Population bottlenecks, this is actually
one of my favorite topics to think about in evolution
since there are some pretty neat examples of bottlenecks.
But I'll go into those specifics later.
For now, let's go over the concept of natural selection.
So if you have a member of population
that has a special trait, like being really strong,
then that person is more likely to live to an age
where he or she can reproduce and pass on that special trait
to offspring.
But you should know that as times change,
so too does the specialness of a trait.
The desirability of a trait is not constant.
Let's say that a new predator comes
into play in our population here and that
can overpower even our strong people.
Maybe now, someone who's tall is more
likely to survive than someone who's strong.
If you're tall, you have longer legs and can run a lot faster.
And since these new predators are too tough to fight off,
the people that are best at running away
are more likely to survive.
So now our tall person is most likely to survive.
And he'll reproduce and pass on that trait
and have lots of tall offspring.
Now, also keep in mind that populations are usually bigger
than just two people.
And this is great for a population
because it means that there are lots of traits in the gene pool
and they can all adapt differently
to changes in the environment.
The more traits in the gene pool,
the more likely that species will survive what we usually
call environmental stresses.
Now, sometimes these environmental stresses
can be massive.
So massive that they almost completely
wipe out a population.
If you were to make a graph looking at a population
size over time, you'd see a huge dip
from its environmental stress.
And we call these population bottlenecks
when the stress wipes out so much of the population
that only a tiny fraction remains.
And we use the word "bottleneck" because we
imagine that we have this bottle.
And all of these blue beads inside
will represent different people in the population.
And because the neck of the bottle is so narrow,
only a couple of beads will make their way out,
just like environmental stress that's significant
will only let a few people survive.
So there are generally two outcomes
that usually result from a big environmental stress.
Either the population can't adapt to the change
and goes extinct or somehow the population recovers
from that stress and can build up its population again.
And we call this a bottleneck.
So let's look at an example of each outcome.
So first, we'll look at an example
where the population couldn't adapt.
Now, we all know that a long time ago dinosaurs
ruled the Earth.
Well, scientists think that one day a giant meteor
came crashing down to the Earth and went kaboom, and completely
changed the environment that the dinosaurs were living in.
So in this case, the meteor and the changes
to the environment that it made was the environmental stress.
And the dinosaurs just couldn't cope with the change.
And they all died and went extinct.
Now, let's look at an example where
a fraction of the population survived the stress.
And this is the story of the peppered moth.
So in the mid-1800s, there was an area of land
with two populations of the peppered moth.
One of them was white colored and the other
was a much darker, black color.
Now, the trees in the area were also white.
And when the moths' predators, which were birds,
came looking for moths to eat, the moths
would hide by attempting to camouflage themselves
on the trees.
And naturally since the trees were white,
the white moths were much better at doing
this that the black ones.
And as a result, around 95% of the peppered moth population
was white, while only 5% of the peppered moth population
was black.
However, a few years later, the Industrial Revolution happened
and factories started to get built in the area.
And they would release huge amounts of soot into the air.
And this soot started collecting on trees and dyed them black.
Now, because of the change, white moths
couldn't camouflage themselves anymore.
And birds ate a huge chunk of the peppered months
since they couldn't avoid their predators.
But what happened was the small population
of moths that did survive this change were
the black peppered moths.
The soot covering the trees is the environmental stress here.
And what's so interesting about this example
is that the population of moths did recover.
But now, most of the moths, 95%, were black,
since they could camouflage themselves
on these newly black trees.
And only a tiny 5% were white.
And this is a great example of the bottleneck effect
since the overall genetic makeup of the peppered moth
population completely changed as a result
of this environmental stress.
So now, let's revisit our graph from before.
If we have a cataclysmic event that
kills a huge number of individuals in a population,
the population will either be unable to cope
with the change and their population will drop to zero
and they'll go extinct, or a small group will survive
and the population can ultimately recover.
So what did we learn?
Well, first we learned that a bottleneck
is an event or change in the environment that
destroys a huge chunk of a population.
It just massacres the population.
But we also learned that following a bottleneck,
a population can either go extinct
if they're unable to cope with the change or a small group
can survive and the population will eventually recover.
But it's important to remember that when a population does
recover from a huge stress like a bottleneck,
the gene pools in that population
will change significantly.