Optimality and Symmorphosis

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Lecture 10 Optimality and Symmorphosis
Started here 8 Feb. 2007
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Organisms are not optimal!
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Why Organisms are not Optimal 1. Organisms are
not "designed" natural selection is not
engineering 2. Biological materials have
limitations 3. Energetic efficiency is not
necessarily what selection maximizes 4.
Environments often change too rapidly 5.
Selection cannot anticipate 6. Genetic drift
operates in all populations 7. Behavior evolves
too rapidly 8. Sexual selection counters natural
selection
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Although engineers have final goals and purposes,
natural selection does not. Moreover, engineers
often design for a single purpose, whereas
organisms must do many things, not just
one. Jared Diamond has used the analogy of
elevators to consider what he likes to call
"safety factors." You can show that freight
elevators have lower safety factors --
smaller cables -- than do passenger elevators.
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Safety factors of some engineered
structures Cable of fast passenger
elevator 11.9 Cable of slow passenger elevator
7.6 Cable of slow freight elevator 6.7 Wooden
building 6 Cable of powered dumbwaiter
4.8 Steel building or bridge 2 Diamond, J.
M. 1993. Evolutionary physiology. Pages 89-111 in
C. A. R. Boyd and D. Noble, eds. The logic of
life the challenge of integrative physiology.
Oxford University Press, Oxford.
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This analogy is spurious, because, whereas
elevators may only carry one thing, such as
freight, and be designed for that purpose alone,
organisms are different. Human "elevators" not
only carry freight, they carry people (including
during pregnancy), they feed themselves, they
avoid becoming food, they mate, and they raise
offspring.
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What if functional demands on a given structure
conflict? Can it be "optimally" designed in any
meaningful sense? Think about skin Should skin
be designed optimally for temperature regulation,
exteroreception, osmoregulation, encryption, as a
barrier to infection, protection from physical
assaults, or rapid healing from
wounds? (modified from Lindstedt and Jones,
1987, p. 292)
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Think about the lung Should the lung be
designed optimally for uptake of O2, elimination
of CO2, temperature regulation, vocalization,
resistance to infection, resistance to
particulates, or rapid healing?
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Another way to view excess capacities "Why
should phenotypes be overdesigned? Statements
that such overdesign represents a 'factor of
safety' (cf. Kummer, 1959) hardly explain its
origin. In any case they implytechnological
planningor prescience and shouldprobably be
discouraged." (Gans, 1979, p. 227)
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"... for most individual organisms the structural
and physiological capacities are likely to be
excessive for the needs of any particular
moment. Obviously, natural selection does not
'look' just at an organism's momentary
utilization of each aspect of the phenotype, but
at the requirements imposed on all phenotypic
aspects of an individual throughout its life
span." (Gans, 1979, p. 227)
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Why Organisms are not Optimal 1. Organisms are
not "designed" natural selection is not
engineering 2. Biological materials have
limitations 3. Energetic efficiency is not
necessarily what selection maximizes 4.
Environments often change too rapidly 5.
Selection cannot anticipate 6. Genetic drift
operates in all populations 7. Behavior evolves
too rapidly 8. Sexual selection counters natural
selection
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Although engineers can start from scratch,
natural selection cannot. Selection works with
pre-existing materials, whatever a species
happened to inherit from its ancestors. And these
may not be the best possible materials for a
particular function. Thus, relatively severe
"constraints" are placed on living systems. If an
engineer wants to build a shell out of titanium,
to maximize strength while minimizing mass, they
can.
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But selection cannot do that with, say,
tortoises, because a mutation conferring the
ability to produce titanium apparently has never
occurred during the course of life on this
planet. Similarly, as compared with natural
materials, engineers can make artificial joints
and heart valves that last far longer than
tissues and are not susceptible to disease.
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Why Organisms are not Optimal 1. Organisms are
not "designed" natural selection is not
engineering 2. Biological materials have
limitations 3. Energetic efficiency is not
necessarily what selection maximizes 4.
Environments often change too rapidly 5.
Selection cannot anticipate 6. Genetic drift
operates in all populations 7. Behavior evolves
too rapidly 8. Sexual selection counters natural
selection
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Often for simplicity, most optimality models are
phrased in terms of energy as the common
currency, and the "goal" of selection is seen as
minimizing absolute energy costs or perhaps
maximizing energetic efficiency. But we have
little empirical evidence that this is what
selection actually tends to do.
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Moreover, what matters in evolutionary models is
relative fitness -- how good one individual is as
compared with others in its population -- not
absolute fitness measured against some external
scale. Thus, selection generally leads to
adequacy or sufficiency, not necessarily
optimality.
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"In spite of occasional statements to the
contrary, there can be little argument that
natural selection is unlikely to be a mechanism
for generating perfection in individual animals.
... that the structure of an animal allows
it to perform particular actions, highly
advantageous under a particular set of
circumstances, does not require perfect matching,
but only adequacy ..." (Gans, 1983, pp.
101-102)
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Why Organisms are not Optimal 1. Organisms are
not "designed" natural selection is not
engineering 2. Biological materials have
limitations 3. Energetic efficiency is not
necessarily what selection maximizes 4.
Environments often change too rapidly 5.
Selection cannot anticipate 6. Genetic drift
operates in all populations 7. Behavior evolves
too rapidly 8. Sexual selection counters natural
selection
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Selection cannot change organisms very rapidly,
for two reasons 1. if selection is too strong,
then population size will be reduced such
that extinction by demographic stochasticity is
likely. 2. the heritability of traits is usually
far less than unity. Quantitative genetics
clearly indicates that evolution often will fail
to keep pace with the rate of environmental
change.
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Why Organisms are not Optimal 1. Organisms are
not "designed" natural selection is not
engineering 2. Biological materials have
limitations 3. Energetic efficiency is not
necessarily what selection maximizes 4.
Environments often change too rapidly 5.
Selection cannot anticipate 6. Genetic drift
operates in all populations 7. Behavior evolves
too rapidly 8. Sexual selection counters natural
selection
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Even if selection could generally track typical
rates of environmental change, it cannot
anticipate major environmental changes, such as
asteroids hitting the earth, severe droughts,
"100-year floods" or even the invasion of a
population by some new pathogenic organism, such
as AIDS in the human population. Thus, what is
"optimal" now may soon not be. Today's adaptation
is tomorrow's constraint.
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Why Organisms are not Optimal 1. Organisms are
not "designed" natural selection is not
engineering 2. Biological materials have
limitations 3. Energetic efficiency is not
necessarily what selection maximizes 4.
Environments often change too rapidly 5.
Selection cannot anticipate 6. Genetic drift
operates in all populations 7. Behavior evolves
too rapidly 8. Sexual selection counters natural
selection
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Random genetic drift operates in all populations
that are of less than infinite size -- which
means all populations. Both theoretical and
empirical studies suggest that random drift is
often strong enough to counter selection. Thus,
drift alone should ensure that average values for
populations or species are rarely if ever at the
optimum dictated by selection.
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You can think of the interplay between selection
and drift with the following analogy. Imagine the
population's mean phenotype as a billiard ball on
a pool table with a lumpy surface. Selection is a
pool cue that aims the population's mean
phenotype at a particular pocket, but drift is
the lumpy surface which makes it go off
course. But, if the cue hits the ball with enough
force, it may get there anyway.
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Another analogy is an unbalanced bowling
ball. The bowler may throw it straight down the
alley towards the pins, but the unbalance, as
genetic drift, will often make it not go straight
to the center pin.
Genetic drift is a key element of Sewall Wright's
shifting balance theory of evolution, in which
drift is argued to often push populations
temporarily in the direction of lower fitness.
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Why Organisms are not Optimal 1. Organisms are
not "designed" natural selection is not
engineering 2. Biological materials have
limitations 3. Energetic efficiency is not
necessarily what selection maximizes 4.
Environments often change too rapidly 5.
Selection cannot anticipate 6. Genetic drift
operates in all populations 7. Behavior evolves
too rapidly 8. Sexual selection counters natural
selection
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Natural Sexual Selection
Act On
Behavior
Constrain
Morphology, Physiology, Biochemistry
Organismal Performance Abilities
Deter- mine
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Cinclus mexicanus
The Dipper example of an organism in which a
behavior, diving to forage, seems to have evolved
more rapidly than underlying morphological and
physological traits that might enhance the
ability to dive.
Figure 3 (page 257) from D. J. Futuyma. 1986.
Evolutionary biology. 2nd. Ed. Sinauer
Associates, Sunderland, Massachusetts.
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"In the case of the water ouzel, the acutest
observer by examining its dead body would never
have suspected its subaquatic habits. ... In such
cases, and many others could be given, habits
have changed without a corresponding change of
structure." Charles Darwin, in The Origin
of Species, Ch. 6
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hard to argue that the dipper is optimally
designed for its current behavior.
Maybe a penguin is close to optimally adapted for
underwater foraging, but even penguins still
reproduce and lays eggs on land, and have not
reevolved gills.
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Quotes about behavior as an evolutionary
pacemaker Ernst Mayr others
(1904-2005)
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"A shift into a new adaptive zone is, almost
without exception, initiated by a change in
behavior other adaptations to the new niche,
par ticularly the structural ones, are acquired
secondarily (Mayr 1958, 1960). With habitat and
food selection - behavioral phenomena - playing a
major role in the shift into new adaptive zones,
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the importance of behavior ... is self- evident.
... Most shifts into new ecological niches are,
at first, unaccompanied by structural
modifications (Robson and Richards 1936)."(Mayr,
1963, p. 604)
Ernst Mayr in 1994, after receiving an honorary
degree at the University of Konstanz.
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"Many if not most acquisitions of new structures
in the course of evolution can be ascribed to
selection forces exerted by newly acquired
behaviors (Mayr, 1960). Behavior, thus, plays an
important role as the pacemaker of evolutionary
change. Most adaptive radiations were apparently
caused by behavioral shifts." (Mayr, 1982,
p. 612)
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"... when a major environmental shift occurs,
natural selection should initially favor
compensatory changes in behavior ..." (Huey
and Bennett, 1987, p. 1098) "These results
indicate that rather striking differences in
ecology and behavior may be accompanied by modest
differences in physiology." (Taigen and
Pough, 1985, p. 991) "Indeed, behavior is in the
vanguard of evolution." (Plomin, 1990, p.
183) "It has often been said that behavior is one
of the most labile traits in animal evolution.
Whether this is so remains to be
demonstrated..." (Bush, 1986, p. 1)
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Why Organisms are not Optimal 1. Organisms are
not "designed" natural selection is not
engineering 2. Biological materials have
limitations 3. Energetic efficiency is not
necessarily what selection maximizes 4.
Environments often change too rapidly 5.
Selection cannot anticipate 6. Genetic drift
operates in all populations 7. Behavior evolves
too rapidly 8. Sexual selection counters natural
selection
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Sexual selection by male-male competition for
access to females, or by female choice of
particular males, can cause the evolution of
bizarre structures and behaviors.
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Among these species of tragopan's, males vary,
but females are similar and presumably adapted
for crypsis, etc. These closely related species
are thought to have diverged and speciated
non-adaptively by sexual selection.
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Sexual selection can even work in direct
opposition to natural selection, because it is a
largely independent process.
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Although we may not generally expect organisms to
be optimal, optimality models nonetheless be
useful tools for understanding the "design" of
biological systems. Symmorphosis is an informal
optimality model, so it may be useful for
understanding how and why organisms work the way
they do.
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Optimality models can indicate the best that
organisms could be, given some explicit
assumption of a design criterion and specified
constraints in other words, one end of a
continuum. Even if we don't believe organisms are
optimal, it is useful to know what an optimal
organism would be like. Establishing the
hypothetical best possible can facilitate
quantitative tests of the degree of departure
from perfect design.
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Symmorphosis
Weibel, E. R., and C. R. Taylor, eds. 1981.
Design of the mammalian respiratory
system. Respiration Physiology 441-164.
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Taylor, C. R., and E. R. Weibel. 1981. Design of
the mammalian respiratory system. I.
Problem and strategy. Respiration
Physiology 441-10. Passage from page 3
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Weibel, E. R., and C. R. Taylor, eds. 1981.
Design of the mammalian respiratory
system. Respiration Physiology
441-164. Passage from pages 151-152
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Weibel, E. R., C. R. Taylor, and L. Bolis, eds.
1998. Principles of animal design the
optimization and symmorphosis debate.
Cambridge University Press, Cambridge, U.K.
xx 314 pp. Weibel, E. R. 2000. Symmorphosis on
form and function in shaping life. Harvard
Univ. Press, Cambridge, Mass. xiii 263
pp.
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Symmorphosis is based on an old idea in
biology "The principle of symmorphosis assumes
that animals incur a selective penalty for
maintaining structures in 'excess' of the
immediate demand. This idea, implied by Aristotle
and Cuvier, was explicitly stated by Darwin in
The Origin of Species (1959) '... natural
selection will tend in the long run to reduce any
part of the organism, as soon as, through changed
habits, it becomes superfluous."
(Lindstedt and Jones, 1987, p. 290)
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This idea was also mentioned by Bennett and Ruben
(1979, p. 651) in their original paper on the
aerobic capacity model for the evolution of
endothermy "It is reasonable to assume, however,
that these coevolved transport and utilization
systems will not differ greatly from each other
within an individual animal in their capacity for
oxygen processing."
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How might we test the general validity of
symmorphosis? Pick a physiological
system. Determine how much excess capacity (if
any) is present. Compare lots of species (or
individual with species).
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This has not been done so we do not know how
well symmorphosis describes biological diversity.
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ANCOVASlope 0.833
Slope 1.025
Using the original data of Taylor, Weibel, and
collaborators, Garland and Huey (1987) found no
matching at the level of deviations from
allometric equations.
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Symmorphosis would predict that all of these
traits should be positively correlated, but they
are not.
Stopped here 8 Feb. 2007