Hi. It's Paul Andersen and this
is disciplinary core idea LS4B. It's on natural
selection. I believe natural selection as
proposed by Charles Darwin is one of the greatest
ideas in all of science. A lot of people think
wrongly that Darwin's famous because he came
up with this idea of evolution. That's not
true. What he really came up with was a mechanism
that explains why evolution occurs. And the
definition I give my students, the ones that
I want them to remember from the beginning
of the year to the end is it's differential
reproductive success. We'll come to that in
just a second. One of the greatest examples
of natural selection that we've seen in the
modern day is the story of the peppered moth.
The peppered moth comes in two different forms.
A dark form, which is actually dominant. And
light form which is recessive. And the trees
that they live in are generally light. And
so if you're a bird trying to find these peppered
moths, this dark one is going to stand out.
So you're going to eat those and then the
light ones are going to be greater in number
in the population. But what happened during
the Industrial Revolution is the factories
started pumping out so much coal dust that
these trees were covered in this black soot.
And so what happened at that point, now the
dark ones were able to camouflage and the
light ones stuck out like a sore thumb. And
so if we look at the data that was collected,
so data in 1848, 98 percent of the moths were
light in color. And by 1895 that number had
dropped to just 5 percent. And so what had
happened, well it's not like the moths had
somehow changed their colors. It's that the
birds had targeted different moths during
those different time periods. And so during
a time when the trees were white they were
eating the dark moths. And when the trees
were dark they were eating the white moths.
And so now as we've cleaned up our act and
the trees have returned to that white color,
we're seeing more of those white moths. And
so natural selection requires two things.
First of all it requires variation in the
population to being with. There have to be
differences. And so a great example that we
can see really over years is the natural selection
that we're seeing in bacteria. And so bacteria,
let's imagine these bacteria right here show
different levels of resistance to antibiotics.
And so some are going to have a high level
of resistance to antibiotics. Antibiotics
like penicillin. And some who are going to
have a low. And so we have to have variation
in those. And that's going to be genetic differences
in those bacteria that are manifested in how
well they respond to an antibiotic. And the
second thing we have to have for natural selection
to occur is a selection process. And so something
has to target those different variations in
traits differently. And so if we were to apply
an antibiotic to this population of bacteria,
it's going to kill all of the ones that have
low resistance to the antibiotics. And so
after selection we're going to only have bacteria
that survive that are resistant to the antibiotic.
And so that population is going to change.
As these reproduce, we're going to have a
different population. And so how do you teach
this? What's the teaching progression? Well
in the low elementary grades you shouldn't.
It's just intentionally left blank. As you
move into the upper elementary grades you
want to talk about differences in organisms.
And the idea that as they look at any population
of organisms you're going to see differences.
So if we have a bunch of giraffes, there are
going to be differences in their neck length.
If we're to look at tigers, differences in
their strip patterns. Or polar bears, difference
in their weight. And what those differences
can lead to is differences in survival. And
so imagine during a drought when there's not
a lot of leaves close to the ground, the taller
giraffes are going to do well. And the shorter
giraffes, not so much. And so that survival
difference is eventually going to allow them
to find mates and reproduce differently. And
so you have to have differences to begin with.
And then those differences are going to played
against their environment. As you move into
middle school you really want to talk specifically
about the two parts of natural selection.
This idea that you have variation in the population.
So variation like we saw in the peppered moths.
Or variation like we see in these bacteria.
And then you have a selection process. A selection
process where something in the environment
is selecting for or against individuals. Now
when Darwin was trying to explain this to
people he started not with nature but he started
with artificial selection. So what's artificial
selection? That's something that we can see
during our life time. Artificial selection
is when you have variation in a population.
So like variation in these wolves. And then
you have a selective pressure, but that selective
pressure is humans. And so all of the different
dog breeds that we have on our planet, all
of the different crops that we have on our
planet have been created by humans. And we've
created them by selecting traits that we like
and getting rid of traits that we don't. So
breeding specific wolves over time created
all of the dog breeds that we have on our
planet. As you move into high school then
you want to talk specifically about these
two things. The idea that there is genetic
variation in the population. So going back
to the peppered moths, there were genetic
differences between these two moths. These
white ones were going to be a recessive trait.
The dark ones were going to be a dominant
trait. And that gene or genetic difference
has to manifest itself in a trait variation.
If we didn't see a difference on the outside
the environment wouldn't be able to select
for or against any of these individuals. But
if we have genetic variation that creates
trait variation. And then we have selection
in our environment, we're always going to
have natural selection occur. And natural
selection can eventually lead to adaptation.
Which is going to be the next video. And I
hope that was helpful.
