Genetic drift, bottleneck effect and founder effect | Biology | Khan Academy

Summary

This video script delves into the concept of evolution, highlighting the distinction between Natural Selection and Genetic Drift as mechanisms of evolutionary change. While Natural Selection focuses on the survival of the fittest traits, Genetic Drift emphasizes random changes in a population's heritable traits over generations. The script uses the example of rabbits to illustrate Genetic Drift, showing how random events can lead to significant shifts in allele frequencies, impacting population diversity. It also introduces the Bottleneck and Founder Effects as instances of extreme Genetic Drift in small populations.

Takeaways

• 🌱 **Evolution Defined**: Evolution refers to the change in heritable traits of a population over generations.
• 🔍 **Natural Selection Clarified**: While often mentioned alongside evolution, natural selection is one mechanism of evolution, focusing on traits that are most fit for the environment.
• 🎨 **Variation in Traits**: Traits in a population can vary, as depicted by different colors in the script, and natural selection favors those that enhance survival and reproduction.
• 🔄 **Genetic Drift Introduction**: The script introduces genetic drift as another mechanism of evolution, distinct from natural selection.
• 🚫 **Randomness in Genetic Drift**: Unlike natural selection, genetic drift is about random changes in heritable traits, not necessarily favoring the fittest traits.
• 🐰 **Genetic Drift Example**: A small population of rabbits is used to illustrate how genetic drift can occur due to random reproductive success.
• 🧬 **Mendelian Genetics**: The script uses a Mendelian genetics example with dominant and recessive alleles to explain genetic drift.
• 📈 **Allele Frequency Changes**: Genetic drift can lead to changes in allele frequencies in a population due to random events, not environmental pressures.
• 🏞️ **Impact of Population Size**: Genetic drift is more pronounced in small populations, which can lead to reduced diversity and the loss of potentially advantageous traits.
• 🌪️ **Bottleneck Effect**: Describes a scenario where a disaster leads to a significant reduction in population, causing an extreme form of genetic drift.
• 🛣️ **Founder Effect**: Explains how a new, smaller population with less variation can be established due to a group becoming isolated from the main population.

Q & A

What is the fundamental concept of evolution discussed in the script?

The script discusses evolution as the change in heritable traits of a population over generations.

What is the difference between Natural Selection and Genetic Drift as mechanisms of evolution?

Natural Selection is about traits that are most fit for the environment and thus more likely to reproduce, whereas Genetic Drift is about random changes in heritable traits without regard to environmental fitness.

How does the script illustrate the concept of variation in a population?

The script uses the example of living circles with different colors to depict variation in a population.

What does the script imply about the traits that are more fit in the context of Natural Selection?

In Natural Selection, the traits that are more fit are those that confer an advantage in survival and reproduction, such as being less likely to be caught by predators or reproducing faster.

How does the script explain the process of Genetic Drift?

The script explains Genetic Drift through the example of a small population of rabbits, where the color gene's frequency changes randomly over generations without any environmental advantage.

What is the significance of the example with the rabbits in the script?

The rabbit example illustrates how Genetic Drift can lead to changes in allele frequency purely by chance, independent of the fitness of the traits.

Why are biologists particularly concerned about Genetic Drift in small populations?

Biologists are concerned about Genetic Drift in small populations because it can lead to less diversity and variation, and even the loss of favorable traits, due to random chance.

What are the two types of Genetic Drift mentioned in the script that cause extreme reductions in population?

The two types of Genetic Drift mentioned are the Bottleneck Effect and the Founder Effect.

Can you explain the Bottleneck Effect as described in the script?

The Bottleneck Effect occurs when a major disaster or event drastically reduces the population size, leading to a significant loss in genetic variation as only a few individuals survive and reproduce.

What is the Founder Effect, and how does it differ from the Bottleneck Effect?

The Founder Effect occurs when a small group of individuals becomes isolated from the main population and establishes a new population with less genetic variation, not due to a disaster but due to the random nature of who becomes the 'founders' of the new population.

How does the script emphasize the role of randomness in Genetic Drift?

The script emphasizes randomness by illustrating how allele frequencies can change drastically in small populations due to chance events, without any correlation to the fitness of the traits.

Outlines

00:00

🧬 Natural Selection vs. Genetic Drift

This paragraph introduces the concept of evolution, focusing on the change in heritable traits within a population across generations. It distinguishes between two primary mechanisms of evolution: Natural Selection and Genetic Drift. Natural Selection is described as the process where traits best suited to the environment increase in frequency due to higher reproductive success. In contrast, Genetic Drift is presented as a random change in heritable traits, unrelated to environmental fitness, and is illustrated with an example involving a small population of rabbits with different color alleles. The randomness of Genetic Drift is emphasized, showing how allele frequencies can change due to chance events rather than selective pressures.

05:00

🐇 Examples of Genetic Drift: Bottleneck and Founder Effects

The second paragraph delves deeper into Genetic Drift, using the example of a small rabbit population to demonstrate how random events can lead to significant changes in allele frequencies. It explains that Genetic Drift is more pronounced in smaller populations, which can lead to reduced genetic diversity and the potential loss of advantageous traits. The paragraph introduces two specific types of Genetic Drift: the Bottleneck Effect, which occurs after a catastrophic event that drastically reduces population size, and the Founder Effect, which happens when a small group becomes isolated from the main population, carrying with them only a subset of the original genetic variation. Both effects are illustrated with the metaphor of marbles being poured from a bottle, highlighting the role of chance in the survival and reproduction of individuals.

10:02

🌟 The Significance of Genetic Drift in Evolution

The final paragraph wraps up the discussion on Genetic Drift, emphasizing its role as a mechanism of evolution that operates independently of Natural Selection. It reiterates that Genetic Drift is driven by randomness rather than the selection of advantageous traits. The paragraph also touches on the implications of Genetic Drift for small populations, where the effects of random changes can be more pronounced and potentially lead to the loss of genetic variation. The summary concludes by acknowledging the importance of understanding both Natural Selection and Genetic Drift in the study of evolution, as they both contribute to the diversity and adaptation of species over time.

Keywords

💡Evolution

Evolution refers to the change in heritable traits of a population over generations. It is the overarching theme of the video, as it sets the stage for discussing how species change over time. The script uses evolution as a framework to explore different mechanisms, such as natural selection and genetic drift, that drive these changes.

💡Natural Selection

Natural selection is a mechanism of evolution that describes how traits that are advantageous for survival and reproduction become more common in a population over time. The video explains that while natural selection is often the focus, it is just one of the ways evolution can occur. An example given is the color of circles representing different traits, where the blue trait might confer a survival advantage.

💡Genetic Drift

Genetic drift is another mechanism of evolution that involves random changes in the frequency of alleles in a population. Unlike natural selection, genetic drift is not driven by environmental pressures but by chance events. The video uses the example of a small population of rabbits with different color alleles to illustrate how genetic drift can lead to changes in allele frequencies.

💡Heritable Traits

Heritable traits are characteristics that are passed down from parents to offspring through genes. The video emphasizes that evolution is about changes in these traits over time. Heritable traits are the building blocks upon which both natural selection and genetic drift act.

💡Alleles

Alleles are different forms of a gene that can exist at the same locus on homologous chromosomes. The script uses the example of a gene for color in rabbits with two alleles, B (brown) and b (white), to explain how genetic drift can affect the frequency of these alleles in a population.

💡Fitness

Fitness, in the context of the video, refers to the ability of an organism to survive and reproduce in its environment. It is a key concept in natural selection, where traits that increase an organism's fitness are more likely to be passed on to future generations. The video contrasts this with genetic drift, where fitness does not necessarily determine which traits become more common.

💡Random Chance

Random chance is a central concept in genetic drift, where the video explains that the survival and reproduction of individuals are not necessarily tied to their fitness but can be influenced by random events. This is illustrated through the example of rabbits where the offspring are determined by random selection, not by the fitness of their traits.

💡Population

A population, as discussed in the video, is a group of individuals of the same species living in a particular area. The script explores how changes in heritable traits within a population can lead to evolution, with a focus on how both natural selection and genetic drift can impact these changes.

💡Bottleneck Effect

The Bottleneck Effect is a type of genetic drift that occurs when a significant event reduces the population size, leading to a loss of genetic variation. The video uses the metaphor of a bottle with marbles to illustrate how only a few individuals, and therefore a limited set of alleles, might survive such an event.

💡Founder Effect

The Founder Effect is another form of genetic drift that happens when a small group of individuals from a larger population establishes a new population, leading to a reduced genetic diversity in the new group. The video describes this through a narrative of a few individuals becoming separated from the main population and starting a new group with a different allele frequency.

Highlights

Evolution refers to the change in heritable traits of a population over generations.

Natural Selection is the primary mechanism of evolution, but it's not the only one.

Genetic Drift is another mechanism of evolution, distinct from Natural Selection.

Natural Selection involves traits that are most fit for the environment to reproduce.

Genetic Drift is about random changes in heritable traits, unrelated to environmental fitness.

An example of Genetic Drift is demonstrated with a small population of rabbits with color genes.

In the rabbit example, the dominance of the brown allele (B) over the white allele (b) is shown.

Genetic Drift can lead to changes in allele frequency due to random reproductive events.

The Founder Effect and the Bottleneck Effect are types of Genetic Drift that cause significant population reductions.

The Bottleneck Effect occurs when a disaster reduces the population, leading to a loss of genetic variation.

The Founder Effect happens when a small group forms a new population with less genetic variation.

Genetic Drift is more likely to occur in small populations and can impact genetic diversity.

Biologists are concerned about Genetic Drift in small populations due to its potential to reduce genetic variation.

Random chance in Genetic Drift can lead to the disappearance of even advantageous traits from a population.

Genetic Drift and Natural Selection are both mechanisms of evolution but operate differently.

The video emphasizes the importance of understanding both mechanisms to grasp the full concept of evolution.

Transcripts

- [Voiceover] We've already made several videos

over evolution,

and just to remind ourselves

what evolution is talking about,

it's the change in heritable traits

of a population over generations.

And a lot of times, you'll hear people say evolution

and Natural Selection really in the same breath,

but what we wanna make a little bit clear in this video is

that Natural Selection is one mechanism of evolution.

It's the one most talked about

because it is viewed as the primary mechanism.

Natural Selection.

But what we're gonna talk about

in this video is another mechanism called Genetic Drift.

So there's Natural Selection,

and there is Genetic Drift.

Now we've done many videos on Natural Selection,

but it's this idea that you have variation in a population,

you have different heritable traits,

and I'm gonna depict those with different colors here.

We have a population of living circles here, (laughs)

and they could come in blue or maybe magenta.

Maybe they come in another variation too,

maybe there is yellow circles,

and Natural Selection is all about

which of these traits are most fit for the environment

so that they can reproduce.

So there might be something about being, say, blue,

that allows those circles to reproduce faster,

or to be less likely to be caught by predators,

or to be able to stalk prey better.

Even if they're only slightly more likely to reproduce,

over time, over many generations,

their numbers will increase and dominate,

and the other numbers are less likely,

or the other trait is less likely to survive,

and so we will have this Natural Selection

for that blue trait.

So this is all about traits being the fittest traits.

Now Genetic Drift is also change in heritable traits

of a population over generations,

but it's not about the traits that are most fit

for an environment are the ones that necessarily survive.

Genetic Drift is really about random.

Random changes.

Random changes,

and a good example of that I have right over here

that we got from,

I'll give proper credit,

this is from OpenStax College Biology,

and this shows how Genetic Drift could happen.

So right over here,

I'm showing a very small population of 10 rabbits,

and we have the gene for color,

and we have two versions of that gene,

or we could call them two alleles.

You have the capital B version,

and you have the lower case B,

and capital B is dominant.

This is kind of a very Mendelian example

that we're showing here.

And so if you have two of lower case genes,

two of the white alleles,

you're going to be white.

If you have two of the brown alleles, the capital Bs,

you're going to be brown,

and if you're a heterozygote,

you're still going to be brown.

So as you can see here, there are several heterozygotes

in this fairly small population.

But if you just count the capital Bs

versus the lower case Bs,

you see that we have an equal amount of each.

And so the frequency,

if you were to pick a random allele from this population,

you're just as likely

to pick a capital B than a lower case B.

Even though the phenotype,

you see a lot more brown,

but these six brown here have both the upper case B

and the lower case B.

Now let's say they're in a population

where whether you are brown or whether you are white,

it confers no advantage.

There's no more likelihood of surviving and reproducing

if you're brown than white,

but just by chance,

by pure random chance,

the five bunnies on the top are the ones

that are able to reproduce,

and the five bunnies on the bottom are not the ones

that are able to reproduce.

And you might be saying hey, why did I pick those top five?

I didn't pick them, I'm just giving an example.

It could've been the bottom five.

It could've been only these two,

or the only two white ones were the ones

that were able to reproduce.

It's by pure random chance,

or it could be because of traits that are unrelated

to the alleles that we are talking about.

But from the point of view of these alleles,

it looks like random chance.

And so in the next generation,

those five rabbits reproduce

and you could have a situation like this,

and just by random chance, as you can see,

the capital B allele frequency has increased

from 50% of the alleles in the population to 70%.

And then it could be another random chance,

and I'm not saying this is necessarily going to happen.

It could happen the other way.

It could happen even though

that first randomness happened,

maybe now all of a sudden this white rabbit is able

to reproduce a lot,

but maybe not.

Maybe these two brown rabbits

that are homozygous for the dominant trait are able

to reproduce,

and one again it has nothing to do with fitness.

And so they're able to reproduce,

and then all of a sudden,

the white allele is completely gone from the environment.

And the reason why this happened isn't

because the white allele somehow makes the bunnies less fit.

In fact, it might have even conferred a little bit

of an advantage.

It might have been,

from the environment that the bunnies are in point of view,

it might have even been a better trait,

but because of random chance,

it disappears from the population.

And the general idea with the Genetic Drift,

so once again, just to compare,

Natural Selection,

you are selecting,

or the environment is selecting traits

that are more favorable for reproduction,

while Genetic Drift is random changes.

Random changes in reproduction of the population.

Now, as you can imagine,

I just gave an example with 10 bunnies,

and what I just described is much more likely

to happen with small populations.

So much more likely.

More likely

with small populations.

And we have videos on statistics on Khan Academy,

but the likelihood of this happening

with 10 bunnies versus the likelihood

of what I just described happening

with 10 million bunnies is very different.

It's much more likely to happen with a small population.

So a lot of the contexts of Genetic Drift are

when people talk about small populations.

In fact, many times Biologists are worried

about small populations specifically

because of Genetic Drift.

For random reasons, you could have less diversity,

less variation in your population,

and even favorable traits could be selected for

by random chance.

There's two types of Genetic Drift that are often called out

that cause extreme reductions in population,

and significantly reduce the populations.

One is called the Bottleneck Effect.

Let me write this down.

So the Bottle,

Bottleneck,

the Bottleneck Effect,

and then the other is called the Founder Effect.

Do that over here.

The Founder,

Founder Effect.

They are both ideas

where you have significant reduction in population

for slightly different reasons.

Bottleneck Effect is you have some major disaster or event

that kills off a lot of the population,

so only a little bit of the population is able to survive.

And the reason why it's called Bottleneck is imagine

if you had a bottle here.

If you had a bottle here and, I dunno,

inside of that bottle,

you had marbles of different colors.

So you have some yellow marbles,

you have some magenta marbles,

you have some, I don't know, blue marbles.

These are the colors that I tend to be using.

You have some blue marbles,

so you have a lot of variation in your original population.

But if you think about pouring them out of a bottle,

maybe somehow there's some major disaster,

and only two of these survive,

or let's say only four of these survive,

and so you could view that as,

"Well, what are the marbles that are getting poured

"out of the bottle?"

It's really just a metaphor.

Obviously, we're not putting populations

of things in bottles.

But after that disaster,

only a handful survive,

and they might not have any traits

that are in any way more desirable or more fit

for the environment than everything else,

but they just by random chance,

because of this disaster,

they are the ones that survived.

And so all of a sudden,

you have a massive reduction not only in the population,

but also in the variation in that population,

and many alleles might have even disappeared,

and so you have an extreme form

of Genetic Drift actually occurring.

Another example is Founder Effect,

which is the same idea of a population becoming very small,

but the Founder Effect isn't because of a natural disaster.

Let's say you had a population.

Once again, you have a lot of different alleles

in that population.

You have a lot of variation,

you have a lot of variation in that population.

So let me just keep coloring it.

You have a lot of variation in this population,

and let's say that,

you know, they're all hanging out in their region,

and maybe, you know,

they are surrounded by mountains.

I'm just making this up as I go,

but let's say a couple of these blue characters were

out walking one day,

and they maybe get separated

from the rest of their population.

Maybe they discover a little undiscovered mountain pass,

and they go settle a new population someplace.

So that's why it's called the Founder Effect.

These are the founders of a new population,

and once again, by random chance,

they just have a lot less variation.

They're a smaller population

and they happen to be disproportionately

or all blue in this case,

and so now this population is going to (mumbles)

Just the process of this was Genetic Drift

where many alleles will have disappeared

because you have such a small population of blues here.

And also because you have such a small population,

you're likely to have even more Genetic Drift.

So it's a really interesting thing to think about.

Evolution and Natural Selection are often talked

about hand in hand,

but Natural Selection isn't the only mechanism of Evolution.

You also have Genetic Drift,

which is really about,

not selecting for favorable traits,

it is about randomness.