Hi. It's Mr. Andersen and in this video I'm going to talk about selection.
Most of us are familiar with natural selection and the work of Charles Darwin. We'll talk
about that. But I'm going to talk about two other forms of selection, both artificial
and sexual selection in this video. When you're trying to use the word selection sometimes
students are confused. And so the best analogy I can come up with is thinking back to your
days playing kick ball or playing baseball when you were young and they'd have everybody
line up and then the two captains would get to choose who they want on their team. And
hopefully you didn't get picked last. I hope they don't do this in elementary anymore,
but that's selection. Selection is when someone is choosing or something is choosing who is
the strongest, the fastest. And then those that aren't chosen, in kickball you just get
chosen last, but in natural selection or artificial selection that means you don't get to pass
your genes on to the next generation. And before you can have selection then you have
to have variation. And so right here are two graphs of variation in human heights. And
so this purple is going to be male and this yellow is going to be females. And so an average
male in the US is going to be around 5 feet 8. The average female is going to be around
5 foot 3. But we have a few people that are much shorter than that and a few people that
are going to be like 6'8", 7 feet tall. And so we got variation. And so in nature we see
that same thing. So these are brassica rapa. It's a fast plant and we were playing with
these in class. Basically you plant a bunch of them. Some come up in two days, some in
three days, some in one and a half days, and so you get variation. And also as they grown
you're going to get variation in size of the leaves, variation in the number of flowers,
how tall they are. They ave little tricomes on the side, so how many of those they have.
And so you have to start with variation, just like in that kickball example, then you're
going to choose based on that variation. And so basically selection we could break down
in this concept map. First of all selection can either be artificial selection. That's
going to be when humans make the choice of who, what genes get passed to the next generation.
And then natural selection, that's when the selection takes place in the environment.
Natural selection we could break down into ecological natural selection. That's going
to be that run of the mill survival of the fittest, differential reproductive success
that you're all familiar with. And then another type of natural selection is sexual selection.
That's when females are making the choice as far as what genes go on to the next generation.
And we could break that down into inter and intra and so that's basically what rest of
the podcast is going to be about. We'll start with artificial selection and we'll end with
sexual selection. So let's start with artificial selection. So let's say this is the mom dog
and she has a bunch a puppies. Well basically what we can do is we can choose which puppies
we want to pass their genes on to the next generation. And so that's how we created a
Chihuahua and St. Bernard. By just choosing the traits that we want in the dogs, breeding
those to create the next generation and the next generation and the next generation. It's
not that we're killing dogs that we don't like. Basically we're choosing the ones that
we do like and we're allowing those to breed and pass their genes on to the next generation.
Humans have been doing this from the beginning time. And so basically once agriculture starts
then we start selecting traits that we want. And we've been doing this for thousands of
years. This guy right here, Norman Borlaug, is somebody who most of you don't know but
he's known as the father of the green revolution. In other words, we were under duress on our
planet. We were running out of food. We weren't able to produce enough food to feed everyone
and so this started 30s, 40s, 50s on. And so basically Norman Borlaug was a farmer and
what he did is produce a number of different traits of wheat. And he started by working
in Mexico. And so he produced wheat. First of all he was able to produce wheat two times
a season. He produced wheat that had bigger grains on the top by doing artificial selection.
He also, when they grew too tall they'd fall over, and so then you would hybridize them
with a shorter kind of a mutant that had short stalks and so they could have a lot of seeds
on that. And so as a result of that we were able to increase wheat yields on the planet.
So look at this. By the work of Norman Borlaug and others, we took production of wheat on
our planet, 1950 up to in 1990 and up to 2000 it's increased you know many fold. And that's
through artificial selection and selecting the traits we want. And so when humans are
making that choice based on variation we call that artificial selection. Ecological selection
is a form of natural selection where nature makes the choice. And this was first described
by Darwin, along with sexual selection. And basically we could break it down in to three
things that can happen to a bell shaped curve. The first of those is going to be called directional
selection. So imagine we have this variation. So we're going to have one end and one end.
So we've got extremes, but most of the averages are going to be in the middle. And so let's
say change takes place in the ecology. Change takes place in the ecosystem, how organisms
are going to be selected for or against. And so an example could be these which are the
glacier lilies. Glacier lilies come up right after the snow leaves. And so basically they'll
start to come out right when the snow melts and then their going to do pollination and
then make the next generation. But basically what's going on right now is that the climate
is getting warmer and warmer and warmer. And so what's happening, well as it gets warmer
and warmer and warmer, the glacier lilies that come out right here aren't going to do
very well. But the glacier lilies that happen to come out a little earlier are going to
do better. And so we're going to see directional selection. In other words, we're going to
see this curve move in one direction. And so how does that work? Basically it works
by, if you're a glacier lily that now comes out much later, you're going to die because
it's going to be way too hot at that point. And so we get a push in this direction. That's
directional selection. We also can have stabilizing selection. A great example of that, if we
think of this red line as before, why do most babies weigh around 7 pounds? Well, if you
were to be born and weigh like one pound back in the day, you'd die. You were premature
and you didn't survive. If you were to be an 18 pound baby and you were to be born right
here, you're going to get stuck in your mom and your genes are going to die and your mom's
genes are going to die. And we're not going to pass that on to the next generation. And
so basically what we have, we're having selection. We're killing on either side of this bell
shaped curve. And so that's going to squeeze the bell shaped curve together. And then the
last thing that can happen is if we think of this red being the bell shaped curve, maybe
the ones in the middle aren't doing that well but the ones on the extremes are doing well.
What's an example of that? Well remember those first finches on Galapagos arrived there and
they started filling different niches. They started eating different foods. And so let's
say one feeds on a small seed, one feeds on a big seed, basically we get disruptive or
diversifying selection. Now what can eventually happen is that can split into two species.
And so basically this is all natural selection. The organisms that are selected, the organisms
that survive pass their genes on to the next generation. And so this made sense to everybody
and the time of Darwin. But the things that was puzzling was sexual selection. So let
me tell you the story of the whiptail lizard. Whiptail lizard lives in the desert Southwest.
They are camouflaged well but what's interesting about them is that they don't have any males.
They only have females in this certain species of whiptail lizard. So how do they reproduce
you might think. Basically what they do is they make a copy of a cell, parthenogenesis
we call this, and that cell is going to be just like every other cell in the lizard except
it can spawn a clone of itself. And so every female lizard in that area is a clone of every
other female lizard in the area. So they've basically gotten rid of males. So you might
think, wow, especially if you're a female. That would be great. We could get rid of all
the males and then life would be easy, but there's problems here. And since they're all
clones than any disease that might target one of them is going to target all of them.
And so basically there is no variation. There's no selection. And so the reason we have males
is to provide new genes to our offspring. In other words, females that share their DNA
with a male produce offspring that are not like them. And so that produces variation.
And so that led to what is called sexual selection. And this was puzzling to Darwin for awhile.
In fact, he's quoted as saying the site of a peacock just makes me ill because he didn't
understand why you'd have these real elaborate things in nature if they're always going to
be selected for ones that fit well in their environment. In other words, these feathers
are going to slow them down, they're going to make them easier to be caught in nature.
And so basically what he set along this idea of sexual selection. In sexual selection the
females are choosing which traits are passed on and which genes go to the next generation.
And so basically we could break that down into Inter and Intra sexual. Inter sexual,
I remember this like an interstate goes from state to state, so it's between states. Inter
sexual is when a female is choosing a male based on characteristics that they have. And
so inter sexual the female peahen is choosing a male that has the brightest of feathers,
because if he has genes that can make bright feathers he also has genes that are going
to make offspring that are able to survive. And so she's mating with him, not just because
she's impressed by the color, but she's impressed by his genes. And so humans do that as well.
And so Angelina Jolie when she's choosing Brad Pitt basically she's looking at all of
his characteristics on the outside and she's saying, oh he has good genes, so maybe he's
going to be able to produce offspring that have genes. Intra sexual, intra means between,
and so that's going to be fighting between, within an individual sex. And so an example
of that would be like these elephant seals battling to control a harem of females or
like bull elk in Montana will fight to control a Harem of females. They're fighting within
that. And so basically what goes on there is that the females just kind of sit back,
they wait until that one male dominates all the other males. If he can win that fight
then he's going to have good genes. He's also going to be able to produce fertile offspring.
And so in both these cases it's the female that's making the choice. It's not nature
it's the female. And once we have a trait that really is a good indication of genetic
health it has a tendency to go out of control. And so some scientists suggest that in humans
it's the brain that's that. In other words females are choosing a mate based on how good
their genes are. And a good indicator of that is your brain. And so when you ask females
what they're attracted to, a lot of the time it's not physical characteristics, although
those are very important, it's, does he make me laugh? Is he willing to take care of me?
And these are really good judges of health. And so in review, what do we have? We've go
three types of selection. Artificial is when humans choose. Natural, ecological natural
selection is going to be when the environment chooses. And then sexual selection is when
the females choose. And why do they get to choose? Well, they have more to lose. They
carry the eggs. They have a limited number of eggs. Lots of times in nature they have
to take care of the young and sperm is cheap. And so it's more important that the females
make that choice. And so that's selection and I hope that's helpful.