How are all of the species living on Earth today related? How does understanding evolutionary science contribute to our well-being? In this course, participants will learn about evolutionary relationships, population genetics, and natural and artificial selection. Participants will explore evolutionary science and learn how to integrate it into their classrooms.
Darwin's Theory of Natural Selection
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Del curso dictado por American Museum of Natural History
Evolution: A Course for Educators
175 calificaciones
De la lección
Darwin's Second Great Idea - Adaptation via Natural Selection
You will learn about Darwin’s second breakthrough: that adaptation via natural selection is the basic mechanism of evolution. You’ll go behind the scenes with Dr. Cracraft to see how evolutionary biologists use the Museum’s collections. Lastly, you’ll choose a topic from the course and explain how to use it as empirical evidence that supports common ancestry and biological evolution.
Conoce a los instructores
Joel Cracraft, Ph.D.Chair and Lamont Curator
Division of Vertebrate Zoology
David Randle, Ph.D.Senior Manager of Professional Development
Education Department
Well welcome to the second week of
the American Museum of Natural History's Evolution course.
Today we're going to talk about Darwin's
really, second big idea, natural selection.
And we'll look at heritability.
We'll look at the variety of natural selection that is out there, and
then we'll investigate how natural selection involves
societal problems and solves some societal problems.
So,
again, we start with Darwin's Theory of Natural Selection.
This is actually a very, very simple concept.
It starts with the idea that within populations, individual organisms within
populations, vary in some phenotypic trait and that means things like body size.
So within humans, body size, all kinds of things vary.
And the second is that some of that variation
has an hereditary basis, that some genetics
underlies some of that variation. Maybe not all the variation, but at least
some of the variation. And then finally, if that variation has
something to do with survival of young or adults then it is natural selection
is just a, an inevitable outcome. And so let's
look at heritability first.
And, and it's not a very difficult concept because all of us know
that we have taken some of our parent's characteristics because of heredity.
We can measure heritability in the wild very simply by saying, we're,
we have, we have a feature, some quantitative feature, say height.
And we can measure the
height of the father, we can measure the height of the
mother, and divide it by 2 to get the mid parent range.
Then we can look at all the offspring and do the very same thing.
And we get a curve between the value for the mid parent range.
And a mean of all the offspring, on, on the Y axis.
And if there's a perfect correlation between the parent and
the offspring values, then it's going to fall on this line where the slope is one.
If there's no relationship whatsoever between this quantitative
thing, trait of the parents and the offspring, and it lies on zero.
That means that there's simply no genetic underpinning, no heritability whatsoever.
So we're looking at variance.
We're looking at the means, and variance is a, a
notion that values are spread brou, a, a, around some mean.
So, take any classroom and the, and the kids in that classroom
have a mean height and they have some variance around that height.
And, if we look at genetics and the phenotypes in terms of the variance,
we say that the variance that we see in some phenotypic trait is
due to genetic variance and environmental variance.
And that's a really, actually a very simple concept.
So, height could be inherited from your, your parents.
But at the same time, you could
be larger, taller, because you've had better nutrition
throughout your life than say somebody else who has not had that.
So, that nutritional difference in, in, in
height would be attributed to environmental variance.
Now, also, heritability, which we call h squared is
simply the proportion of genetic variance to all of that variance.
The genetic variance plus the environmental variance.
And so, the h2 value tells us just how heritable certain traits are.
Now this diagram right here, is showing all the
traits, say for chickens that have been looked at.
And it, and you might imagine that
heritability is a very big thing in agriculture.
And so you can see
here that this is egg production, right there.
And, basically the heritability for egg
rate or sexual maturity when they nest
and lay eggs and so forth, is very low.
But up here, egg weight, and shell-color, and albumin, which is
part of the part of the yolks in eggs. Those have very, very high heritability,
and so egg size is very easy to select. And we'll, we'll get into that.
So another way we can, we can do this is again, if,
if, if heritability is perfect. In other words, it's all genetic,
then we're going to have a heritability line of, of one.
If there is no contribution of genetics to, to the traits,
trait variation, it's going to be around zero, or close to zero.
So, here's a real world example of Galapagos finches.
this, this wonderful study by Peter and
Rosemary Grant showed that the bill depth of
Galapagos finches is very highly heritable.
So, you can see that the heritability is about 90% of all the variation.
90% of it's explained by heritability. So, this is why
beak size can change within a species, from year to year
to year, can change very rapidly because selection is very high.
Now, let's look at the components of selection because selection can
happen at, at many levels of the life history of organisms.
And we'll start with, we'll start with the the
gametes, which are simply the eggs and the sperm.
Between this point and this point there
could be male/female genetic compatibility or incompatibility.
So there's
a, a, a function of compatibility selection.
Then there's viability selection.
So a lot of fertilized eggs are produced, but
not all of them survive, and, and to adulthood.
So for instance, in a very large fish, might produce millions of
eggs, many of which get fertilized. Yet most of those young zygotes do
not make it to adulthood. And then, when
adults get together to mate, there could be sexual selections.
So not all males, not all females, breed in the next generation.
And then finally, fecun-, fecundity we know, for instance, with
tunas, big fish, large older adult females produce way,
way, way more eggs than do young fish. And so therefore, there is a very severe
fecundity, selection in some, some kinds of species.
So, let's talk about the categories in natural selection.
population geneticists divide them into kind of three
general categories, one of which is called
directional selection, stabilizing selection, and then diversifying.
And it all depends upon where selection is acting.
So, for instance, if there's a phenotypic
variance of some sort of character, say height.
and selection is, is against everything in this dark brown.
Or it may be for something in here, in the light, in, in, the light brown.
Then the mean shifts, and the variance shifts toward the right.
So, you can say these
are smaller individuals, and those are not favored, and so the
populations get slightly bigger in the next, in the next generation.
Then there's something called stabilizing selection,
where we are selecting against the extremes.
And then in the second generation, in the F1 generation, the,
the character value has less variance and, and the mean is,
is is, more, more, focused, but with less variation.
So the third kind of selection is diversifying selection.
And there's a phenotypic distribution.
But this time selection is against individuals toward the mean.
And it begins, then, in the next generation, to split
the part the population apart a little bit in this
phenotypic distribution.
So positive selection is selection that spreads an allele.
So then an allele is a genetic
component that, that gives positive effects to fitness.
In other words, survivability and reproduction.
And so positive selection spreads these alleles
through the population, where as purifying selection eliminates
deleterious mutations.
Now, this is a busy slide but, but, it, it does have some
imp, important information. Here is the response to selection.
So this little r right there is how easily or
do, do a pop, does a population respond to selection.
Then here is s, the selection differential.
How strong
is that selection?
So we have three different cases here, and one
of them is high heritability with moderate selection differential.
So here's a moderate selection differential right here.
And so we want to know, what's the response to
selection with high heritability, and just a moderate selection differential?
And that response is from this mean
to that mean. And we can see here that the response
is, is very strong because we have high heritability.
Now if we go to low heritability with that moderate selection
differential, then the, the response to selection is very small,
because heritability is, is very, very low, and a, a population
cannot respond to a selection differential.
But if we have high heritability, and high selection differential,
we can move the population means a lot, lot more.
Because there's stronger selection, and heritability is very high.
And so the slope here, is, is higher.
And the difference between these two
parental means, from one generation to another
is stronger.
There are things in England called pepper moths, in Europe called pepper moths.
Now, back in the 30's and 40's there was a lot of air pollution, and this air
pollution from coal plants, puts coal dust all
over the trees in, in, in, much of England.
Certainly Central England. And there
are two morphs to the peppered moth.
One is white, which is easy to see, but right
here if you look very closely, is a dark morph.
And so this is a kind of a genetic variation within
a population where there are two very different kinds of morphotypes.
So what they did over time is clean up Britain.
The trees got lighter, and lighter, and lighter.
And at a point in time when rain and everything had swept
away all of the coal dust, then you had reverse selection.
So here, the white morph is very cryptic on this lighter background.
Whereas the black morph is very easy to see.
And it's selection on these morphs at different stages against
an environmental background by birds that made all, all the difference.
And here is the curve. So we start with, we start with,
in 1960 and earlier on, melanism began to decrease.
In other words the, the black morph began
to decrease with the improvement in air quality.
And so they've measured, they've measured the frequency
of these forms, and as air quality increased, trees became lighter.
Birds were able to see the dark morphs and preyed on them rather than on the cryptic
light morphs. This example is very recent,
by, a colleague at, Harvard, Hopi, Hoek, Hoekstra, and her lab.
And it's a wonderful, wonderful example of, adaptation, on, on beach
mice, along the Gulf Coast of Florida. And you can
see, that there's variation from here, where the backs of the mice are very dark.
And they, in different areas they get lighter.
And that's correlated with the background.
In other words, the sand color varies across this area in, in ways in which
these mice are trying to be cryptic
because they're trying to avoid predation from birds.
And there's a number of, of markers that can be, these are
all different kinds of traits that they, that they studied across these beach mice.
But you can really see that along this beach are, different phenotypes,
and that's a, a, a, a major response to, to natural selection through predation.
And this old field mouse
is something that occupies inland, away from, from the beach.
Now, it's really important to understand how
natural selection is operating in the wild, but
also what humans are doing in terms
of, of, of creating problems with natural selection.
because we, for instance, we use a lot of pesticides.
And pests are, have very, very high reproductive rates and
so they've learned how to adapt. And so, over time, they become
more and more resistant to these different kinds of insecticides.
And the same thing, the same principle is true
with anything, for instance, that kills bacteria on your hands.
The more and more you put antibiotics on your hands or in your system,
the more quicker bacteria can adapt to em. So
it's very important to understand how natural selection
operating on these microorganisms can have effects with humans.
So the take-home message is here.
Again, phenotypic variation is pervasive. And a lot of that phenotypic
variation, we see in organisms and populations has a genetic basis.
And all, many, many characters, we can
just easily examine our own characteristics, in
terms of hair color, height, eye color,
and so forth, these are heritable var, variation.
Much of this variation has fitness consequences.
Now, hair color and eye color may or
may not have fitness vari, consequences today.
But a lot of variation we
see in populations does have fitness consequences.
So it's affecting the survival and the reproduction of
the individual organisms that have or don't have this variation.
So natural selection is operating in wild populations all the time.
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