What is genetic drift?

Key points

  • Genetic drift is a mechanism of evolution in which allele frequencies of a population change over generations due to chance (sampling error).
  • Genetic drift occurs in all populations of non-infinite size, but its effects are strongest in small populations.
  • Genetic drift may result in the loss of some alleles (including beneficial ones) and the fixation, or rise to 100\%100%100, percent frequency, of other alleles.
  • Genetic drift can have major effects when a population is sharply reduced in size by a natural disaster (bottleneck effect) or when a small group splits off from the main population to found a colony (founder effect).

Introduction

Natural selection is an important mechanism of evolution. But is it the only mechanism? Nope! In fact, sometimes evolution just happens by chance.

In population genetics, evolution is defined as a change in the frequency of alleles (versions of a gene) in a population over time. So, evolution is any shift in allele frequencies in a population over generations – whether that shift is due to natural selection or some other evolutionary mechanism, and whether that shift makes the population better-suited for its environment or not.

In this article, we’ll examine genetic drift, an evolutionary mechanism that produces random (rather than selection-driven) changes in allele frequencies in a population over time.

What is genetic drift?

Genetic drift is change in allele frequencies in a population from generation to generation that occurs due to chance events. To be more exact, genetic drift is change due to “sampling error” in selecting the alleles for the next generation from the gene pool of the current generation. Although genetic drift happens in populations of all sizes, its effects tend to be stronger in small populations.

Genetic drift example

Let’s make the idea of drift more concrete by looking at an example. As shown in the diagram below, we have a very small rabbit population that’s made up of 888 brown individuals (genotype BB or Bb) and 222 white individuals (genotype bb). Initially, the frequencies of the B and b alleles are equal.

Genetic drift at work in a small population of rabbits. By the third generation, the b allele has been lost from the population purely by chance.

What if, purely by chance, only the 555 circled individuals in the rabbit population reproduce? (Maybe the other rabbits died for reasons unrelated to their coat color, e.g., they happened to get caught in a hunter’s snares.) In the surviving group, the frequency of the B allele is 0.70.70, point, 7, and the frequency of the b allele is 0.30.30, point, 3.

In our example, the allele frequencies of the five lucky rabbits are perfectly represented in the second generation, as shown at right. Because the 555-rabbit “sample” in the previous generation had different allele frequencies than the population as a whole, frequencies of B and b in the population have shifted to 0.70.70, point, 7 and 0.30.30, point, 3, respectively. 

From this second generation, what if only two of the BB offspring survive and reproduce to yield the third generation? In this series of events, by the third generation, the b allele is completely lost from the population.

Population size matters

Larger populations are unlikely to change this quickly as a result of genetic drift. For instance, if we followed a population of 100010001000 rabbits (instead of 101010), it’s much less likely that the b allele would be lost (and that the B allele would reach 100\%100%100, percent frequency, or fixation) after such a short period of time. If only half of the 100010001000-rabbit population survived to reproduce, as in the first generation of the example above, the surviving rabbits (500500500 of them) would tend to be a much more accurate representation of the allele frequencies of the original population – simply because the sample would be so much larger. 

This is a lot like flipping a coin a small vs. a large number of times. If you flip a coin just a few times, you might easily get a heads-tails ratio that’s different from 505050 – 505050. If you flip a coin a few hundred times, on the other hand, you had better get something quite close to 505050 – 505050 (or else you might suspect you have a doctored coin)!

Allele benefit or harm doesn’t matter

Genetic drift, unlike natural selection, does not take into account an allele’s benefit (or harm) to the individual that carries it. That is, a beneficial allele may be lost, or a slightly harmful allele may become fixed, purely by chance.

A beneficial or harmful allele would be subject to selection as well as drift, but strong drift (for example, in a very small population) might still cause fixation of a harmful allele or loss of a beneficial one.

The 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.

A population bottleneck yields a limited and random assortment of individuals. This small population will now be under the influence of genetic drift for several generations.

The 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).

Simplified illustration of the founder effect. The original population consisting of equal amounts of square and circle individuals fractions off into several colonies. Each colony contains a small, random assortment of individuals that does not reflect the genetic diversity of the larger, original population. These small colonies will be susceptible to the effects of genetic drift for several generations.

In the figure above, you can see a population made up of equal numbers of squares and circles. (Let’s assume an individual’s shape is determined by its alleles for a particular gene).

Random groups that depart to establish new colonies are likely to contain different frequencies of squares and circles than the original population. So, the allele frequencies in the colonies (small circles) may be different relative to the original population. Also, the small size of the new colonies means they will experience strong genetic drift for generations.

Summary

Unlike natural selection, genetic drift does not depend on an allele’s beneficial or harmful effects. Instead, drift changes allele frequencies purely by chance, as random subsets of individuals (and the gametes of those individuals) are sampled to produce the next generation.

Every population experiences genetic drift, but small populations feel its effects more strongly. Genetic drift does not take into account an allele’s adaptive value to a population, and it may result in loss of a beneficial allele or fixation (rise to 100\%100%100, percent frequency) of a harmful allele in a population.

The founder effect and the bottleneck effect are cases in which a small population is formed from a larger population. These “sampled” populations often do not represent the genetic diversity of the original population, and their small size means they may experience strong drift for generations.

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