Phenotypic Plasticity

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!