Transcript 174-16-Winter_17_1-M..

Lecture 16:
Phenotypic
Plasticity
1
How do organisms respond to
environmental change?
At the individual level:
Behavior
Physiology
Plasticity
At the population level:
Behavior
Performance
Physiology, Biochem, Morphol
Plasticity
2
All of these involve complex traits
Complex traits are:
Common at relatively high levels of
biological organization
Comprised of many subordinate traits
Capable of exhibiting emergent properties
Often modular
Affected by many genes and
environmental factors
http://complextrait.org/
regarding human diseases and disorders
3
Emergent Properties 1
http://dictionary.reference.com/browse/emergent+property
noun
any unique property that "emerges" when component
objects are joined together in constraining relations to
"construct" a higher-level aggregate object, a novel
property that unpredictably comes from a combination
of two simpler constituents
Examples
The familiar taste of salt is an emergent property with
respect to the sodium and chlorine of which it is
composed.
4
Emergent Properties 2
http://www.nature.com/scitable/topicpage/biological-complexity-and-integrative-levels-oforganization-468
When units of biological material are put together, the
properties of the new material are not always additive,
or equal to the sum of the properties of the
components. Instead, at each level, new properties
and rules emerge that cannot be predicted by
observations and full knowledge of the lower levels.
Such properties are called emergent properties
(Novikoff, 1945). Life itself is an example of an
emergent property.
5
Modularity
"Although their meaning varies, modules generally are
components, parts, or subsystems of a larger system
that contain some or all of the following features:
(i) identifiable interfaces (usually involving protocols)
to other modules,
(ii) can be modified and evolved somewhat
independently,
(iii) facilitate simplified or abstract modeling,
(iv) maintain some identity when isolated or
rearranged, yet
(v) derive additional identity from the rest of the
system."
Csete, M. E., and J. C. Doyle. 2002. Reverse engineering of biological
complexity. Science 295:1664-1669. Page 1665.
6
Classic complex traits:
7
The ultimate complex trait:
Behavior
Organismal
Organ Performance
Systems
Organs
Tissues
Cells
Organelles
Proteins, etc.
DNA
8
Selection acts hierarchically:
Behavior
Organismal
Organ Performance
Systems
Organs
Tissues
Cells
Organelles
Proteins, etc.
DNA
In animals,
selection
generally acts
more directly
on behavior
than on the
subordinate
traits that
determine
performance
abilities
Fig. 1 in Garland, T., Jr., and S. A. Kelly. 2006. Phenotypic plasticity and experimental evolution. Journal of Experimental Biology 209:2234-2261.
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At any level of organization ...
Behavior
Organismal
Organ Performance
Systems
Organs
Tissues
Cells
Organelles
Proteins, etc.
DNA
Phenotypes may
be affected by
environmental
factors, i.e., their
expression may
be "plastic"
10
Phenotypic Plasticity:
The ability of an individual organism to
alter its phenotype in response to
changes in environmental conditions.
or
The modification of developmental events
by the environment.
or
The ability of one genotype to produce
more than one phenotype when exposed
to different environments.
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The ability of one genotype to produce
more than one phenotype when exposed
to different environments.
Trait
Environment
Trait
Plasticity
Trait
No Plasticity
Highly Variable
Plasticity, strong
Genotype-byEnvironment
Interaction
Environment
Environment
Each of the colored lines is a "Reaction Norm"
12
Features of "Phenotypic Plasticity"
1. Something in the internal and/or external
environment changes (usually)
2. Organism senses that change
3. Organism alters gene expression
4. Usually, the altered gene expression
yields additional observable phenotypes
Includes "acclimation" and "acclimatization"
as well as learning and memory.
13
Features of "Phenotypic Plasticity"
1. Something in the internal and/or external
environment changes (usually)
Changes in ambient temperature, humidity or
oxygen concentration would constitute external
environmental factors, and many organisms respond to
these with phenotypic plasticity that involves multiple
organ systems and multiple levels of biological
organization.
Mechanical overload of the heart is an example of an
environmental change that occurs within an organism,
and it leads mainly to organ-specific changes that
necessarily involve fewer levels of biological
organization.
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Features of "Phenotypic Plasticity"
2. Organism senses that change
Some changes may occur without any formal
sensing by the organism, e.g., as a result of
direct (and possibly differential) effects of
temperature on the rates of ongoing
biochemical and physiological processes.
15
Features of "Phenotypic Plasticity"
3. Organism alters gene expression
Some plastic responses need not involve
changes in gene expression (transcription) but
instead could occur via phosphorylation of
existing proteins, changes in protein levels
caused by variation in protein ubiquitination, or
stimulation of existing microRNAs.
16
Features of "Phenotypic Plasticity"
4. Usually, the altered gene expression
yields additional observable phenotypes
In principle, lower-level traits might change in
offsetting ways, such that a higher-level trait
could show little or no apparent change.
For example, it would be theoretically possible
(though perhaps unlikely) for exercise training to
cause an increase in maximal heart rate but a
reduction in maximal stroke volume, such that
maximal cardiac output was unchanged.
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Hierarchical masking effects:
Behavior
Compensatory
plasticity at
lower levels
could lead to
reduced
plasticity at
higher levels
Organismal
Organ Performance
Systems
Organs
Tissues
Cells
Organelles
Proteins, etc.
DNA
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Features of "Phenotypic Plasticity"
The changes may or may not be reversible.
The changes may or may not be adaptive in
the sense of increasing the organism's
reproductive success (Darwinian fitness).
The idea that environmentally induced modifications are
adaptive in the sense that they improve organismal
function and/or enhance Darwinian fitness has been
termed the "beneficial acclimation hypothesis."
In general, non-adaptive plasticity might be expected to
occur any time that an organism is exposed to
environmental conditions with which it is "unfamiliar" in
terms of its evolutionary history.
Humans taken to high altitude?
Any wild animal brought into captivity?
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Features of "Phenotypic Plasticity"
In some cases, behavioral plasticity
(compensation) can shield lower-level traits
from selection.
For example, gravid lizards or snakes
may become more wary.
Bauwens, D., and C. Thoen. 1981. Escape tactics and vulnerability to predation associated with
reproduction in the lizard Lacerta vivipera. J. Anim. Ecol. 50:733-743.
Brodie, E. D., III. 1989. Behavioral modification as a means of reducing the cost of reproduction.
Am. Nat. 134:225-238.
At the population level, phenotypic plasticity
in behavior and other traits can facilitate
invasions of new habitats.
e.g., "willingness" to eat new foods or nest in
unusual spots
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Classic Cases of Phenotypic Plasticity
Two genetically identical
water fleas, Daphnia
lumholtzi.
The helmet and extended tail
spine of the individual on the
left were induced as a result of
chemical cues from a
predaceous fish and serve as
protection.
This figure is recreated from Agrawal, A. 2001.
Phenotypic plasticity in the interactions and
evolution of species. Science 294:321-326,
Figure 1. Shown in Kelly et al. (2012)
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Classic Cases of Phenotypic Plasticity
Poorly fed and well-fed sibling
echinopluteus larvae of the sea urchin
Lytechinus variegatus on day 4 of development.
Note greater investment in ciliated band and
internal skeleton under low food conditions.
Photo by J. S. McAlister.
No Predator
Predator
http://www.zoo.ufl.edu/mccoy/Quantifyingplasticity.htm
http://www.unc.edu/~podolsky/plasticity.htm
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Classic Cases of Phenotypic Plasticity
Carotenoid coloration is
phenotypically plastic, and diets
lacking carotenoids result in very
little color in normally pigmented
species, such as the house finch
(Carpodacus mexicanus).
Population differences in
[carotenoid] have been related to
the presence of specific food
plants.
Price, T. D. 2006. Phenotypic plasticity,
sexual selection and the evolution of colour
patterns. J. Exp. Biology 209:2368-2376.
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Taylor, C. R., and E. R. Weibel. 1981. Design of the
mammalian respiratory system. I. Problem and
strategy. Respiration Physiology 44:1-10.
Passage from page 3:
We will discuss symmorphosis in a later lecture.
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Many such examples do
seem to be adaptive, i.e., to
confer higher Darwinian
fitness (or at least they
increase organismal
performance at some task),
so we can proceed to ask ...
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To be or not to be:
when
should
plasticity
evolve?
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When should plasticity evolve?
Intuitively:
Not in a constant environment.
Not if variation in environmental
factors is entirely unpredictable.
In those cases, the optimum genotype is
likely to be one that results in a single
phenotype that confers high Darwinian
fitness with respect to the long-term
average environmental conditions.
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Formal Theoretical Models:
Gabriel, W. 2005. How stress selects for reversible
phenotypic plasticity. J. Evol. Biol. 18:873-883.
"As a null model I assume
that plasticity is not costly. ...
costs would usually enter as
constant factors that do not
alter the optimal values of
mode and breadth."
28
Gabriel, W. 2005. How stress selects for reversible
phenotypic plasticity. J. Evol. Biol. 18:873-883.
"Phenotypic plasticity ... can be an adaptive strategy to
cope with variable environments ... and is a common
phenomenon for many traits in almost all organisms."
"Stress occurring in periods shorter than life span
strongly selects for reversible phenotypic plasticity, for
maximum reliability of stress indicating cues and for
minimal response delays."
Implicitly, he seems to define "stress" as anything
that threatens homeostasis, survival or other
components of Darwinian fitness.
29
Gabriel, W. 2005. How stress selects for reversible
phenotypic plasticity. J. Evol. Biol. 18:873-883.
"Analytic expressions are given for optimal values of mode and
breadth of tolerance functions for stress induced and noninduced phenotypes depending on
(1) length of stress periods,
(2) response delay for switching into the induced phenotype,
(3) response delay for rebuilding the non-induced phenotype,
(4) intensity of stress, i.e. mean value of the stress inducing
environment,
(5) coefficient of variation of the stress environment and
(6) completeness of information available to the stressed
organism.
Adaptively reversible phenotypic plastic traits will most probably
affect fitness in a way that can be described by simultaneous
reversible plasticity in mode and breadth of tolerance functions."
30
Gabriel's (2005) Conclusions:
"reversible phenotypic plasticity would be expected
for all organisms [if]:
they are exposed to stress periods that last shorter
than life span;
stress appears in the long run with some regularity
so that natural selection can shape ... plastic traits.
... given the predicted huge fitness advantages,
the cost of plasticity would have to be unexpectedly
high ... to counteract selection for reversible ...
plasticity."
Are these predictions supported?
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Ways to study evolution:
Compare extant species (or populations)
to infer what
has happened
in the past ...
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Ways to study evolution:
Environmental Variability
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Gap
More Variable
Understory
Comparisons of Species:
Less Variable
Mean Plasticity
Valladares, F., S. J. Wright, E. Lasso, K.
Kitajima, and R. W. Pearcy. 2000. Plastic
phenotypic response to light of 16
congeneric shrubs from a Panamanian
rainforest. Ecology 81:1925-1936.
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Gap
Comparisons of Species:
Understory
More plastic for
gas exchange
traits than for
structural traits
Mean Plasticity
Valladares, F., S. J. Wright, E. Lasso, K.
Kitajima, and R. W. Pearcy. 2000. Plastic
phenotypic response to light of 16
congeneric shrubs from a Panamanian
rainforest. Ecology 81:1925-1936.
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Comparisons of Species:
Some generalities:
Plant morphology is more plastic than
animal morphology.
In animals, behavior is very plastic.
In vertebrates, skeletal muscle is more
plastic than the lung.
Skeletal muscle is more plastic in
mammals than in lizards.
Snake guts are very plastic.
Carp are very plastic.
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Comparisons of Species:
Fig. 1. Small intestinal wet mass, intestinal
nutrient uptake rates, and intestinal nutrient
uptake capacity of fasted snakes presented as
a percentage of those variables measured
from digesting individuals of four species of
infrequently-feeding snakes (A) and of
four species of frequently-feeding snakes
(B). For nutrient uptake rates and uptake
capacities, bars represent the average fasted
percentages (+ SE) for the uptake of Lleucine, L-proline, and D-glucose. Note that
with fasting infrequently-feeding snakes
reduce intestinal mass, nutrient uptake rates,
and therefore uptake capacity by much
greater magnitudes than frequently-feeding
species. Source of data is Secor and
Diamond (2000).
From Secor (2005) Integr. Comp. Biol. 45:282-294
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Comparisons of Species:
Fig. 3. Phylogenetic assessment of the
postprandial increase in intestinal
nutrient uptake capacity for 24 families
and 4 subfamilies of amphibians and
reptiles. …
Bar lengths represent the mean factorial
increase for the uptake capacity of Lleucine, L-proline, and D-glucose.
For families represented by multiple
species (see Table 1), bar length and
error bars signify mean and 1 SE of
averaged factorial increase in uptake
capacities among those species.
From Secor (2005) Integr. Comp. Biol. 45:282-294
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Another way to study evolution:
The longestrunning
vertebrate
artificial
selection
experiment:
Body Mass (g)
Impose selection in an experimental
population and observe evolution
in real time ...
Male mice
at 42 days
of age
67 grams
100
gens.
30 grams
Generation
Bunger, L., A. Laidlaw, G. Bulfield, E. J. Eisen, J. F. Medrano, G. E. Bradford, F. Pirchner, U. Renne, W. Schlote,
and W. G. Hill. 2001. Inbred lines of mice derived from long-term growth selected lines: unique resources for
mapping growth genes. Mammalian Genome 12:678-686.
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Example: Selection on Plasticity
Scheiner, S. M., and R. F. Lyman. 1991. The genetics of phenotypic
plasticity. II. Responses to selection. J. Evol. Biol. 4:23-50.
Difference in thorax size of Drosophila melanogaster at 19 & 25oC
"We used a family selection scheme to select on the trait of
phenotypic plasticity of thorax size in response to temperature.
That is, the phenotype of a group of full-sibs as expressed in two
environments was the selected trait. We realize that this form of
selection will not be the usual form of selection in nature.
However, the purpose of this experiment was to explore aspects of
the genetic basis of the trait rather than to mimic natural selection."
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Example: Selection on Plasticity
Scheiner, S. M., and R. F. Lyman. 1991. The genetics of phenotypic
plasticity. II. Responses to selection. J. Evol. Biol. 4:23-50.
Difference in thorax size of Drosophila melanogaster at 19 & 25oC
41
Example: Selection on Plasticity
Scheiner, S. M., and R. F. Lyman. 1991. The genetics of phenotypic
plasticity. II. Responses to selection. J. Evol. Biol. 4:23-50.
Difference in thorax size of Drosophila melanogaster at 19 & 25oC
"We have demonstrated that phenotypic plasticity is a trait that can
respond to selection. This response is partially independent of
change in the mean of that trait; selection on plasticity of thorax
size did not result in a change in mean thorax size but selection on
mean thorax size did change plasticity. The complex pattern of
direct and correlated responses to selection show that the
phenotypic plasticity of a trait can be considered a character upon
which evolution can act but in ways which will interact with
selection on the mean of the trait."
42
Scheiner, S. M., and R. F. Lyman. 1991. The genetics of phenotypic
plasticity. II. Responses to selection. J. Evol. Biol. 4:23-50.
43
Plasticity may also evolve even
when it is not an intentional
target of selection.
Any time the selective event is more than
instantaneous, plasticity may evolve.
For example, many selection experiments
with Drosophila involve desiccation,
temperature or starvation "stress" that
lasts for hours or days. Survivors may be
those that were innately more tolerant at
the start of the stress and/or that rapidly
increased their tolerance.
44
Example: Selection Not on Plasticity
Harshman, L. G., J. A. Ottea, and B. D. Hammock. 1991. Evolved
environment-dependent expression of detoxication enzyme activity
in Drosophila melanogaster. Evolution 45:791-795.
Reared on standard medium (3 Control lines)
or lemon (3 Selected lines) for 20 generations.
For the Selected lines:
1. flies were placed in bottles with fresh lemon for 7-10 days;
2. 50% mortality occurred;
3. survivors were placed into a new bottle with fresh lemon
and vermiculite to produce the next generation.
All test flies were reared on standard medium for 1 generation.
All were transferred to either lemon or fresh medium for 24 h.
Epoxide hydrolases and glutathione S-tranferase assayed.
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Example: Selection Not on Plasticity
Interaction P = 0.0068
Greater Induction
= "Genotype-by-Environment Interaction"
Control
Selected
46
Example: Selection Not on Plasticity
Harshman, L. G., J. A. Ottea, and B. D. Hammock. 1991. Evolved
environment-dependent expression of detoxication enzyme activity
in Drosophila melanogaster. Evolution 45:791-795.
"In the present study the culturing regime
used was ostensibly continuous, unless the
process of lemon rotting every generation
constitutes temporal variation. Normally, one
would anticipate selection for change in
environment-dependent enzyme expression
to occur in variable environments but the
results of the present study suggest it can
evolve in a relatively constant regime."
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"Self-Induced Adaptive Plasticity"
Swallow, J. G., J. S. Rhodes, and T. Garland, Jr. 2005. Phenotypic and evolutionary plasticity
of organ masses in response to voluntary exercise in house mice.
Integrative and Comparative Biology 45:426-437.
A behavior under selection causes changes
in subordinate traits that in turn enhance
the ability of the organism to perform the
behavior.
48
"Self-Induced Adaptive Plasticity"
Possible examples in nature:
Animals that feed on particular foods may experience
shifts in digestive enzymes that facilitate their ability to
eat those foods.
Birds that engage in altitudinal migration might make "trial
runs" that would induce physiological changes that would
improve their ability to function at high altitude.
In rats, maternal behavior is hormone-dependent in firsttime mothers, but is less so in experienced mothers.
Similarly, male-male agonistic interactions in vertebrates
may result in the winners experiencing elevated
testosterone levels, which could facilitate their
subsequent performance in such interactions.
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Extra
Slides
Follow
50
This was about 20 minutes short in 2011.
Add more real examples, e.g., Hicks snake guts and hearts.
Human/rat training studies.
For 2012, I added Dapnia picture from Kelly et al. (2012) CPHY
paper and also Secor on snake guts. It was then just about right on
time.
However, in 2013 it was about 8 minutes too short.
Includes "acclimation" and "acclimatization" as well as
learning and memory.
For 2014, it was 15 min short???
For 2015, need to move GLUT4 plasticity into here - Ted forgot to
do this!
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Example: Selection Not on Plasticity
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Updates for 2007 Winter:
Pigliucci, M. Phenotypic plasticity 101. From http://www.genotypebyenvironment.org/
Pigliucci, M. 2005. Evolution of phenotypic plasticity: where are we going now? Trends Ecol. Evol.
20:481-486.
53
Gomez-Mestre, I., and D. R. Buchholz. 2006. Developmental plasticity mirrors differences among
taxa in spadefoot toads linking plasticity and diversity. Proceedings of the National Academy of
Sciences, USA 103:19021-19026.
Developmental plasticity is found in most organisms, but its role in evolution remains
controversial. Environmentally induced phenotypic differences may be translated into adaptive
divergence among lineages experiencing different environmental conditions through genetic
accommodation. To examine this evolutionary mechanism, we studied the relationship between
plasticity in larval development, postmetamorphic morphology, and morphological
diversity in spadefoot toads, a group of closely related species that are highly divergent in the
larval period and body shape and are distributed throughout temperate areas of both the New and
the Old World. Previous studies showed that accelerated metamorphosis is adaptive for desertdwelling spadefoot toads. We show that even under common garden conditions, spadefoot
toad species show divergent reaction norms for the larval period. In addition, experimentally
induced changes in the larval period caused correlated morphological changes in
postmetamorphic individuals such that long larval periods resulted in relatively longer
hindlimbs and snouts. A comparative analysis of morphological variation across spadefoot toad
species also revealed a positive correlation between the larval period and limb and snout lengths,
mirroring the effects of within-species plasticity at a higher taxonomic level. Indeed, after 110 Ma
of independent evolution, differences in the larval period explain 57% of the variance in
relative limb length and 33% of snout length across species. Thus, morphological diversity across
these species appears to have evolved as a correlated response to selection for a reduced larval
period in desert-dwelling species, possibly diverging from ancestral plasticity through genetic
accommodation.
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"Self-Induced Adaptive Plasticity"
Swallow, J. G., J. S. Rhodes, and T. Garland, Jr. 2005. Phenotypic and evolutionary plasticity
of organ masses in response to voluntary exercise in house mice.
Integrative and Comparative Biology 45:426-437.
The behavior under selection causes changes in
subordinate traits that in turn enhance the ability of the
organism to perform the behavior.
Could be a threshold effect, could be quantitative
within both linetypes, or could be difference
between S and C (like neurogenesis)
As wheel running increases across generations,
that leads to greater training effects, which in
turn support the higher wheel running.
55
When should plasticity evolve?
Gabriel, 2005:
"Stress occurring in periods shorter than life span strongly selects for
reversible phenotypic plasticity, for maximum reliability of stress
indicating cues and for minimal response delays. ... Analytic expressions
are given for optimal values of mode and breadth of tolerance functions
for stress induced and non-induced phenotypes depending on
(1) length of stress periods,
(2) response delay for switching into the induced phenotype,
(3) response delay for rebuilding the non-induced phenotype,
(4) intensity of stress, i.e. mean value of the stress inducing environment,
(5) coefficient of variation of the stress environment and
(6) completeness of information available to the stressed organism.
Adaptively reversible phenotypic plastic traits will most probably affect
fitness in a way that can be described by simultaneous reversible
plasticity in mode and breadth of tolerance functions."
56
57
Experimental Evolution:
"By experimental evolution we mean
research in which populations are studied
across multiple generations under defined
and reproducible conditions, whether in
the laboratory or in nature."
58
Possible wheel-running trajectories:
16000
Selected
Control
Revolutions
14000
12000
10000
S/C = 3 on
days 2- 6
8000
6000
4000
2000
0
0
1
2
3
4
5
6
Day
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JEB Discussion Meeting - "Phenotypic Plasticity"
Hans Hoppeler, Ken Lukowiak, M. Flueck, and Ewald R. Weibel
Downing College, Cambridge UK
September 10 - 14, 2005
Goal and Structure of Conference
Phenotypic plasticity is a fundamental biological phenomenon common to all
organisms. Phenotypic plasticity allows individuals of a species to adapt to
environmental challenges and thus to improve fitness and eventually reproductive
success. Over the last decade the mechanisms of phenotypic plasticity have become a
very active area of (comparative) biological research. It looks like an excellent time to
look at Phenotypic Plasticity in terms of its structural and physiological expression as
well as the molecular mechanisms that drive it.
The symposium brings together scientists of diverse orientations, from
molecular biology to comparative physiology and ecology, engaged in both experimental
and theoretical work. It offers a platform for the open discussion of concepts and
observations on the variation in diverse biological processes associated with phenotypic
plasticity.
Each of the four sessions has four speakers with 25 minutes of lecture and 10
minutes discussion each. The session is concluded with a general discussion period of 20
minutes. Participation is by invitation only.
60
Phenotypic plasticity of skeletal muscle. Chair: Hoppeler
0900-0925 Flueck M Functional, structural and molecular
plasticity of mammalian skeletal muscle
0935-1000 Johnston IA Environment and the plasticity of
skeletal muscle in fish
1030-1055 Hood DA Coordination of metabolic plasticity in
skeletal muscle
1105-1130 Anderson JE The satellite cell in skeletal muscle
plasticity
1140-1200 General Discussion
Sunday 61
Phenotypic plasticity of the brain. Chair: Lukowiak
1500-1525 Nguyen PV Comparative plasticity of brain
synapses in inbred mouse strains
1535-1600 Magistretti PJ Neuron-glia metabolic coupling
and plasticity
1630-1655 Weiss S Pregnancy-stimulated neurogenesis in
the adult female forebrain mediated by prolactin
1705-1730 Syed NI The brain plasticity at the base of
neuron – silicon interface
1735-1800 General Discussion
Sunday 62
Molecular mechanisms of phenotypic plasticity. Chair: Flueck
0900-0925 Swynghedauw B Phenotypic plasticity of adult
myocardium. Molecular mechanisms
0935-1000 Cossins AR Genomic insights into the
mechanisms of environmentally-induced
phenotypic plasticity
1030-1055 Schrader J Molecular strategies compensating
loss of gene function: lessons from the myoglobin
knockout mice
1105-1130 Goldring CE Plasticity in cell defence: access to
and reactivity of critical protein residues and DNA
response elements
1140-1200 General Discussion
Monday 63
Role of phenotypic plasticity in evolution. Chair: Garland
0900-0925 Fordyce JA The evolutionary consequences of
ecological interactions mediated through
phenotypic plasticity
0935-1000 Garland T Jr Phenotypic plasticity and
experimental evolution
1030-1055 Pigliucci M Evolution by genetic assimilation:
much ado about nothing?
1105-1130 Price TD Phenotypic plasticity and the evolution
of colour patterns
1130-1200 General Discussion and End of Conference
Tuesday
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Crucial Point!
Any particular instance of phenotypic
plasticity may or may not be adaptive!
However, many examples do seem to be
adaptive (i.e., to confer higher Darwinian
fitness), so we can proceed to ask ...
65