Why isn't every animal the same?
Why isn't there just one type of bird, or
one type of fish?
There are different pressures in the world
which means that different animals succeed
in different places.
And the result is called natural selection.
Let's select some examples and see what we
learn.
If we look at a very simple organism like
bacteria, they reproduce by splitting in half.
The DNA is divided into two equal parts and
the cells both make copies to pass on to their
daughters when they divide.
If copying is perfect, and there are no other
forces at play, then the bacteria should copy
itself identically every time forever, until
it covers the Earth.
Clearly this is not how things work.
Copying isn't perfect.
And generations do change over time as a result.
We have talked about mutations before.
Every now and then there is a change in the
genetic code.
Most of those changes are not useful.
Some of them do nothing.
And the very rarest ones are actually useful.
Here's a simple example.
No offense, but in your kitchen there are
mold spores everywhere.
And that's not because of whether you clean
your kitchen or not.
There are mold spores basically everywhere
on Earth.
So when these mold spores land on, say, a
piece of toast, the mold grows, digests the
toast, and makes new mold spores.
But that mold has to compete for space and
access to toast with every other mold spore
and bacterial colony that landed on the toast.
It's a hard life.
But there could be one mold spore that, for
whatever reason, has the ability to make a
new chemical.
Penicillin.
This chemical makes it so that every mold
in the area, instead of growing, dies.
This is also true in a science lab.
You can see in a petri dish when there's a
penicillin-making mold present, because it
makes a moat of empty space around itself.
And the more it advances, the further back
the other mold gets pushed.
All that space and food is now available for
the pink mold, for free.
Same thing on the toast.
Every mold which makes penicillin has an enormous
advantage in getting food and making mold
spores.
The pink mold gets more food and makes more
spores.
The yellow mold gets less food and makes fewer
spores.
The spores are what start the next mold generation,
and that generation starts with more pink
mold, and the process repeats.
So over a few generations, more of the mold
population becomes the descendants of pink
mold, with the ability to make penicillin.
That's natural selection.
More useful traits become more common.
Less useful traits become less common.
It just happens... naturally.
Some examples of natural selection in action.
Camouflage.
Fur.
Teeth.
And really, really big tails.
Each one gives some advantage to the animal
that has it, and that makes it become more
common.
Every trait is selected this way in every
single animal.
They want their useful traits to outweigh
their not useful traits.
If they do, great, they become more common.
If they don't, sorry, they become less common.
Natural selection changes populations all
the time.
And it gradually shapes the population to
be more and more adapted to their environment.
This is why so many animals have found so
many ways to succeed in the world.
We're just starting to understand all the
ways that animals have adapted to their environments.
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selection.
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