Evolution due to chance events. The bottleneck effect and founder effect.

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\% 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 88 brown individuals (genotype BB or Bb) and 22 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.
Image credit: "Population genetics: Figure 2," by OpenStax College, Biology CC BY 3.0.
What if, purely by chance, only the 55 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 0.70.7, and the frequency of the b allele is 0.30.3.
In our example, the allele frequencies of the five lucky rabbits are perfectly represented in the second generation, as shown at right. Because the 55-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 0.70.7 and 0.30.3, respectively.
Nope! This is another way in which alleles are "randomly sampled" in populations of finite size. Whenever two individuals reproduce, they have a certain probability of producing offspring with a particular genotype. But if they produce a non-infinite number of offspring, those offspring may deviate by chance from the expected ratios (and thus, may not accurately reflect their parents' allele frequencies).
For example, let's consider the case where two Bb rabbits reproduce with each other.
On average, they should produce BB, Bb, and bb offspring in a ratio of 1:2:11:2:1, as we could predict with a Punnett square. But that's the case on average. In any given case where two rabbits have a litter, their non-infinitely sized litter (which might be around 66 88 baby rabbits) may not fit the 1:2:11:2:1 ratio perfectly1^1. In many cases, the ratio will be off by 11 or 22 rabbits, and in some cases, it may be off by considerably more.
If we have a huge group of reproducing rabbits, these differences will get balanced out, but if we have a small group of rabbits, they can have a significant impact on allele frequencies.
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 10001000 rabbits (instead of 1010), it's much less likely that the b allele would be lost (and that the B allele would reach 100%100\% frequency, or fixation) after such a short period of time. If only half of the 10001000-rabbit population survived to reproduce, as in the first generation of the example above, the surviving rabbits (500500 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.
To see how this works, let's focus on how the white allele was lost from the 1010-rabbit population. In this small population, there were only two white rabbits (genotype bb), and both of these were unable to reproduce and pass on their alleles. This was a major reason why the b allele was lost from the small population (40%40\% of the b alleles belonged to just these two rabbits!).
In a population of 10001000 rabbits with the same allele frequencies, there would be 200200 white rabbits rather than just 22. How likely is it that every single white rabbit, by chance alone, would fail to pass on its genes to the next generation? What if the population was 100,100,000000 rabbits (20,20,000000 white) or 1,1,000,000,000000 rabbits (200,200,000000 white)? As we can see, larger populations are increasingly “buffered” against the effects of genetic drift
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 5050 5050. If you flip a coin a few hundred times, on the other hand, you had better get something quite close to 5050 5050 (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.
Natural selection and genetic drift both result in a change in the frequency of alleles in a population, so both are mechanisms of evolution. However, the two processes differ in how they cause allele frequencies to change. Genetic drift causes evolution by random chance due to sampling error, whereas natural selection causes evolution on the basis of fitness.
In natural selection, individuals whose heritable traits make them more fit (better able to survive and reproduce) leave more offspring relative to other members of the population. That is, an individual with higher fitness is more likely to pass on its genetic material (alleles) to the next generation. The alleles that helped make this individual more fit will likely benefit the offspring in a similar way and should increase in frequency in the population over time. Thus, evolution by natural selection is not dependent on chance; it depends on an allele’s effect on reproductive success. Alleles that improve fitness are likely to increase in frequency, while alleles that reduce fitness will decrease in frequency.
Genetic drift does not take into account an allele’s effect on fitness because it is a random process. Think back to the rabbit population discussed above. What if the white rabbits were more fit than the brown rabbits (better able, on average, to survive and reproduce in the environment in which they lived)? In the example, the only two white rabbits in the population failed to reproduce, resulting in a loss of the beneficial alleles they carried. This result was purely due to chance and illustrates how genetic drift can result in the loss of beneficial alleles from a small population.

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 indviduals 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.
Image credit: "Population genetics: Figure 3," by OpenStax College, Biology, CC BY 3.0.
During the 19th century, Northern elephant seals were nearly hunted to extinction for their oil-rich blubber2^2. The U.S. government stepped in to protect the seals and give the population a chance to bounce back. The population has rebounded from a size of about 100100 individuals to over 30,30,000000 today.
Over-hunting acted as a bottlenecking event, decreasing the population to a small number of individuals that represented only a miniscule fraction of the genetic diversity in the original population. Because Northern elephant seal colonies typically contain a single dominant male that mates with up to 100100 females, all of the Northern elephant seals alive today may be able to trace their ancestry back to a single male!
Although the population has experienced an incredible recovery over the past century, scientists are concerned about the long-term survival of the seal population due to its reduced genetic diversity. It’s generally accepted that an extreme reduction in genetic diversity results in a population that is more susceptible to disease. If a pathogen spread through the population, the seals could potentially be wiped out, as all of them might be similarly non-resistant (that is, there might not be any existing alleles that conferred resistance).
In a normal, genetically diverse population, there would be higher levels of standing genetic variation, making it more likely that some individuals would happen to have a gene variant that conferred resistance.

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.
_Image credit: "Founder effect," by qz10, public domain._
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.
The genetic composition of certain human populations illustrates the founder effect3^3. For example, Ellis-Van Creveld syndrome (whose symptoms include polydactyly, or extra fingers, and other physical abnormalities) is much more prevalent in the Amish population of eastern Pennsylvania than the rest of the United States population.
The current Amish population can trace its ancestry back to a founding group that was composed of roughly 200200 individuals, and since this founding event, the Amish population has remained more or less reproductively isolated from the rest of the American population. It is believed that a single couple out of the original 200200 founders carried a recessive allele for Ellis-Van Creveld syndrome.
Genetic drift, in combination with reproductive isolation, caused this allele to increase in frequency in the population. This led to a much higher prevalence of the syndrome among the Amish relative to the rest of the American population.

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\% 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.

Attribution:

This article is a modified derivative of the following articles:
The modified article is licensed under a CC BY-NC-SA 4.0 license.

Works cited:

  1. Krempels, Dana. (2006). Why spay or neuter my rabbit? In Houserabbit adoption, rescue, and education. Retrieved from http://www.bio.miami.edu/hare/scary.html.
  2. Haw, J. (2013, May 24). Northern elephant seals: Increasing population, decreasing biodiversity. In Scientific american. Retrieved from http://blogs.scientificamerican.com/expeditions/northern-elephant-seals-increasing-population-decreasing-biodiversity/.
  3. Genetic drift and the founder effect. (2001). In Evolution. Retrieved from http://www.pbs.org/wgbh/evolution/library/06/3/l_063_03.html.

Additional references:

Genetic drift. (2016, April 19). Retrieved May 19, 2016 from Wikipedia: https://en.wikipedia.org/wiki/Genetic_drift.
Purves, W. K., Sadava, D., Orians, G. H., and Heller, H. C. (2003). Genetic drift may cause large changes in small populations. In Life: The science of biology (7th ed., pp. 468-469). Sunderland, MA: Sinauer Associates, Inc.
Reece, J. B., Urry, L. A., Cain, M. L., Wasserman, S. A., Minorsky, P. V., and Jackson, R. B. (2011). Genetic drift. In Campbell biology (10th ed., pp. 488-490). San Francisco, CA: Pearson.
University of California Museum of Paleontology. (2016). Bottlenecks and founder effects. In Understanding evolution. Retrieved from http://evolution.berkeley.edu/evolibrary/article/bottlenecks_01.
University of California Museum of Paleontology. (2016) Genetic drift. In Understanding evolution. Retrieved from http://evolution.berkeley.edu/evolibrary/article/evo_24.
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