. Events like natural disasters (earthquakes, floods, fires) can decimate a population, killing most individuals and leaving behind a small, random assortment of survivors.
The allele frequencies in this group may be very different from those of the population prior to the event, and some alleles may be missing entirely.
The smaller population will also be more susceptible to the effects of genetic drift for generations (until its numbers return to normal), potentially causing even more alleles to be lost.
How can a bottleneck event reduce genetic diversity? Imagine a bottle filled with marbles, where the marbles represent the individuals in a population.
If a bottleneck event occurs, a small, random assortment of individuals survive the event and pass through the bottleneck (and into the cup), while the vast majority of the population is killed off (remains in the bottle). The genetic composition of the random survivors is now the genetic composition of the entire population.
2. Founder Effect
The founder effect is another extreme example of drift, one that occurs when a small group of individuals breaks off from a larger population to establish a colony.
The new colony is isolated from the original population, and the founding individuals may not represent the full genetic diversity of the original population.
That is, alleles in the founding population may be present at different frequencies than in the original population, and some alleles may be missing altogether.
The founder effect is similar in concept to the bottleneck effect, but it occurs via a different mechanism (colonization rather than catastrophe).
Effects of Genetic Drift
Loss of Variance
Genetic drift tends to be more pronounced in smaller populations. As a result, chance events can lead to the loss of specific alleles from the gene pool. This reduction in genetic diversity can limit the adaptive potential of a population, making it more susceptible to environmental changes.
Increased Homozygosity in a Population
Genetic drift increases the frequency of homozygous individuals in a population. Homozygosity is found to occur when an individual possesses two identical alleles at a particular gene locus.
As genetic drift reduces the number of alleles, individuals are more likely to inherit identical copies from their ancestors.
Loss of Beneficial Alleles and Fixation of Deleterious Alleles
Genetic drift in small populations can increase the risk of inbreeding, as closely related individuals may contribute disproportionately to the next generation.
Inbreeding can cause the expression of recessive deleterious alleles and result in reduced fitness and health. Beneficial alleles might be lost, and deleterious alleles might become fixed purely due to chance factors.
Gene Flow vs. Genetic Drift
Gene flow is the movement of genes between populations, species, or organisms. E.g., bacteria can transfer genes between different cells. On the contrary, genetic drift refers to the random selection of genes in a population.
When individuals from one population migrate to some other population and breed there, gene flow occurs.
Unlike genetic drift, gene flow does not evaluate the allele frequencies.
Genetic Drift Example
Genetic drift can be observed in the following examples:
The American Bison was once hunted to such an extent that it became endangered. The population which has recovered today show very few genetic variations.
Consider a population of rabbits with brown fur and white fur, white fur being the dominant allele. Due to genetic drift, only the brown population might remain, with all the white ones eliminated.
A couple with brown and blue eyes has children with brown or blue eyes. Even if there is a 50% chance of having blue eyes, brown eyes being the dominant allele, all the children might have brown eyes in the future generations as a matter of chance.
A bird has an allele for two different sizes of beaks. Genetic drift might eliminate one of the beak sizes from the population, thus reducing the genetic variations of the gene pool of birds.
Hypothesize a plant that produces blue or yellow flowers. If the yellow flowers are destroyed in a fire and the blue allele is the dominant one, the plant will produce only blue flowers.