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AP®︎/College Biology
Genetic drift, bottleneck effect, and founder effect
Evolution has multiple mechanisms, including genetic drift, which involves random changes in trait frequency. In particular, genetic drift is more likely in small populations. Examples include the bottleneck effect, where a disaster reduces population size, and the founder effect, where a small group starts a new population; both result in less genetic variation. Created by Sal Khan.
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- would the extinction of dinosaurs be considered a bottleneck effect?(20 votes)
- It would not.
The principle idea of the bottleneck effect is that a sharp reduction in the size of a population due to some event changes the allele frequencies in surviving or future generations.
In an extinction level event, it is true that a population would be reduced, but it does not satisfy the second condition - that the alleles are reproduced or passed on. This is because there is no surviving population of dinosaurs.(34 votes)
- Why is it that genetic drift is more likely in small populations?(14 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 calledA
and 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 10AA
s?
Does this help?(25 votes)
- What is the difference between genetic drift and gene flow?(9 votes)
- Genetic drift has to do with the randomness of reproduction and the resulting allele frequencies. In this video, it's by pure chance that the brown bunnies reproduce and over a few generations all of the bunnies end up being brown. That's genetic drift. Gene flow has to do with the migration of organisms. Say we have a population of all brown bunnies and a white bunny decides to migrate into that population. Now there will be new genes (for white fur) in the population.(14 votes)
- Can you distinguish between if it is an example of GENE FLOW or GENETIC DRIFT FOUNDER EFFECT?
: Extra fingers occur more often than normal among the old Amish in Pennsylvania because the original founding population was only 200.
My study guide showed it as a Founder Effect, but I still do not doubt that it can be an example of Gene Flow because it is result of migration, and considered as microevolution, too. Please help me with this question.(5 votes)- They are two different concepts.
Gene flow: when an individual enters or exits a population, this changes the allele frequency for the population the individual entered/exited. For example if 200 people left England to start an Amish colony, this would have a gene flow effect on ENGLAND.
Founder effect: a small group of individuals splits off and starts a new population with less variation than the larger population they came from. Using the same Amish example, their new colony of 200 people would be subject to a genetic drift founder effect as you say.
In summary, the gene flow effect is what happens to the population they came from (England), the founder effect refers to the new smaller population that they started (Amish colony).(16 votes)
- I'm trying to understand how these terms relate to each other. In this video it is stated that the bottleneck effect and the founder effect are the two main types of genetic drift. Other sources mention that the founder effect is a type of population bottlenecking, which makes it sound more like a type/subtype relationship. Is it that the subtype (founder effect) is also considered a separate main type, in a way?(5 votes)
- The type (Genetic Drift) refers to an event in which the allele frequency of a population changes. The subtypes, Bottlenecking and Founder effect, are two different concepts. Imagine a colony of ants, half is red and half is black, if you step on the half dominated by red ants, then you have caused a bottleneck catastrophe which lead to the genetic drift from an equal phenotypic frequency of red and black ants, to a population dominated by mostly black ants. Imagine that same colony as it hasn't gone through any disasters. Let's say a group of red ants rebel against the queen and leave to start their own colony. This founder's effect disturbed the original colony because now there are less red ants to contribute their red alleles to the gene pool: allowing for the black ants to dominate in this scenario as well. Simply put, something has to have happened which caused part of a population to decline for it to be considered bottlenecking; part of the population has to have left for it to be Founder's effect. I hope this answers your question!(8 votes)
- How do we determine if a gene allele is recessive or dominant? Can a recessive gene become dominant and vice versa?(2 votes)
- I haven't heard of it.
An allele is a version of a gene. Like it can be either dominant, recessive or co-dominant.
If we speak about classic dominance patterns, a gene could be either dominant or recessive.
You just observe phenotype and occurrence at which gene is displayed. If an allele is visible in phenotype in most cases (homozygous and heterozygous) then it is dominant. If an allele is masked by another one (dominant) and is visible only in homozygous situation, then it is recessive.(4 votes)
- Are Mendelian genetics accurate or is it just a huge oversimplification of heredity?(2 votes)
- Nowadays, Mendelian genetics seem to be an oversimplification of heredity. In reality, most traits found in a complex organism have been discovered to be non-Mendelian in nature. There are several types of non-Mendelian genetics as well, like incomplete and co-dominance, polygenic inheritance, and sex-linked inheritance. However, there are still some traits that follow Mendelian genetics, such as sickle-cell anemia (Mendelian genetics are also a useful gateway to pedigree charts and the more complex genetic topics!).
Did this help?(4 votes)
- I remember watching a YouTuber who raised mantises. He hatched a mantis egg and raised the babies, then releasing them in a greenhouse (with permission from the owner) because he couldn't just raise 100 baby mantises. Was putting them in the greenhouse an example of the Founder Effect?(3 votes)
- Can the phenotype of an organism be changed by the environment?(3 votes)
- In this answer I'm assuming you meant direct effects rather than evolutionary effects.
Yes there are many examples where the environment can have a very strong influence on phenotype.
One example of this is how while monozygotic ("identical") twins show very strong similarities in many measures of personality and behavior, but they are not identical.
One reason for these differences is that experience and environmental differences (even within the womb) can affect many phenotypes including personality. This article helps explain some of this:
https://learn.genetics.utah.edu/content/epigenetics/twins/
A second example of this is human height — this has increased with increasing wealth (and food availability) in most countries even those with relatively low immigration levels (e.g. Japan). These changes have occurred much faster than can be explained by evolutionary changes. An article on this:
https://www.scientificamerican.com/article/how-much-of-human-height/
A third example of this is sex determination in some reptiles and fish — some temperatures will result in all male hatchlings while others will result in all female hatchlings (intermediate temperatures result in a mix of males and females). Wikipedia has a decent article on this here:
https://en.wikipedia.org/wiki/Temperature-dependent_sex_determination
There is a Khan Academy video on environmental effects on phenotype here:
https://www.khanacademy.org/science/ap-biology/heredity/environmental-effects-on-phenotype/v/gene-environment-interaction
This is also an interesting article with examples of environmental effects on phenotype:
https://www.nature.com/scitable/topicpage/environment-controls-gene-expression-sex-determination-and-982(1 vote)
Video transcript
- [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.