To be or not to be:

1
To be or not to be
when shouldplasticity evolve?
Stopped here 15 Feb. 2007, start here 20 Feb. 2007
2
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.
3
Formal Theoretical Models
Gabriel, W. 2005. How stress selects for
reversible phenotypic plasticity. J. Evol.
Biol. 18873-883.
"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."
4
Gabriel, W. 2005. How stress selects for
reversible phenotypic plasticity. J. Evol.
Biol. 18873-883.
"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.
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Gabriel, W. 2005. How stress selects for
reversible phenotypic plasticity. J. Evol.
Biol. 18873-883.
"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."
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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?
7
Ways to study evolution
Compare extant species (or populations)to infer
whathas happenedin the past ...
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Ways to study evolution
9
Comparisons of Species
More Variable Less Variable
Understory Gap
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
811925 -1936.
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Comparisons of Species
More plastic for gas exchange traits than for
structural traits
Understory Gap
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
811925-1936.
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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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Another way to study evolution
Impose selection in an experimental population
and observe evolutionin real time ...
Male miceat 42 days of age
The longest- runningver tebratear tificial
selection experiment
100gens.
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 12678-686.
13
Example Selection on Plasticity
Scheiner, S. M., and R. F. Lyman. 1991. The
genetics of phenotypic plasticity. II.
Responses to selection. J. Evol. Biol.
423-50. 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."
14
Example Selection on Plasticity
Scheiner, S. M., and R. F. Lyman. 1991. The
genetics of phenotypic plasticity. II.
Responses to selection. J. Evol. Biol.
423-50. Difference in thorax size of Drosophila
melanogaster at 19 25oC
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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.
423-50. 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."
16
Scheiner, S. M., and R. F. Lyman. 1991. The
genetics of phenotypic plasticity. II.
Responses to selection. J. Evol. Biol. 423-50.
17
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.
18
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 45791-795.
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
"Genotype-by-Environment Interaction"
Control Selected
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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 45791-795.
"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 45426-437. A behavior under
selection causes changes in subordinate traits
that in turn enhance the ability of the organism
to perform the behavior.
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"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 first-time 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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Lecture 14 Measuring Selectionin the Wild
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An influential paper Lande, R., and S. J.
Arnold. 1983. The measurement of selection on
correlated characters. Evolution
371210-1226. "Natural selection acts on
phenotypes, regardless of their genetic basis,
and produces immediate phenotypic effects within
a generation that can be measured without
recourse to principles of heredity or evolution.
In contrast, evolutionary response to selection,
the genetic change that occurs from one
generation to the next, does depend on genetic
variation. ... Upon making this critical
distinction ... precise methods can be formulated
for the measurement of phenotypic natural
selection."
This verbal definition of selection, inheritance,
and evolution is crucial, because it allows clear
operational definitions of the three things
consistent with r h2s. Many discussions of
natural selection actually confound phenotypic
selection and inheritance, so be careful when you
are reading!
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"It cannot be too strongly argued that the
problem of animal evolution is essentially a
statistical problem ..." Weldon, W. F. R. 1893.
On certain correlated variations in
Carcinus moenas. Proc. Roy. Soc. London
54318-329.
Raphael Weldon was a pioneer in the application
of statistics to biology and a founder of the
journal Biometrika. Worked with Karl
Pearson. http//www-groups.dcs.st-and.ac.uk/histo
ry/Mathematicians/Weldon.html
"He was by nature a poet, and these give the best
to science, for they give ideas." (K. Pearson,
1906)
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"... the questions raised by the Darwinian
hypothesis are purely statistical, and the
statistical method is the only one at present
obvious by which that hypothesis can be
experimentally checked." (Weldon, 1894)
Testing for a correlation between phenotype and
lifetime fitness is best, but we can also get
interesting information by correlating phenotype
with some component of fitness, such as survival
from year to year. The basic question Are
survivors a random sample of the original
population before selection?
27
Miles, D. B. 2004. The race goes to the swift
fitness consequences of variation in
sprint performance in juvenile lizards.
Evolutionary Ecology Research 663-75.
From November to June
P
lt 0.03
0.14
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Miles, D. B. 2004. The race goes to the swift
fitness consequences of variation in
sprint performance in juvenile lizards.
Evolutionary Ecology Research 663-75.
r 0.56, P lt 0.0001
? Survivors o Non-survivors
Multiple logistic regression indicates speed, not
size, predicts survivorship
29
Miles, D. B. 2004. The race goes to the swift
fitness consequences of variation in
sprint performance in juvenile lizards.
Evolutionary Ecology Research 663-75.
However, multiple logistic regression with all
traits indicated that stride length was the best
predictor of survivorship.
30
Miles, D. B. 2004. The race goes to the swift
fitness consequences of variation in
sprint performance in juvenile lizards.
Evolutionary Ecology Research 663-75.
"Two key patterns emerged from the analysis of
differential survivorship in juvenile U. ornatus.
First, larger individuals were more likely to
survive than smaller individuals. However,
inclusion of the performance data also indicated
that size alone did not completely explain the
patterns of survival. Including body size and
burst velocity simultaneously in a regression
analysis resulted in only burst velocity
significantly predicting survivorship. This
result indicates that performance is a better
predictor of survivorship in juvenile lizards
than body size. Second, the selection analyses
revealed a complex pattern of linear, quadratic
and correlational selection on locomotor
performance. There was evidence for strong
directional selection on stride length. However,
the quadratic selection analyses revealed a
disparate pattern of selection acting on
locomotor performance."
31
Stabilizing or directional selection can be
detected by including the quadratic terms for
traits in the multiple regression
model. Similarly, correlational selection can be
detected by including cross-products terms.
Example Brodie, E. D., III. 1992. Correlational
selection for color pattern and antipredator
behavior in the garter snake Thamnophis
ordinoides. Evolution 461284-1298.
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Jackson, J. F., W. Ingram, III, and H. W.
Campbell. 1976. The dorsal pigmentation pattern
of snakes as an antipredator strategy a
multivariate approach. American Naturalist
1101020-1053. "Irregularly banded and
blotched-spotted patterns are associated with an
antipredator strategy of defense rather than
flight, and these patterns likely function
disruptively to minimize initial detection by
predators. Striped and unicolored-speckled
patterns are associated with antipredator
strategies emphasizing flight more than defense."
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Endler, J. A. 1986. Natural selection in the
wild. Princeton University Press, Princeton, New
Jersey. 336 pp.

• Conclusions of Endler's book
• Selection intensities in nature often are as
  strongas those implemented by animal breeders.
• Differences in fitness of gt 10 are not
  uncommonfor polymorphic traits.
• Selection related to survival is generally less
  strong than selection related to mating ability,
  fertility or fecundity.
• As of yet, few if any cases have quantified
  selection acting over the entire lifecycle of an
  organism (e.g., zygote to zygote).
• If quantitative traits, such as size, shape or
  metabolic rate are affected by many genes, then
  even relatively strong selection on them may
  still allow genetic drift to determine the fate
  of most mutations at such loci.

35
Kingsolver, J. G., H. E. Hoekstra, J. M.
Hoekstra, D. Berrigan, S. N. Vignieri, C. E.
Hill, A. Hoang, P. Gibert, and P. Beerli. 2001.
The strength of phenotypic selection in natural
populations. American Naturalist 157245-261.

• Abstract
• We tabulated 63 published studies of 62 species
  that reported over 2,500 estimates of linear or
  quadratic selection. More than 80 of the
  estimates were for morphological traits there is
  very little data for behavioral or physiological
  traits.
• Most published selection studies were
  unreplicated and had sample sizes below 135
  individuals, resulting in low statistical power
  to detect selection of the magnitude typically
  reported for natural populations.
• The absolute values of linear selection gradients
  b were exponentially distributed with an
  overall median of 0.16, suggesting that strong
  directional selection was uncommon.

36
Kingsolver, J. G., H. E. Hoekstra, J. M.
Hoekstra, D. Berrigan, S. N. Vignieri, C. E.
Hill, A. Hoang, P. Gibert, and P. Beerli. 2001.
The strength of phenotypic selection in natural
populations. American Naturalist 157245-261.

• Abstract
• Comparisons of estimated linear selection
  gradients and differentials suggest that indirect
  components of phenotypic selection were usually
  modest relative to direct components.
• The absolute values of quadratic selection
  gradientsg were exponentially distributed with
  an overall median of only 0.10, suggesting that
  quadratic selection is typically quite weak.
• The distribution of g values was symmetric about
  0, providing no evidence that stabilizing
  selection is stronger or more common than
  disruptive selection in nature.

Stopped here 20 Feb. 2007 End of Material for
2nd Midterm Exam