Hi. It's Mr. Andersen and welcome to biology essentials video number 9. This
is on populations. How they came to be through evolution. How they continue to evolve every
day. One of my favorite theories in all of evolution is the idea of the Red Queen
Theory. And it essentially comes from Alice Through the Looking Glass or Through the Looking
Glass where Alice remembers running around with the red queen. And she eventually stops
and says "Well, in my country, you'd generally get somewhere if you run as fast as you can
like we've been doing." And the red queen says, "You live is a slow sort of country.
Here it takes all the running you can do just to stay in the same place. If you want to
get somewhere else you must run at least twice as fast." And so what's interesting about
that is it's a really good metaphor for how natural selection and evolution takes place.
Species are constantly in competition against all of the other organisms around them. And
they may not look like they're changing but they continue to change and it's shaped the
life that we have on our planet. And so what am I going to talk about? Well I'm going to
start with evolution and the idea is that it's been around from day one. So from day
one until today all life on our planet has been created through evolution or change.
And also it continues on a daily basis. Species are evolving every day. Populations are evolving.
Because remember all evolution is is a change in the allele frequency of a gene pool. It
can lead to speciation but it's constantly happening every day. What are the mechanisms
by which it occurs? Well we've already talked about a lot of these, genetic drift, mutations
and we'll talk more about them in the next pod cast. But today I want to specifically
talk about selection. And if you don't have a good understanding of how natural selection
occurs, today you're going to get it. And so natural selection is when nature selects
which organisms are able to survive or die. And a quintessential example is in the beak
of the finch and the work of Darwin, but more importantly the work of the Grants. Sexual
selection is a different type of selection and that's when mates are selecting based
on the other gender. An example I'll give you is a peacock. And then the one thing that
I forgot to put here is artificial selection, which is another important one. Artificial
selection is another type of selection and that's actually where Darwin starts explaining
the mechanisms by which natural selection could occur. One thing that I want to drive
home is the idea that all life evolves. So once life shows up on our planet it's been
evolving ever since then. And so archaea live here in the hot pots of Yellowstone Park and
they look very similar to what the first organisms on our planet probably looked like. But the
archaea that live here and the animals that live in the savannah and the forest all of
these things all life came from one common ancestor and they got to be the way they are
through a process of evolution. So let's talk specifically about the mechanisms by which
that occurs. And when Darwin is explaining in the Origin of the Species he actually starts
with artificial selection. Cause it really makes sense to us how that might occur. If
we were to look at this Great Dane and we were to look at this Chihuahua and if we were
to look at a wolf, the DNA of all of them is almost identical. And the reason why is
that these dog breeds that we have on our planet were created by humans, artificially
choosing traits that they want. And you can see the explosion of tiny little breeds of
dogs that I am seeing around right now. It's humans making those choices, selecting which
offspring can pass on their traits. In class this year we will be using Brassica rapa which
is a Wisconsin fast plant. What you can do is, you can actually do the selecting, you
can choose the traits you want to pass on to the next generation. And then you can see
that artificial selection taking place. And actually Brassica is a type of plant that
led to a lot of the different foods that we have today. In fact a lot of the foods that you
eat were created through a process of artificial selection. Today I want to spend a lot of
time however talking about natural selection, the mechanisms by which that occurs. The most
famous studies on Galapagos were done by a Peter and Rosemary Grant. And they studied
Geospiza fortis. Hope I am pronouncing that right. And they studied those on Daphne Major,
which is this rock. I've seen it from a distance. It's just this rock in the Galapagos but it's
a great natural laboratory. So you can study the bird populations on it. Now natural selection
is going to select species to either survive or die and then the traits are passed on to
the next generation. And so here's some data from Peter and Rosemary Grant. In 1976 they
sampled all of the birds on Daphne Major that they could find and they measured their beak
depth. In other words they measured this distance from here to here. So they captured them probably
in nets they measured them with a caliber and they measured the beak depth all the way
across in 1976. And they studied that in 1977 and they studied the beaks for 30 years they've
been studying the beaks of the finches on the Galapagos. But what they found is in 1977
there's a drought and so they go back in 1978, and so let me get the numbers right so you
can see them, in 1976 they sampled 751 birds. So there were 751 birds here, but they come
back in 1978 and they found there were only 90 birds left. And so lots of times when we
draw these graphs you have a tendency to not realize what the end number or how many species,
how many individuals were actually there. And so what happened in that drought of '77?
It's like a bird apocalypse. Almost all the birds died from 751 down to 90. Which ones
didn't die? The ones that didn't die are ones that had a slightly bigger beak. And so why
is that? Well, what the Grant's found is that they were feeding on seeds that were slightly
bigger and harder to break. And so the ones that were able to break those seeds were able
to survive, not die and they were able to pass their genes on to the next generation.
And so what happened to our bell shaped curve, our bell shaped curve moved to the right.
And so we call that directional selection. Now are they going to stay this way? No, because
if you have a big beak you have to make that beak, you have to fly it around and so when
the weather gets good what we'll see is we'll actually see, when there's no drought, we're
actually going to see it move back in the other direction or directional change. And
so this moving over and over and over, that's the first type called directional selection.
We can also have disruptive selection. Disruptive selection happens when you have a bell shaped
curve of a trait and then it somehow gets split in the middle. And so an example of
that would be the idea that all of the birds on the Galapagos, this is a picture that's
actually from Darwin's book, all of the finches on the Galapagos started from one ancestral
species. In other words there was one finch that flew there or population that flew there
maybe 2-3 million years ago and the fourteen beaks, or the 14 species that we now have
on the Galapagos Islands actually came through disruptive selection. In other words if they
start feeding on different seeds you eventually can have two different species and they quit
interbreeding and then you can have different varieties. And so even though these look quite
a bit different, they all come from one common ancestor. My favorite are the ones that are
actually feeding on the, the vampire finch, that feeds on the, they suck the blood off
of the blue footed boobies in the Galapagos. Pretty crazy. Last type is stabilizing selection.
Stabilizing selection occurs when you have, here's our original bell shaped curve, it
happens when you have selection on either side. In other words you're squeezing that
bell shape a little bit closer. I couldn't come up with great example of the finches on the
Galapagos of that stabilizing selection, however, it would probably be what shows why one species
stays the same. But a great example of this would be in the birth of children. In other
words if you were a tiny baby, let's say back in the day you weighed one pound when you
were born you probably were born premature and you died. If you were like eighteen pounds
as a baby you probably didn't get out of your mother and you died and her genes died with
you. And so that's going to push that bell shaped curve together. That's artificial selection
excuse me, natural selection. And then the last type is going to be sexual selection.
Sexual selection, it was come up with by Darwin was well, but not really tested until the
19th century or the 20th century. In sexual selection the mate is actually doing the choice.
And lots of times we'll see sexual dimorphism. In other words, this is a peahen and this
is a peacock. And so we see a huge difference in their appearance and the reason why is
that the female is making the choice. It's not nature that is making the choice she's
actually making the choice. And so why does a peacock have this beautiful train that it
does? It's just going to get in the way. It's doing that to impress the mate. In other words
this female is only going to choose a peacock that has a really big train. Now why is she
doing that? She's looking for herself. She's only going to choose the trait that's going
to pass on, or it's a clear indication of the health of that mate. And so if you can
make a beautiful train like this you probably have good DNA. And there were studies done
where they would actually eliminate, so they would cut off some of these eyes and they
would find the female, the peahens would actually ignore them. They also found in the studies
that if the males have a really beautiful train like this, they actually produce offspring
that are more likely to survive. And so in sexual selection the female is actually doing
the choice, not nature. And so if we ever see that sexual dimorphism, like in Montana
we have elk and the elk have these huge antlers but their doing that to impress their mate.
Now Geoffrey Miller has taken this to an extreme, not to an extreme, but he is saying that what
is it in humans that is like the peacock's feathers? What is it that's like that in us?
It's grossly in excess. What you'd see in humans is that it's our brain. It's how smart
we are. And why are we so smart? Well, we're trying to impress females and so it's interesting
when you look at males and ask them what are they attracted to in females, a lot of times
it's physical characteristics but when you ask females it's more behavior. He makes me
laugh or he makes me feel good. And so those things are clear indications of brain and
Geoffrey Miller will go as far to say that culture and all of those things are just made
through a process of sexual selection. I think he's headed in the right direction.