So today we're going to talk about a topic that's
very central to the idea of evolution,
and that's natural selection.
But before we get into that, I want to talk about what
evolution isn't.
So evolution isn't when some organism like this monkey
magically transforms into a human.
And it's also not when an organism
changes in some way when it's in trouble, like a person growing
wings after jumping off a building.
And I want to clarify that this is not
what we're referring to when we think of evolution.
And evolution is a process that occurs to populations
of an organism, not individual members.
And it occurs over huge amounts of time,
and we're talking millions and millions
of years for even small changes.
So natural selection is one of the forces
that ultimately drives evolution,
but what is natural selection exactly?
Well, why don't we jump right in and look at an example?
Let's say it's 10,000 years ago and people survived by hunting
and gathering, but they also have
to worry about being chased around by wild animals.
So in order to survive, these people
need to be able to find food.
But they also need to be able to escape from predators.
Well, let's say that one of the people of these two
has a special genetic trait and has
slightly longer legs than the other guy.
Now, these longer legs put him at an advantage,
because his legs are longer and he can run,
let's say two times as fast as everyone else.
And because of this, he's more likely to survive
when a predator like this bear chases him down.
So what this also means is that the guy with the long legs
is more likely to reach an age where he's old enough
to find a mate, reproduce, and have children who would also
have this special trait of longer legs
because it's genetic.
And because he's more likely to have kids than everyone else,
over long periods of time soon more and more of the population
will have this special trait.
Now, let's look at this idea again but a little more deeply.
And let's say there are six people in the world
and two of them have longer legs than everyone else,
And let's say that the ones with the longer legs
have a 50% chance of surviving and reproducing
while the shorter-legged people have
only a 25% chance of surviving and reproducing.
So that means one of our two long-legged people
and one of our four short-legged people
here will reach an age where they can reproduce,
so now these people who survived will each have four children.
And naturally, these children will resemble their parents.
And the children of long-legged people
will also have long legs, and the children
of short-legged people will have short legs.
So now in our next generation, we
have four people with long legs and four with short legs.
And you can already see that more of the population
has long legs than when we started.
But let's take it another generation further.
So half of our long-legged people
will reproduce, whereas only 1/4 of our short-legged people
will reproduce.
And this means that by our third generation,
we'll have eight long-legged kids
and only four short-legged ones.
Now, if we number our generations,
generations one, two, and three, we
can see that in generation one, 33% of the population
was long legged.
In generation two, 50% of the population was long legged.
And by generation three, 67% of our population was long legged.
And this is all because that special trait of having longer
legs made those people more likely to survive and reproduce
than those with short legs, and this
is the crux of how natural selection works.
So why is it called natural selection in the first place?
Well, let's say with our example of the short- and long-legged
people.
Now, we use the word selection because one trait
is advantageous over another and is
selected to be passed on to future generations
more than other traits.
On the other hand, selection can also
apply to a disadvantageous trait.
If we have people who have really short legs
and run really slowly, then those people
will be selected against and won't
pass on traits to offspring as frequently.
Now, we use the word natural because there
isn't an individual who's physically selecting which
traits are good and which ones are bad.
It all has to do with whoever has the greatest
probability of surviving.
There's no one actually doing the selecting
except nature itself.
Now finally, I just want to point out
that natural selection does not apply
to acquired characteristics.
If a father teaches his son how to hunt
and this makes a child more likely to survive,
that isn't a trait that's selected for us since it's not
genetic and it's not absolutely passed on to children.
So that's why we say that natural selection only
applies to heritable traits, with heritable traits being
any genetic trait.
So what did we learn?
Well, first we learned about the concept of natural selection
and how traits that help an organism survive
are more likely to get passed on to offspring.
Next we learned that evolution, which
is driven by natural selection, occurs to populations, not
individuals, and occurs over a huge period of time.
And finally, we learned that natural selection only
applies to heritable traits, ones
that are genetic and passed down from generation to generation.