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Biology library
Course: Biology library > Unit 25
Lesson 2: Population genetics- Allele frequency
- Hardy-Weinberg equation
- Applying the Hardy-Weinberg equation
- Discussions of conditions for Hardy-Weinberg
- Allele frequency & the gene pool
- Mechanisms of evolution
- Hardy-Weinberg
- Genetic drift, bottleneck effect, and founder effect
- Genetic drift
- Natural selection in populations
- Selection and genetic drift
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Genetic drift
AP.BIO:
EVO‑1 (EU)
, EVO‑1.H (LO)
, EVO‑1.H.1 (EK)
, EVO‑1.I (LO)
, EVO‑1.I.1 (EK)
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, 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 8 brown individuals (genotype BB or Bb) and 2 white individuals (genotype bb). Initially, the frequencies of the B and b alleles are equal.
What if, purely by chance, only the 5 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, point, 7, and the frequency of the b allele is 0, 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 5-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, point, 7 and 0, 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 1000 rabbits (instead of 10), it's much less likely that the b allele would be lost (and that the B allele would reach 100, percent frequency, or fixation) after such a short period of time. If only half of the 1000-rabbit population survived to reproduce, as in the first generation of the example above, the surviving rabbits (500 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 50 50. If you flip a coin a few hundred times, on the other hand, you had better get something quite close to 50 50 (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.
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).
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, 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.
Want to join the conversation?
Does this mean that evolution can actually take a step back in cases were adaptive genes are lost and genes with harmful effects stay?(1 vote)
Evolution doesn't go in a direction, it is a continuing process.(13 votes)
I still don't understand. How come that genetic drift is beneficial for endangered species, isn't genetic drift reducing the allele frequencies and thus creating less variation where natural selection could wipe out the entire population?(7 votes)
Genetic drift is random and doesn't decrease the genetic diversity of a species. If anything it would increase the diversity since the genetic changes are not the same throughout the population.(2 votes)
- How could genetic drift ever create some type of allele that hampers a species or organism from living or reproducing? Wouldn't natural selection kick in over a few generations and take out the gene hampering these actions, no matter the severity of the genetic drift, or bottleneck event? If so, such "bad genes" would not last long, even in extreme bottleneck scenarios. Am i right?(5 votes)
Genetic drift is more common in smaller populations. Imagine an island where 5 white rabbits and 10 grey rabbits live. Perhaps grey rabbits have better camouflage against the island's rocks. If a storm randomly kills 10 grey rabbits and 2 white rabbits, only the white rabbits survived to pass on their genes. Even though grey is preferred, it obviously would not give them an advantage against a storm.
Natural selection is dependent upon variation. If all the rabbits possessing the grey allele are killed, that gene (and that phenotype) could literally be lost forever in the small island population.(2 votes)
lets say that there is a population of equal no.s of alleles of blue ,yellow and red. now 2of each alleles migrate to a different place . and the new population consists of 2 blue alleles ,2 yellow alleles and 2 red alleles.will it still be called a founder effect?(4 votes)
No, it would be called founder effect if you isolated only one allele. From what we can see, again, all types of alleles are present (regardless of the quantity of each).(2 votes)
why Genetic drift effect is strongest in small populations ?(3 votes)
In small populations it is more likely that chance events will significantly change the frequencies of alleles in the population.
For example:
Imagine a population of 4 organisms which have one gene for color with two alleles - lets say a dominant allele calledAand a recessive allele calleda.
The individuals have the following genotypes:
#1=AA
#2=Aa
#4=aa
A storm happens and by chance a tree falls on individual 1 and kills it – so sad!
What has happened to the frequency of the alleles?
What would happen if the tree had fallen on #4? How about #2 or #3?
Now imagine there were 40 organisms with the same mix of genotypes – even if something killed off 1/4 of the population what are the chances it would get all 10AAs?
Does this help?(4 votes)
- It may sound pedantic, but is there any sort of practical occasion where genetic drift acts as a truly random evolutionary mechanism? This article states that "allele benefit or harm doesn't matter" for genetic drift, but is this even possible? I mean even during virtually random events, like an asteroid hitting the Earth and causing a major extinction, natural selection can still act upon allele fitness for this post-apolytiptic scenario - as for non-avian dinosaurs during the K–Pg extinction event.
I can't really think of an event that creates that supposed "random" allele selection - and as such I can't see much of a pratical, tangible difference between genetic drift and natural selection. How can allele frequency, which heavily depends on genotype survival/fitness, not be dictated by natural selection?(3 votes)
In the scenario of the asteroid causing a mass extinction, the asteroid wipes out many of the alleles present in the gene pool, regardless of whether they are beneficial or not. The 'fitter' alleles of this reduced gene pool are passed down to the subsequent generation. Natural selection without the asteroid (i.e. genetic drift) would have produced significantly less alteration in the gene pool, (and subsequently allele frequency), at least for the same time period . This is not to say that genetic drift (here, the bottleneck effect) occurs independently of natural selection, just that in scenarios such as natural disasters, it has a much greater impact. Also, in some cases (e.g. color of fur and eyes) there really is no such thing as a 'beneficial' allele. Evolution in this case is solely dependent upon genetic drift.(3 votes)
- How does population size affect allelic frequency?(1 vote)
- Small population is more affected than a big population because, in small populations, you don't have many alleles, so if a catastrophic event happens and kills some of the small population organisms, you will have fewer alleles. In a big population, you have many alleles so if genetic drift happens, it will not affect as much.(3 votes)
- In the above example for founder effect,
'It is believed that a single couple out of the original 200 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.'
can someone please explain this.
how does recessive allele increase in frequency? is the dominant one having the syndrome or the recessive one?(1 vote)
if the couple has several has several children, then all of them would carry the recessive allele. if the other couples have an average offspring count less than that of the aforementioned couple, the allele would increase in freq. then if the children grow up and have offsprings, their children would also carry the recessive gene. the example is talking about the allele freq., not the actual number of people who have the syndrome (just to make that clear)
I think the recessive one would cause the syndrome.(2 votes)
- Some of this is right. See if a population continued to stay separated from each other (like in a colony as mentioned above) the chance for genetic drift could end up taking away bad genes or creating new ones. But if the population was already mostly wiped out it stands little chance of survival.(1 vote)
- but they would still be alive, and a smaller population means more resources for individuals. some creatures do not need to be in large groups to survive. also, as long as the remaining population has offsprings, then the populations won't die out.(1 vote)
- What if the founding population say migrates to another environment and is not fit for that environment? Will they just die off? Is the case where beneficial alleles can be lost and harmful ones can be fixed only if the founding population is fit for its new environment, and therefore can survive and reproduce?(1 vote)
- In that case, what has been labeled as 'beneficial' is no more beneficial and same applies for harmful alleles.
New environment dictates new mutations and new rate of survival.
No, not the whole population will die off.(1 vote)
