Transcript E3_Plasticity_2011 - MicrobialEvolution.org

The Evolution of Plasticity & Learning
E3: Lecture 17
Phenotype versus Genotype
• Even before the rediscovery of Mendel’s results, August Weismann
proposed the separation of germ cells and somatic cells. The former
carried the hereditary material and the latter performed various body
functions.
• Mendel’s famous experiments with peas suggested the existence of
discrete particles of inheritance (this was the best way to explain the
reemergence of a phenotype in the F2 generation not found in the F1
generation).
P
yy

YY
F1 Yy

August
Weismann
Yy
Gregor Mendel
F1 Yy
100%
F2
yy
YY
Yy
25% 75%
• Not long after the rediscovery of Mendel’s work, Wilhem Johannsen
introduced the terms “genotype” and “phenotype” to refer to the
composition of the hereditary particles and the physical appearance
of the organism (in which environmental influences played a role).
Wilhem
Johannsen
Nature versus Nurture
• In 1581, educator Richard Mulcaster wrote about “abilities
…what were in children, by nature emplanted, for nurture to
enlarge.”
• The roles of genotype versus environment in producing
phenotype echoes a long-standing debate over the
importance of “nature” versus “nurture”
• Certain phenotypic traits (e.g., eye color) seem fairly
insensitive to environmental differences but fairly sensitive to
genetic differences (nature over nurture).
• Other phenotypic traits (e.g., spoken language) seem fairly
insensitive to genetic differences, but fairly sensitive to
environmental differences (nurture over nature).
• Of course, any phenotypic trait is influenced by both genetic and environmental
factors, but…
 Why might certain traits show more flexibility with differences in one factor over the
other?
 Is there a general framework in which we can dissect the influence of various factors?
The Evolution of Plasticity & Learning
Lecture Outline
• Introduction to phenotypic plasticity
• Experimental investigation of plasticity
• Learning as a form of plasticity
• Experimental evolution on learning
• Summary
The Evolution of Plasticity & Learning
Lecture Outline
• Introduction to phenotypic plasticity
• Experimental investigation of plasticity
• Learning as a form of plasticity
• Experimental evolution on learning
• Summary
Genotype Does Not Completely
Determine Phenotype
• Organisms do not develop in a vacuum; their
environment can be critical to their development.
Thus, the same genotype can look different in
different developmental environments.
dry season
wet season
• Examples of sensitivity to the environment:
 The butterfly Bicyclus anynana has both a dryseason morph (dull brown without dramatic eyespots) and a wet-season morph (with a set of
eye-spots and bands).
 The annual plant Impatiens capensis will
exhibit elongated stems under low red:far-red
(R:FR) light ratios relative to high R:FR growth
conditions.
 The barnacle Chthamalus anisopoma curves
its shell when a gastropod predator is present
during its development
low R:FR ratio
no predator
high R:FR
predator
• As an analogy, consider the
way a cake looks and tastes.
• A recipe alone does not bake
the cake; moreover, different
cakes can be made using the
same recipe if the
“environment” changes (e.g.,
cook, oven, ingredients, etc.)
environment
recipe
• In this case, the heriditary
material (e.g., genes) does not
completely specify the wing
pattern; rather environmental
influences (e.g., temperature)
are crucial.
cake’s
“environment”
• Consider again our butterfly
that exhibits polyphenism
(multiple morphs).
inherited
material
A Cooked-Up Analogy
Phenotypic Plasticity
• A flat norm of reaction represents a
relatively non-plastic genotype; whereas
a steep norm of reaction represents a
genotype that is phenotypically sensitive
to the environment.
• For instance, the size of barking tree
frogs at metamorphosis declines with
temperature, whereas the squirrel tree
frog seems less plastic.
genotype B
phenotype
• This is represented by the genotype’s
norm of reaction, a function that relates
the developmental environment to the
phenotype produced.
genotype A
genotype C
environment
size at metamorphosis
• When a given genotype produces
different phenotypes under different
environmental circumstances, the
genotype is termed plastic
Hyla gratiosa
Hyla squirella
temperature
Nature versus Nurture: Revisited
• If different genotypes give the same
(environment-dependent) phenotypes (e.g., a
series of non-flat completely overlapping norms
of reaction), this corresponds to a situation in
which “nurture” is given causal power.
genotype A
phenotype
• If different genotypes give different
(environment-independent) phenotypes (e.g. a
series of flat displaced norms of reaction), this
corresponds to a situtation in which “nature” is
given causal power.
genotype A
phenotype
 All statements of nature (e.g., genetic, innate,
inherited, inborn, etc.) and nurture (e.g.,
environmental, learned, experiential, etc.) are
relative to the set of genotypes, environments
and phenotypes measured.
genotype C
environment
• There are two important things to note:
 The situations in which either of these
situations strictly apply are few and far between.
genotype B
genotype B
genotype C
environment
Why Be Plastic?
wet season
brightness
of eye spots
dry season
Dry morph→Camoflauge in
dry litter during inactive
season
Wet morph→Eyespots
“deflect” predators during
active season
temperature
Tall plant→Advantageous
to elongate when shaded
out by neighbors
high R:FR
plant height
low R:FR ratio
Short plant→Advantageous
to avoid elongation when
light is abundant
R:FR ratio
predator
Dudley & Schmidt 1996
Unbent morph→More
fecund without predators
present
shell curvature
no predator
Brakefield & French 1999
predator density
Bent morph→Physically
deters predation by
gastropods
Lively 1986
• Phenotypic plasticity may not be
adaptive.
• Consider our butterfly again
• The brightness of the eye spots
also changes at 100C, but it would
be hard to call this adaptive…
brightness
of eye spots
Plasticity Need Not be Adaptive
18C
Very dull
eye spots
(on a very
dull butterfly)
24C
temperature
100C
Take 5 minutes to talk to your neighbor about the following:
Assume you have found plasticity in a species you are studying
in response to some environmental cue. How might you
experimentally demonstrate that the plasticity is adaptive?
dry
wet
shady
sunny
no predator
predator
The Evolution of Plasticity & Learning
Lecture Outline
• Introduction to phenotypic plasticity
• Experimental investigation of plasticity
• Learning as a form of plasticity
• Experimental evolution on learning
• Summary
Addressing the Adaptive Value of Plasticity
plant height
• For plasticity to be valuable, the
following should apply:
 The environment should be
heterogeneous
• One experimental approach to
addressing the adaptive value of plasticity
involves the separation of the
developmental environment from the
selective environment.
developmental
environment
 The environmental cue for
development should be a reasonable
predictor of later fitness of the
phenotype.
R:FR ratio
selective
environment
 There should be no single phenotype
that is optimal in all environments (i.e.,
what works in one environment does
not work in another).
high density
low density
Plasticity Experiment
ambient
light
• Dudley & Schmitt (1996)
tested whether elongation in
Impatiens capensis is
adaptive.
• By filtering light, they grew
plants under two different
developmental environments:
Elongation (low R:FR) and
Suppression (high R:FR).
• By transplanting plants from
different cue environments to
both low and high density
selective environments, the
fitness of different
phenotypes could be gauged.
developmental
environment
Johanna Schmitt
Elongation treatment
Suppression treatment
High density treatment
Low density treatment
selective
environment
Susan Dudley
filtered
light
Demonstration of Adaptive Plasticity
• The authors measured the
total number of reproductive
structures (flowers, fruits, and
pedicels) produced by each
plant in the selective
environments.
• They found that elongated
plants had higher fitness at
high density; whereas
suppressed plants had higher
fitness at low density.
• These findings are
consistent with an adaptive
form of plasticity: Elongation
is valuable in high density (in
order to gather light), but
detrimental when sufficient
light is available.
Elongated
-High
Suppressed
-High
Elongated
-Low
Suppressed
-Low
Adaptive Shade Avoidance
 The environment should be heterogeneous
These annual plants live in both woodland
and open habitats where light levels vary.
 There should be no single phenotype that is optimal in
all environments (i.e., what works in one environment
does not work in another).
Dudley & Schmitt experimentally demonstrated that
each environment has a different optimal
phenotype.
 The environmental cue for development should be a
reasonable predictor of later fitness of the phenotype.
Low R:FR ratios are found in dense patches of
competitors (where elongation is likely to pay off).
High R:FR ratios are found in open patches (where
suppression pays off).
plant height
• For plasticity to be valuable, the following should apply:
R:FR ratio
The Evolution of Plasticity & Learning
Lecture Outline
• Introduction to phenotypic plasticity
• Experimental investigation of plasticity
• Learning as a form of plasticity
• Experimental evolution on learning
• Summary
 Irreversible plasticity
occurs when phenotypes
become fixed after a critical
window (e.g., wing pattern)
• One type of very special
reversible plasticity is
neuronal-based learning.
glucose
glucose
t=1
brightness
of eye spots
lactose
t=10
lactose
temperature
100C
irreversible plasticity
brightness
of eye spots
occurs when different
phenotypes are continually
expressed throughout the
lifetime (e.g., lac operon)
reversible plasticity
non-adaptive
plasticity
24C
t=1
18C
brightness
of eye spots
 Reversible plasticity
adaptive
plasticity
R:FR ratio
lac expression
• Phenotypic plasticity can
also be either reversible or
irreversible.
lac expression
• Phenotypic plasticity can be
adaptive (e.g., shade
avoidance) or non-adaptive
(e.g., cooked butterflies)
plant height
Types of Plasticity
temperature
24C
t=10
18C temperature 24C
Learning: Building Your Reaction Norm
• Learning can be a tremendously effective
way to approach a changing or novel
world.
• Consider the case of Betty the crow:
This crow learned to fashion a
hook into a wire in order to grasp
a handle of a bucket of food.
• What Betty and other animals are able to
do is build a reaction norm through
experience:
 Stimuli are the “environment”
 Behavior is the “phenotype”
• This is a case of reversible plasticity
(e.g., different stimuli are continually able
to elicit different responses), but also this
is a case where the very nature of the
plasticity can change over time.
3
2
1
The Crow’s Urban Nutcracker
Why Learn?
• As with the early argument about plasticity, learning should be
adaptive when:
 The environment changes over time
 There is no single behavior that is optimal in all
environments (i.e., what works for one stimulus at a given time
does not necessarily work for another stimulus).
 A given behavioral response to a given stimulus is
reasonably reliable over time.
• The rate of change in the environment (i.e. stimuli) is important
to the value of learning:
 If the environment changes too slowly, then innate
behaviors may be favored
 If the environment changes too quickly, then what is learned
today has no use tomorrow.
• Environmental change needs to be “just right”–models have
shown that low rates of change within generations and high
rates of change between generations are ideal.
t=1
t=24
Goldilocks Principle
The Evolution of Plasticity & Learning
Lecture Outline
• Introduction to phenotypic plasticity
• Experimental investigation of plasticity
• Learning as a form of plasticity
• Experimental evolution on learning
• Summary
Learning Experiment
Trial 1
Gen i
Frederic Mery
Tadeusz Kawecki
Trial 1
Gen i+1
Q
Q
• Mery & Kawecki (2002)
explored real-time evolution of
learning in D. melanogaster.
• Within each generation in their
treatment, they paired one of
two fruit flavors with an aversive
chemical cue (quinine) during a
“learning trial”
• They removed the cue during
subsequent “selection trials,”
picking eggs from the flavor not
previously associated with
quinine for the next generation.
• Quinine-paired flavors
alternated between generations;
whereas control populations
lacked quinone.
Trial 2
Gen i
Trial 2
Gen i+1
Trial 3
Gen i
Trial 3
Gen i+1
Measuring Learning: Conditioning Response
• The authors split the lines into three treatments to test for conditioning effects:
 Quinine paired with pineapple in first trial
 Quinine paired with orange in first trial
 No quinine in first trial (used to measure “preference”)
• Number of eggs on each agar flavor (without quinine) from the third trial were scored.
• They also did this with two new flavors (tomato and apple), using the same design.
Trial 1
Trial 1
Q
Trial 3
Trial 1
Q
Q
Trial 3
Trial 1
Trial 3
Trial 3
Measuring Learning: Rate
• The authors split the experimental and control lines into groups exposed to a quininepaired flavor for variable amounts of time:
• Number of eggs on each agar flavor (without quinine) from a choice Trial were scored.
Q
Q
Q
Q
Q
Q
Measuring Learning: Memory
• The authors split the experimental and control lines into groups exposed to a quininepaired flavor for a set amount of time and then variably delayed the time until the choice.
• Number of eggs on each agar flavor (without quinine) were scored.
1 hr
Q
Q
Q
2 hrs
Q
Q
Q
3 hrs
Q
Q
Q
The Evolution of Learning
• The experimental flies showed preferential egg laying
in the medium not paired with quinine.
• Response to conditioning: Experimental flies showed
strong increases in oviposition on the flavor not originally
paired with quinine (whereas the control and stock flies
did not); this result held for novel flavors too.
• Rate of learning: Experimental flies showed a stronger
aversion away from the previously quinine-paired flavor
with shorter forced exposure to it.
• Decay of conditioned response: The decay of
conditioning was faster in the control flies than in the
experimental flies.
QQQ-
Q-
Q-
Q-
Adaptive Learning
• Learning should be adaptive when:
 The environment changes over time
The flavor paired with quinine was alternating in the experiment.
 There is no single behavior that is optimal in all environments
Flies in the experimental treatment could not always simply avoid a single flavor.
 A given behavioral response to a given stimulus is reasonably reliable over time.
Avoiding the flavor that was paired with quinine gave a reliable prediction of
fitness (as eggs were selected from the alternative flavor).
• Mery & Kawecki found that under their experimental treatment (relative to their
control), flies evolved a faster rate of learning, a longer memory and a pronounced
conditioned response in avoiding a flavor paired with an aversive cue.
Take 5 minutes to talk to your neighbor about the following:
What might be potential costs to learning? How might you
experimentally test for these costs? Could you use Mery
& Kawecki’s system to do this?
The Evolution of Plasticity & Learning
Lecture Outline
• Introduction to phenotypic plasticity
• Experimental investigation of plasticity
• Learning as a form of plasticity
• Experimental evolution on learning
• Summary
Summary
• Organismal development is generally influenced by environmental
inputs– such sensitivity is termed phenotypic plasticity and is
represented graphically by the norm of reaction (a genotype-specific
graph giving phenotype as a function of environment).
• Plasticity can be irreversible or reversible (this depends on whether
continual changes in environmental exposure produce changes in
phenotype). Morphological development in animals is often irreversible,
whereas physiological reactions are often reversible.
• Neuronal-based learning is a special type of reversible plasticity.
• Whether plasticity and learning is adaptive depends on a number of
factors: the costs of plasticity, heterogeneity in the environment,
reliability in cues, and phenotypic trade-offs.
• Experimentalists have shown shade avoidance plasticity to be
adaptive in annual plants and learning to evolve de novo in fruit flies.