In this video, I'll be talking about
different forms and different directions
of natural selection. Often we just talk
about natural selection as if it's a
uniform thing, but it can take on, once we
start considering different traits,
especially different quantitative traits,
it can start to take on lots of
interesting variations. So as stated in
previous videos, phenotypic traits--they
can be discrete or quantitative. By
discrete, that's things like eye color or
whether you have sickle-cell anemia or
not. Those can often be determined by
fairly simple Mendelian genetics, alleles
at a single locus. Quantitative traits
they're measured numerically-- height,
weight, blood pressure, length of a beak
on a Finch. Those are quantitative traits
and they're typically determined by many
genes. In order to study quantitative
traits, what we often do is we plot
fitness or a fitness component like
number of offspring or size of offspring.
But we plot the quantitative trait
against the fitness component. So in this
case here, we have on the x-axis, we're
going to plot our trait. And on the
y-axis, we're going to plot our fitness
component. In this particular example,
we're going to have the mass of the
mother. So for many organisms, the size of
the mother indicates how many resources
she has stored. So this could be the
height of a tree, or the mass of a
herbaceous plant, the mass of a mammal.
Those things often indicate
the storage of lots of resources. That's
a heritable quantitative trait. Often
being largely, having the ability to
store large amounts of fat, or to be a
robust strong organism-- that's heritable.
A fitness component could be the mass of
the child, producing a big healthy baby
helps the baby survive into the future.
So we have our fitness component that we
can measure on mom to determine her fitness, and then
we can measure a trait. And in this
case, there's generally a positive
correlation. The bigger the mom, the
bigger the baby. So a positive
correlation, a upward slope, between a
trait and a fitness indicates that
selection can increase the population
mean of that trait. So in this case, we
have a positive correlation here. So we
would indicate that selection would act
to increase--Selection would increase the
mean size of the mom in the population
because bigger moms produce bigger
babies and biggers babies
are more likely to survive and pass
their genes on. So the moms down here,
small moms, moms on the lower end of the
x-axis down here at 2.5, they are going
to produce small babies. Small babies
aren't going to survive well and pass on
their genes, so those genes are going to
disappear from the population. Large moms
are going to produce large babies, so
those large baby genes and large momma
genes are going to move into the future.
And that's going to produce results in
more individuals being large. That's
gonna move the average size of moms up
as generations pass. So again, a positive
correlation between a trait and fitness--
that indicates that natural selection
can increase the mean of the trait, the
population of the mean. Again, any given
individual doesn't evolve over time.
Populations evolve. So we'll often show
this by showing a histogram. On the
x-axis here is the mean of the trait. The
frequency of the trait in the
individuals in the population is shown
here. So here there's being a small mom
at unit 1 here. Those are relatively rare.
Large moms --12, 13, 14--are relatively rare.
Most moms in the population are around 7.
So fitness is not actually on this graph.
Fitness is going to be represented
separately by showing a line, by showing
the positive correlation. As mom's get bigger,
their fitness goes up. Now we can overlay
these graphs. You'll often see this in
books, where we've got what's
called a dual y-axis over here. We're
showing the frequency of moms at
different height.
So this left y-axis relates to the
histogram, and then the right y-axis
relates to fitness. So this indicates
that individuals down here on this end--
they are rare, but when they do occur,
they are going to have low fitness.
Individuals here are also rare, but they
have the highest fitness. So these few
individuals that are very big--they have
the highest fitness. Most individuals are
average sized and they have average
fitness. Now when you think about how
these graphs relate to here, these
individuals that are small, these one or
two individuals that are small,
individuals here are small, they have
relatively low fitness. So they are not
going to pass their genes for small size
on to the future. These individuals here
are not very common currently, but they
are going to be most likely to survive
and pass their genes on. They're gonna
increase the frequency of the genes that
cause being big into the future. So again,
individuals on this end are going to
have relatively low fitness. Individuals
on the high end are going to have higher
fitness. And the population is going to
be having an increasing frequency of the
genes for large size. So what we see here
is that over time, this histogram can
shift. So originally, the mean was a
little bit less than 7. Its mean
was 6.8. If there's a
positive correlation between this trait
and fitness, larger individuals
are more likely to survive and produce.
They're gonna pass their genes for being
big into the future. This is going to
drive up the mean. And here, the
distribution has shifted, and it's
7.5. These individuals and
their genes didn't move into the future.
These individuals and their genes did
move into the future. So this is called
directional selection because it's
moving...
Evolution, natural selection has a
directionality to it, which is larger
size. And there's a consistent push, a
consistent direction of moving towards a
larger size. Now a directional selection
can be either positive or it can be
negative. So we can talk about positive
selection or negative selection. Positive
general directional selection or
negative directional selection.
Here's another representation of this in
terms of negative selection. Here we've
got some sort of trait. It's not defined
in this figure from this book, but here
individuals highest on the x-axis
here have the lowest fitness, whereas
smaller individuals on the left-hand
here have the highest fitness. So there's
a negative correlation. So this is going
to drive the distribution to be smaller.
The green here is the original
distribution, and selection is going to
move... shift the peak this direction
after selection. In this case-- in a
previous example, I used histograms to
show natural... to show
distribution of the trait. Here we're
just using a smooth curve. Those are just
simple, small difference in a way of
presenting the data. So again, negative
correlation. Here is the original
distribution of the trait after
selection, the mean moves down, moves to
the left.
So selection on a quantitative
trait can be directional, as we just
shown. But it also take on the
characteristic that's called stabilizing.
Or there can also be disruptive
selection. So the way I'll lay out these
graphs for these different examples-- I'll
first show the distribution of the trait
that's our starting point. Then I'll show
the correlation between the trait and
fitness. And then I'll show what happens
afterwards. So again, here for
positive directional selection, we have
the distribution of our trait, we have
a positive correlation between that
trait and fitness. So natural selection
is going to move up the mean, the
population mean, in the next generation.
It's going to move it up slightly
because these individuals here had the
highest fitness, and their genes are
passed more frequently into the future.
So stabilizing selection-- we've
start off with our same distribution of
traits-- we have a curve here. This
shouldn't be confused with the shape
here. This is showing the relationship
between fitness and the trait. So when
the trait is on the left hand here -- let's
call it size because it's easy -- so say
when an organism is small, it has low
fitness. As it increases in size, it has
high fitness. But then... as it
decreases, it's low fitness again. This
actually matches with... infant
body size and fitness. Size of children
is relatively heritable, size at birth.
And because of limitations on the size
of the human birth canal -- even
though large babies... are good
because they're more robust, they have
more resources -- if they're too big, they
get stuck in the birth canal. And small
babies are bad because they are not
robust. They lack resources for survival.
So in humans, the highest fitness is on
intermediate sized babies. That's why we
don't see 15 pound babies, why we don't
typically see 6 pounds babies except
in extreme circumstances. Most babies -- 6
7, 8 pounds, somewhere in there. So
what this does is this eliminates the
extremes from the populations. Women who
give birth to medium-sized babies are
most likely to produce the most babies. Those babies are going to go on to
produce medium sized babies of their own.
And the distribution is going to become
pointier, so the... extremes on
either end have been removed. You can ask
what this is going to do to the standard
deviation of the distribution of baby
weight. And what it's going to do is it's
going to reduce, it's going to shrink the
amount of variation in the population. So
human birth weight is actually pretty
pretty consistent, pretty constant.
Full-term healthy birth weight has a
pretty restricted range. So disruptive
selection has the opposite--So again, we
have a opposite pattern as
stabilizing selection. So we have our
trait distribution here. It's been the
same. Here, the curve has been flipped.
The highest fitness value is on a low
trait. The lowest fitness value is here.
And the highest -- this should say high -- the highest value is over here. So it
goes high, low, high. This value is wrong
here. So what this does is this
eliminates the middle. These individuals
in the middle here -- we're starting off
with them being abundant. But they are
going to have the lowest fitness. And
it's the individuals at the extremes
that are going to have the highest. So
this is going to shrink the middle here,
and it's going to make either end more
common. And this is going to impact the
standard deviation, the amount of
variability. This is actually going to
increase the amount of variability in
the population. Here's another
representation of it. One thing you can
do is it can produce a bimodal
distribution. So here is fitness relative
to the trait. High fitness, low fitness,
high fitness. The green represents the
original distribution of the trait in
the population. Then after natural
selection, those in the middle had the
lowest fitness, so we would create a
little dip in here. And it can result in
a bimodal distribution. So directional
selection, for review, is that selection
acting against one extreme-- either, for example,
really small or really big-- and it either
increases or decreases the mean.
Stabilizing selection is selection
against both extremes, so the mean stays
about the same, but the variation
decreases. Human birth weight is an
example of this. Imagine birth weight for
most mammals is going to be this, going
to be... going under stabilizing
selection. Disruptive selection is the
opposite of stabilizing selection. It's
selection against the middle. It
increases variation, and it can even
create a bimodal distribution. One thing
with bimodal distributions is that the
mean can actually stay the same, but the
standard deviation increases. There's
also other forms of selection that we
talked about. One of these is frequency-dependent selection. This is an
interesting form of selection. I'll have
an example in a second.
Another one is balancing selection. Frequency-dependent selection is selection against
the most common form of a trait. And it
often happens when there are
interactions between species. In the
classic example that tends to be in all
the textbooks is of a fish in a lake that
has a parasite. And this parasite-- this
comes in two forms to morphs. The jaws
can either be right-oriented or left-
oriented. And these parasitic fish have
evolved in order to come up and bite
scales off and flesh off of one side or
the other. So the right jawed fish is
good at chewing off scales from the left
side of a prey fish. And the left jawed
fish is good at chewing scales off the
right side of a fish. If you're this fish,
you're going to be less likely to-- less
able to get it off of the other side of
the fish. So over time these fish, the
prey fish, learn to to protect themselves
and avoid one side or the other. So when right-jawed fish...
become really common, the prey fish
become really good at shaking them off.
Spotting them and shaking them off. In
that case it becomes really advantageous
to be a left-jawed fish because your
prey fish are all looking to the other
side. They're looking at the opposite
side for a parasite coming from that
direction, so these left-jawed fish can
come in, can sneak in. However, these left-
jawed fish are going to... have higher
fitness when this happens. They're going
to produce more offspring. The population
is going to become dominated by left-
jawed fish. And then the prey fish are
going to learn to watch their right
flank and to protect their right side.
And so these left-jawed fish are going
to be at a disadvantage. And the right-jawed
fish, who have become rare, they are going
to be able to better sneak up on the
fish. And so it's going to go back and
forth, back and forth, back and forth.
That's whatever the rarest form
of the jaw is going to have the
advantage. But then it's going to go back
and forth as the fish the prey fish
learn. Frequency-dependent selection is
always going to maintain variation
because one genotype is never favored
permanently. It's never good to just be
one... As soon as
one phenotype becomes common, its fitness
is actually going to reduce, be lower,
and it's going to be advantageous to be
the other phenotype. Balancing selection
is somewhat similar to stabilizing
selection, but stabilizing selection
applies to quantitative traits-- it
applies to traits that we measure with a
ruler or weigh on a scale. Things like
weight, mass, height, birth weight, size.
Balancing selection refers to
qualitative traits or discrete traits,
like having a sickle cell anemia or not,
and it's when the heterozygous have the
highest fitness. So... because of
this, the heterozygous possess both the
the dominant allele and the recessive
allele. So they have the they are going
to produce both homozygous recessive,
homozygous dominant, and heterozygous
children. So all of the alleles -- both
alleles -- are going to be maintained in
the population. So while it's great to be... if you're living in a place with
malaria, it's great to be a heterozygous-- to be Ss. You're gonna
pass those genes on to your children
though. You're gonna be successful.
However, some of your children are gonna
be SS and susceptible to
malaria. Some of your children are gonna
be ss and have
sickle cell anemia. And only a portion of
your children are going to be
heterozygotes (Ss).
