Hey there! For this video we're going to
be talking about phenotypic plasticity;
which is how individual phenotypes can
vary in response to the environment.
We're gonna go through a number of terms
associated with plasticity. Some of the
key ones among those, are reaction norms
and polyphenisms. And we're also going to
be talking about a special form of
plasticity known as performance curves.
So, if you'll remember back to some of
our earlier videos, there are three main
sources of phenotypic variation among
individuals. The first are genetic
differences which we talked about quite
a bit in our last couple of videos. But the
second which is just as important,
sometimes more important, are
environmental effects and this includes
things like phenotypic plasticity. This
is any situation in which the
environment is inducing differences in
phenotype rather than the genes. You can
kind of think of this as twins or clones
having the exact same genetic background
but showing different phenotypes. And
this may be adaptive meaning it has a
positive effect on fitness or
maladaptive. And the final of these is
random chance. Today, because we're
talking about plasticity, we're really
going be focusing on number two: The
effects of the environment on the
phenotype. And so let's take a moment to
talk about what I like to call the
beastie area of plasticity jargon there
are a lot of words that gets thrown
around and they all mean almost the same
thing. They're highly synonymous but not
perfectly synonymous. So, the first of
these is the more overarching umbrella
term and that is phenotypic plasticity.
And we use the word plasticity in a
sort of older sense. What plastic was
mean it was named for. And it's the idea
of something be able to being able to
take any form. And so plastics we call
plastic because you can melt them down
and mold them and make them into any
form you want. That's why we call them
plastic. And so when we talk about a
phenotype being able to take multiple
forms even though an individual has the
same genotype, we call this phenotypic
plasticity. So that's sort of the
broadest overarching term for any time
the environment can be affecting
organisms. Again it can be adaptive or
maladaptive. We
usually think of plasticity though as
being active responses to the
environment. So if an organism you know
freezes to death it being so cold has
caused its body to turn into a solid
which you could say is a change in its
phenotype but that's not in any way an
active response. That's just the wreck
damage induced by the environment. That's
not really what we're talking about when
we talk about plasticity. We're instead
talking about the kind of things that
have been actively in response to the
environment. A nice example that most of
you have probably experienced is if you
go outside and put your skin in the Sun
it's likely to change color. For me, I'm
quite pale, I have the the the problem if
I do this too long my skin turns bright
red and hurts. That would not be
plasticity.
That's just damage. But if I go out in
the Sun and in response to being in the
Sun my skin produces more melanin,
becomes more brown, that actually helps
protect my skin and my DNA and my cells
from the damage caused by ultraviolet
radiation. That would be an example of
adaptive plasticity. Really anytime the
organisms are having an active response
to environmental variation, we can turn
phenotypic plasticity. Now another word
that's often used very synonymously with
phenotypic plasticity is acclamation.
Acclamations a little different than
just plasticity writ large.
Because plasticity writ large can
include variation that is reversible or
irreversible. An example of an
irreversible form of phenotypic
plasticity is temperature dependence sex
determination in many reptiles. These
animals, their genes do not determine
whether they're male or female instead,
it's the temperatures they experience
when they're in the egg. But once they
develop into a male or a female, that
can't go backwards. So, that still
phenotypic plasticity but it is not a
form of acclamation. Acclamation is
something that is reversible and tends
to be in response to extend it or
chronic exposure. Tanning is actually a
pretty good example of acclimation.
Another term that gets thrown
around is phenotypic flexibility. This is
somewhat similar to acclimation, but in a
way is an even broader term. This really
is talking about any form of response to
the environment that can reverse itself.
This is just having a generally flexible
phenotype. Tanning would be an example of
having a generally flexible phenotype as
would bodybuilders. The fact that you can
put stresses on your muscles cause them
to grow and get bigger. If that can
rapidly reverse, if you stop working out
but then if you start working out you
can build up your muscles again would be
a good example of phenotypic flexibility.
And finally, developmental plasticity are
consequences of the environment that
tend to be irreversible because they
influence how the organism develops from
a zygote to a full adult organism. So,
temperature dependent sex determination
is an example of developmental
plasticity. Other examples have to do
with things like I'm sure you've heard
that parents/when pregnant women
can't adjust their diets and things in a
way that might be beneficial to their
baby's development, brain development,
learning. Those would all be examples of
developmental plasticity, the kind of
influence of the maternal environment on
the developmental trajectory of the
babies. So these are sort of four big
terms we're going to use to describe.
Very similar things, this is all the
environment affecting the phenotype of
individuals, but with slightly different
meanings as I've just explained. Another
set of important plasticity jargon, are
the difference between a reaction norm
or a polyphenism. This totally depends
on whether the phenotype changes
continuously in response to the
environment or if there's a threshold
response so the phenotype is in one
state in one environment and a different
state in a different environment. So,
a reaction norm, sometimes also called a
norm of reaction sort of flipping the
word around, is any time we have a
continuous response to the environment,
An example is shown by the growth
heights
these plants and responds to temperature
and this shows that when it's really
really cold the plants don't grow it all,
when it's moderately cold the plants are
kind of short, when it as it warms up the
plants get bigger and bigger and bigger
until it gets too hot and the plants
start to get shorter again. This is a
continuous response of these plants to
the environment. Totally driven by the
environment you could have totally
clonal plants with the exact same genes
and their phenotype would vary in this
way. Polyphenism, an example of a
polyphenism includes temperature
dependent sex determination, as I
mentioned before. If you have one set of
temperatures, the baby develops into a
male. If you have another set of
temperatures it develops into a female.
Another really good example is in
honeybees, whether or not the individuals
develop into a regular worker bee or a
queen bee. And this is totally dependent
on the diet that the larvae are fed by
the other worker bees. If they're fed a
standard diet they grow up and develop
into a worker bee. However, if they're fed
something called royal jelly, which is a
really nutrient-rich form of the diet,
that diet induces the larvae to develop
into a much larger, very fertile honeybee.
Very different phenotype. That's the
queen bee, but the queen bee is
genetically identical to her worker bees.
This difference, this polyphenism is
completely driven by the environment.
We're generally going to be thinking
more about reaction norms than polyphenism. So I want to dive into it just a
little bit more. This graph shows sort of
the stylized version of a reaction norm.
Our x-axis is the environment, with two
extremes on the left and the right, and
the y-axis is the value for our trait.
And in this case, what we see is we have
two genotype. So, these two individuals
with different genes or these could be
two populations of clonal individuals
where they all have the exact same
genetic makeup. Either the red group
all has the exact same genetic makeup
and the blue group all has the exact
same genetic makeup. And the idea is that,
what phenotype they show is largely
driven by their environment. So in one
extreme, we see that genotype A
has a smaller value for its phenotype
than genotype B. But at the other extreme
maybe this is a hot environment, this
flips and so we can see market
phenotypic variation on the landscape
that's driven more by the the phenotype
than just the pure genotype. In this case
you actually have both sort of driving
things because the genotypes differ and
how they plastically respond to the
environment. Which we'll be talking about
a bit more in our next video. And it also
highlights that patterns of phenotypic
plasticity can vary among genotypes.
Sometimes, you have them all responding
to the environment in the exact same way,
sometimes you have the molix responding
in about the same way, but sort of with
different intercepts so it's like they
have different starting points. And
sometimes you have
genotypes whose reaction norms cross
that's when you have both plasticity and
genetic effects. And this suite of
options is really interesting and in his
important mode for evolution, but we're
going to talk about this again more in
our next video. For today and for this
video I really just want you to
understand that phenotypic plasticity is
variation and phenotype caused by the
environment. And different genotypes can
have different reaction norms. Another
example of a reaction norm that we are
going to talk about a lot in this class
and you should have already seen SimUText is the performance curve. An
idealized version is shown here in this
case we have the pressure or difficulty
of your class and the y-axis is how well
you perform. And the curve is just sort
of this hump shaped curve and the idea
is that if your class is too easy,
you're not going to perform all that
well because you're just switched off. If
the class is just challenging enough,
your performance will be optimized it's
gonna be the highest. And if the class is
way too hard ,you're gonna stress out and
your performance is going to drop. So,
that's totally dependent on how hard
your class is and has nothing to do with
your genetic propensity for class.
This would be an example of a
performance curve and thus an example of
phenotypic plasticity of a sort. We touched
on thermal performance a little bit
earlier in our very first example of a
reaction norm. But it's a nice sort of
classic example of a performance curve
so we're gonna dive into it just a
little bit more. So in this case, we see a
thermal performance curve which again is
an example of a reaction norm. Our x-axis
is body temperature and our y-axis is
relative performance. This could be how
many babies and individual makes, it
could also be something like how fast
they can run. And thermal performance
curves tend to be this hump shaped curve.
They're left skewed. The skew is the
direction the tail of the distribution
points. So, that means the humps more to
the right, a little confusing but is the
left skewed curve. And this curve like
all performance curves can be
characterized by a handful of parameters
including an optimum. So, in this case
it's the body temperature at which
performance is its highest. Two critical
limits, in this case the body
temperatures were performance is the
very lowest for ectothermic animals when
we measure these critical thermal limits
is often is when they really when they can
stand up. If you get some too hot or too
cold a bit before they could die, you
actually have them sort of slip into a
coma-type state and they can't work
their muscles anymore. So those are the
critical thermal limits and then some
parameter that describes how wide the
performance breath is because you could
have a situation where you have your
critical limits and basically a plateau
where everything in between you have
high temperatures or this could be
extremely narrow where basically they
have high performance at the optimum and
then it rapidly drops off on both sides.
So, this parameter is telling you sort of
how wide that high performance area is.
In this case it's a "B", which means
that it's the range of temperatures at
which the organism is able to have at
least 80% of its peak performance. So,
that's a performance curve, in this case
a temperature or a thermal performance
curve.
And we think a major reason for these
performance curves, particularly in
response to temperature has to do with
how biochemical reactions work.
Biochemical reactions are quite
temperature sensitive. As temperature
goes up, proteins become a bit more
flexible. Enzymes and other proteins are
pretty much always sort of wiggling
around and changing shapes. And that's
sort of how an enzyme works, is it'll
wiggle into one shape that allows it to
grab a substrate it'll then wiggle into
another shape with that substrate and
that's what it does to catalyze a
reaction. So it sort of snapping between
states between shapes. As you heat them
up, they're able to wiggle a bit,
a bit more rapidly. All compounds,
all molecules move around a little more
when they're at a higher temperature. And
so then they're able to have the
reactions go much more quickly. But at
some point they get so hot, that they're
wiggling around so rapidly that they
stop doing a very good job of catalyzing
reactions. And that is then when your
performance would drop back off. So, this
is just showing how the phenotype of the
organisms can vary in response to the
environment just because of how
biochemical reactions are sensitive to
the environment. In this case, sensitive
to the temperature environment. However,
lots of other factors can influence
enzymes too. And this is only one route
that can cause phenotypic plasticity but
it is one of the important routes,
especially for flexible phenotypic
plasticity, the kind of thing that can be
reversible. So in this case we see an
environmental variable on the x-axis and
we have enzymatic activity on the y-axis
This is really how fast it can catalyze
a reaction. We already talked about the
thermal performance curve which is just shown
in the far left. The middle one shows you
an example for pH. But this basically is
showing you is that when you have
relatively neutral pH's around 7,
the enzymes function well but if pH gets
either too acidic or too alkaline, you
get a very reduced enzymatic activity.
Showing how a phenotype can respond to a
pH environment. Finally, on the right is
just showing the response of enzymes to
substrate concentration. As the amount of
stuff for them to work on goes up,
their activity goes up. And it goes up
quite rapidly, but at some point there's
enough substrate in the environment that
the enzymes are are being able to use
substrate as fast they can, they can't
use it any faster, so you have this
asymptote. And this and it stops changing
in response to the environment at that
point. In this case, it's almost more like
a threshold response. But these are three
different forms of reaction norm. Again,
continuous phenotype variation in
response to an environmental variant. So,
quick wrap-up. The environment can pretty
dramatically influence the phenotype of
individuals. This is a really important
and it's something we often skip over in
biology classes. Although, it's something
that I'm sure you intuit, many of you
know about you all know about tanning,
you all know about working out, so you
have some notion that phenotypes
can vary in response to an environment.
But we tend to talk about how genes are
everything and a gene will code for a
trait, then that individual has that
trait as like a fixed thing. However, it
turns out there's a very large fraction
of the variation among individuals that
can be explained by the environment. As a
whole, we call this phenotypic plasticity,
although there's a number of sort of
other terms with similar meanings,
such as acclimation phenotypic
flexibility, developmental plasticity, so
on. An important thing about phenotypic
plasticity that's quite different from
evolutionary change, is plastic changes
can occur within the lifespan of an
individual, I mean they can respond to
the environment very rapidly and more
rapidly than evolutionary change can. But
there is a bit of a trade-off
in this, because if a phenotype can be
fully changed just by the environment,
that means natural selection cannot
effectively act on it because there's no
heritable trait variation. In this case,
if we think of our breeder's
equation, we would have 'h' squared a
heritability of zero, if it's a fully
plastic trait. Meaning it could not
evolve generation after generation. Now
that's not to say that phenotypic
plasticity is completely
removed from evolution by natural
selection. Because although a specific
phenotype cannot evolve, reaction norms
can evolve. So, how each individual
genotype responds to the environment is
something that natural selection can act
on. And we're talking about that as well
as how we can distinguish the fraction
of variation caused by the environment,
and the fraction of variation that's
caused by genes in our next video. Thank you!