Genetic drift | Heredity & Evolution | Biology | Khan Academy


– [Instructor] Natural
selection is a process in which if there is a trait
which has an advantage, meaning higher chances of survival, then automatically, it gets more passed on because more chances of reproducing, and over generation, its
number can start increasing, and this is one way in
which evolution can happen. And we’ll get back to this in a second. But in this video, I wanna
talk about a different way in which evolution can take place and this is called genetic drift. So let’s find out what genetic drift is. So, I like to think of a genetic
drift as evolution by luck. Evolution by luck. And you will see why I say that. So let’s get back to our story of beetles. So, in a previous video, we were focusing on this group of beetles, which are mostly red in color,
but then due to mutation, some beetles were born differently. Some had green color
and some had blue color. And these crows were an
important part of the story because they eat on these beetles, and this is where the green
beetles had an advantage. They are hard to see even in this picture. They are hard to notice, right? Which means less chance of getting eaten, more chances of their survival, and so more chances of them reproducing and passing on those genes. And so, as time passed on, their
numbers started increasing, and that’s what we call natural selection. And, of course, this has
been explained in more detail in our previous videos on
evolution and natural selection, so if you need more clarity, definitely you can go back and watch that. But in this video, let’s
consider a different scenario. Let’s say before natural
selection has time to kick in, some calamity strikes. Maybe there’s a fire or an earthquake or lightning strike or cyclone, or something like that happens. Let’s consider something simple. Let’s say some animal comes
in, stomps it, and goes. That can totally happen. So let’s say an elephant
stomps on it, thud! And this kills almost all the beetles. Only a few ones over here survives. What happens next? Well, a couple of things can happen. Now maybe, the crows will eat all of them and they just die out so
our population is vanished. That’s one scenario. Another scenario could be if they survive, they can start reproducing
and repopulating. But now what’s interesting to see is there are more blue-colored beetles. That means as time passes
by, there’s a good chance that more blue-colored
beetles will be found. And if we compare this with
the previous situation, what do you see? Well, earlier, the red
beetles were more in number which means the genes
responsible for red color, they were more frequently seen and more frequently being passed along. So red color genes had a higher frequency. But afterwards, see what happened. Now, blue color are in majority. That means their genes
are more frequently seen, the genes responsible for blue color. That’s in high frequency now which means our beetles have evolved. That’s the definition of evolution. When the gene frequency changes over generation in a population, that’s what we call as evolution so the beetles have definitely evolved. But think about what
caused this evolution. Did they evolve because the blue beetles had some kind of an advantage? Absolutely no. They have no advantage
compared to red beetles. At least in this scenario,
they can be easily spotted. It’s the green ones that
had an advantage, right? But the only reason this happened, this whole evolution
happened to blue beetles is because in that
stampede when that elephant stomped the group of beetles, they just happen to be in the right place at the right time purely by
luck and so they survived. Most of the red beetles perished. The green beetles also perished and it is for that reason
they evolved this way. And that’s why I like to
call this evolution by luck. So it’s called genetic drift
because in this disaster, most of the red genes just drifted away, meaning they just vanished
because they died. The green genes, the genes
responsible for green color, which would have been naturally selected, they also vanished. They just died and drifted away. So most of these genes drifted away because of some calamity, some disaster. That’s why it’s called genetic drift. So evolution can also happen. So this is evolution. Let me just write that. This is evolution but it’s not caused due to natural selection, it’s not because they have an advantage, but it happened purely by chance. And so the basic moral of the story here is that in certain populations, you might see certain traits
which are being passed along even though they have
absolutely no advantage in that environment. And that happens mainly
because of genetic drift, meaning some random event
caused all the other genes to just vanish away. Now, before you wind up,
one question for you. Do you think genetic drift
happens in a large population or in small population? What do you think? Okay, let’s see. So let’s imagine this was a
large population of beetles, say thousands of beetles. And the way I’m gonna show that is I’m just gonna take
that same elephant foot and make it smaller. This does not mean that the
elephant has become smaller. Think of it as, since we’re dealing with large number of beetles, they are occupying a much larger area, so I’m zooming out to show you this. And from that perspective, the
elephant foot looks smaller. Think of it that way. Now, in this situation, do you see that there
is a very good chance that almost all the genes might survive? Even if the elephant stomps
multiple times, think about it. Because they are more
spread out over here, when that stampede happens, definitely a lot of red will survive but maybe there were some of these greens were in this corner, some of the greens were in this corner, some of the blues may
have been in that corner. So because now there are more
in number compared to before, there’s a very good chance that all those genes might survive. So yeah, the beetles will die definitely but all these genes might still survive and if they survive this
particular catastrophe, then it’s the green one that
will get naturally selected. Ooh, so what does this mean? This means in large population, we don’t expect a genetic drift to happen. Genetic drift only happens
in small populations. So let me just write that down. That’s super-important. You would expect genetic drift to happen in small populations. Small population. That’s pretty much it. So what did we learn in this video? We learned something called genetic drift. It’s a process in which random events can make certain genes just drift away and then automatically, the genes that survive that random event will get more passed on and
ends up becoming majority in that population. Because of this, in certain population, even though certain traits
have absolutely no advantage, they might still be
found majority in number. And remember, genetic drift
happens in small population. Smaller the population, more chances of having genetic drift.

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1 Response

  1. obeytanush says:

    bam bingewatching over

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