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Studying Evolution
Adaptation – a trait or suite of traits that increases the fitness of its
possessor.
 The adaptive significance of some traits are not obvious.
 This lecture and chapter will:
• Points out the pitfalls of studying putative adaptations
• Cover 5 methods for inferring that a trait is adaptive for a
particular function.
Studying Evolution
The use of the words adaptive, and adaptation is a short
cut for explaining a much larger evolutionary journey.
 However, the shortcut implies that an organism is somehow
perfectly matched to it environment.
 That somehow, the phenotypic traits that we observe are in
some way responsible for the success of the species.
• Cacti are adapted to hot, dry climates.
• Birds are adapted to live in trees..
• Frogs are adapted to live in water.
Studying Evolution
The phrase ‘X is adapted to live in Y’ may not mean more than:
X lives in environment Y.
That properties of X constrain it to live only in Y.
For an evolutionary biologist – ‘X is adapted to live in Y’ means
that environment Y has provided forces of natural selection
that have affected the life of X’s ancestors, and have molded
and specialized the evolution of X.
For an ecologist – ‘X is adapted to live in Y’ means that
individuals of X have themselves had some prior experience
(e.g. been cold-hardened) that makes life in Y (e.g. an
extremely cold winter) possible.
Canalization – is a measure of the ability of a population to produce the
same phenotype regardless of variability of its environment or genotype
(Wikipedia). (robustness)
Studying Evolution
Alternatively, the statement ‘X is adapted to live in Y’ can mean
that a property (an adaptation) has been identified that X
possesses and that has been shown to (or at a reasonable guess
might) explain why X can live in Y.
(e.g. Melanic peppered moths can live in polluted environments
because they can avoid predation). Such a property could be
either genotypic or phenotypic.
Studying Evolution
Gould and Lewontin – adaptationist programme – faith in the
power of natural selection as an optimizing agent to explain the
adaptive value of a phenotypic trait.
• This lecture and chapter is about making a connection
between a phenotypic trait and it’s adaptive value.
• It is not about demonstrating natural selection, but whether a
particular trait we observe confers the fitness that we would
predict.
• This adaptationist programme by itself, (supposedly) takes
care and rigor in determining the relationship between form
and function.
Studying Evolution
 The adaptationist programme is frequently overexploited. It
assumes that all traits have been selected for some fitness
value.
• It assumes that biological processes appear to have goals
 Birds fly south to avoid winter
 Birds have wings so they can fly
• The adaptationist programme is often teleological.
(Gk. Teleos – “end”) – Using design or purpose to explain a
phenomena (particularly natural).
 The adaptationist programme does a poor job of determining
whether the presence of a trait is really the product of
selective forces that do not work towards some end.
Studying Evolution- the seductiveness of ‘Just So’ stories.
 It is fairly easy to contrive an
evolutionary story to explain a
particular trait of an organism.
• The greater difficulty is to
ascertain whether that the
trait as any adaptive value.
• And whether our hypothesis
of the nature of that adaptive
value is supported.
Studying Evolution- the seductiveness of ‘Just So’ stories.
 Oxpeckers & their hosts – a mutualism?
• Benefits to the oxpeckers:
 Easy meal of ticks
 Safe place to eat ticks
 Clean open wounds
• Benefits to the host:
 Removal of parasites
 Careful observation reveals that:
• Oxpeckers rarely fed on ticks, instead
 Licked blood from open wounds
 Probed host’s ears for wax
 Gleaned dead skin from host’s hair
• Hosts attempted to shoo oxpeckers away 1-2 times/minute
Need to design and conduct and experiment to test the hypothesis.
Studying Evolution- the seductiveness of ‘Just So’ stories.
 3 month experiment with two treatment groups
• Business as usual
• Oxpeckers excluded (shooed away)
Treatment groups switched
Treatment groups
randomized.
Studying Evolution- the seductiveness of ‘Just So’ stories.
 3 month experiment with two treatment groups
• Business as usual
• Oxpeckers excluded (shooed away)
Treatment groups switched
Treatment groups
randomized.
Studying Evolution- the seductiveness of ‘Just So’ stories.
 3 month experiment with two treatment groups
• Business as usual
• Oxpeckers excluded (shooed away)
Treatment groups switched
Treatment groups
randomized.
Studying Evolution- the seductiveness of ‘Just So’ stories.
 Conclusion: oxpeckers are vampires and consumers of earwax
Treatment groups switched
Treatment groups
randomized.
Studying Evolution- Other considerations
1) Differences among populations or species are not always
adaptive.
Yellow billed oxpecker
Red billed oxpecker
• That is, mutations may arise in separate populations, and become
fixed by genetic drift.
• The trait may be selectively neutral.
Studying Evolution- Other considerations
2) Not every trait is an adaptation. Some traits may be used for
functions there were not originally selected for.
• While feeding on an animal, an oxpecker may find a mate.
• This does not mean that feeding on large animals evolved because it
creates mating opportunities.
Studying Evolution- Other considerations
3) Not every adaptation is perfect.
• Feeding on large mammals provides high-quality meals.
• Large mammals migrate long distances, exposing oxpeckers to
potentially unpredictable food supply.
Studying EvolutionRecognizing the many factors other than adaptation that may
account for an organism’s features, Williams (1966) wrote that
although “adaptation is often recognized in purely fortuitous
effects, and natural selection is invoked to resolve problems that
do not exist,” we should consider adaptation “an onerous concept
that should be used only where it is really necessary.”
That is, we all too often consider features to be adaptations (from
selection), but we should not assume this unless we have
convincing evidence to support this.
Studying Evolution5 approaches for studying whether a trait is an adaptation for a
particular function.
1)
2)
3)
4)
5)
Complexity*
Design*
Experiments
Observational methods
Comparative method
* Products of inference, and do not demonstrate any advantage of a trait.
Studying Evolution- Complexity
Even if we cannot immediately guess the function of a trait, we
often suspect it has an adaptive function it if is complex, for
complexity cannot evolve except by natural selection.
A peculiar, highly
vascularized structure
called a pectin projects
in front of the retina in
the eyes of birds.
Function is uncertain.
Generally assumed to play
some important functional
role because of:
• it’s complexity,
• it is ubiquitous among
bird species (conserved).
Studying Evolution- Complexity
The histological complexity of the lateral line system of fishes, and
its uniformity within many clades, implied to physiologists that it
had an adaptive function, which after extensive study, proved to
be sensation of differences in water pressure.
Assumptions were correct!
Studying Evolution- Design
The function of a morphological structure is often inferred from
its correspondence with the design an engineer might use to
accomplish some goal, such as locomotion, or the dissipation or
retention of heat.
• Artemisia carruthii, a sage that grows in
the hot regions of the American
Southwest has finely dissected leaves.
• Artemisia ludoviciana is more widely
distributed in America and Eurasia, has
broad, less dissected leaves
The trait could also be present due to
selection in an ancestor.
Studying Evolution- Design
Indeed, many structures are analogous in design to human
implements.
Villi in the small intestine
Terry cloth
Studying Evolution- Design
Mathematical models, based on
principles of engineering,
explain why, if fleetness is
advantageous, horses should
have evolved the one-toed
condition.
A single bone has greater
resistance to bending stresses
than several bones of the same
cross-sectional area and weight
– it can bear more weight when
running at greater speeds.
Studying Evolution- Experiments
Experiments are among the most powerful tools in science.
Contingent on:
• Phrasing the question/hypothesis as specifically as possible.
• Listing and testing as many competing hypotheses as
possible.
(Testing multiple hypotheses divides the affection for an single hypothesis.)
Studying Evolution- Experiments
What a predator sees
Do the wing markings and
wing waving of Zonosemata
flies mimic the threat
displays used by jumping
spiders on each other?
Studying Evolution- Experiments
Erik Greene’s experimental design
Studying Evolution- Experiments
Erik Greene’s results
Studying Evolution- Observational Methods
Not all hypothesis can be tested under controlled conditions, nor can
all organisms be manipulated like flies.
However, some observations do not necessarily lead to cause and effect.
That we always find iguana’s with body temperatures that are always near their
physiological optimum does not prove that iguana’s are behaviorally selected to
actively maintain their body temperature by thermoregulating.
It could be that they are just always found in environments in which the temperature is
within their thermal optimum.
Studying Evolution- Observational Methods
A successful observational experiment may require some
manipulation so that it can be demonstrated that iguanas make the
appropriate choice when provided with a range of temperatures that
they may actively choose.
• It then remains to demonstrate that such choices are adaptive –
that they increase fitness.
Studying Evolution- Comparative Method
The comparative method tests for patterns across species.
• Comparative methods take advantage of natural evolutionary
experiments provided by convergent evolution.
• If a trait evolves independently in many lineages because of a
similar selection pressure, then it suggests that the trait has some
adaptive value.
• Rigorous methods can produce results consistent with hypothesis.
Grey-headed flying fox
Relative testes size (adjusted for body size) as a function of roost
group size for 17 species of fruit bats and flying foxes.
Studying Evolution- Comparative Method
A simple correlative analysis provides weak evidence for the
relationship between testes size and size of roost group.
Theoretical population of six species.
Scatterplot based on independent
data points.
Not strong evidence.
Each species could have inherited testes
size from a common ancestor – selection
not a factor. Testes size not an
independent factor.
Studying Evolution- Comparative Method
Phylogenetically independent contrasts – a more robust approach
Studying Evolution- Other pitfalls
Pleiotropic effects – when a gene influences more than one trait.
Albinism -Individuals lack pigment in skin and hair.
 Crossed eyes occur at a greater frequency than pigmented individuals. The gene for
albinism can cause defects in the nerve connection between the eyes and the brain.
Albinism and crossed eyes are not always linked.
Phenylketonurea (PKU) - Defect in the liver enzyme phenylalanine hydroxylase
leads to excess plasma levels of phenylalanine. This in turn affects myelination of
axons in the brain, and ultimately to mental retardation.
 Decreased pigmentation
 eczema
Marfan’s syndrome – defect in the production of fibrillin-1, resulting in problems with
connective tissue throughout the body. Includes long arms, legs, fingers. Tall, thin body.
 Aortic enlargement
 Eye problems (nearsightedness, early glaucoma, early cataracts, dislocated lens)
Mouse lethal yellow (Ay) – increased activity of the agouti coat color gene.
 Adult onset obesity
 Increased somatic growth
 Increased susceptibility to tumor growth
 Premature infertility
Studying Evolution- Phenotypic plasticity
Phenotypic plasticity – variation in the phenotype due to
environmental influences. (Chapter 5).
Plasticity itself is a trait that can evolve.
Studying Evolution- Phenotypic plasticity
Phenotypic plasticity – variation in the phenotype due to
environmental influences. (Chapter 5).
Reaction norms – graphical representation of a genotype’s change
in phenotype across a range of environments.
Phototactic behavior of
Daphnia from different
environments (densities of
fish predators)
Studying Evolution- Phenotypic plasticity
On your own
10.6 -Trade-offs and constraints
10.7 - Selection Operates at Different Levels
The Spandrels of San Marco
The Spandrels of San Marco
Panselectionism – natural
selection of individuals is the only
cause of evolution.
The Spandrels have become
a euphemism for the
excessive use of Natural
Selection and adaptation to
explain the purpose of all
traits.
The Spandrels of San Marco
Fan-vaulted ceiling at King’s College chapel.
• The nature of the architecture
dictates the creation of the
spaces.
• The functionality of the space
does not dictate the architecture.
The Spandrels of San Marco
Dr. Pangloss from Voltaire’s Candide
“Things cannot be other than they are…. Everything is made for
the best purpose. Our noses were made to carry spectacles, so
we have spectacles. Legs were clearly intended for breeches,
and we wear them.”
The Spandrels of San Marco
The Panglossian paradigm emphasizes the near omnipotence of
Natural Selection to select the best design (with few constraints)
to the exclusion of other possibilities.
Some constraints are acknowledged –
Phyletic inertia - that humans are not optimally designed
for upright posture because so much of our Bauplan
evolved for quadrapedal life.
But other constraints are considered unimportant or
entirely dismissed.
The Spandrels of San Marco
Studies using the adaptationist programme proceed using the
following two steps:
1) An organism is atomized into ‘traits’, which are then
explained as being optimally designed for their function by
Natural Selection.
2) If part-by-part optimization fails, then an interaction is
acknowledged. The optimizing of one feature cannot be done
at the expense of another. The notion of trade-offs is
introduced, and the optimality of one part is acknowledged to
be limited by constraints of another part.
• The sub-optimality of a part is explained as its contribution to
the best possible design for the whole. The sub-optimality of a
part is attributed to the force of Natural Selection and nothing
else.
The Spandrels of San Marco
The narrowness of their adaptationist interpretation of how traits
arose results in the following progressive set of explanations for a
given trait. (p 586)
 If one adaptive argument fails, try another.
• Zig-zag commissures of clams, once considered as
strengthening devices, now become sieves.
• Either explanation may be correct.
• But, the failure of one explanation
should not be the basis for creating
a new explanation. Each trait may
not be for a specific purpose.
The Spandrels of San Marco
 If one adaptive argument fails, assume another exists, albeit
weaker.
• In the absence of a good adaptive argument, point out the
lack of understanding of the organism and its interactions
with its environment.
• Emphasize immediate utility, and exclude other attributes of
form.
• e.g. males of T. rex may have used their small fore limbs to
titillate female partners, but G & L argue, they didn’t arose de
novo for that end. They are more likely the reduction of a
homologous structure in ancestors.
The Spandrels of San Marco
 The problem is, the adaptationist programme often amounts to
nothing more than story telling, and does not have a structure in
place that would permit the rejection of the story in the face of
evidence.
As G. & L. point out, “Since the range of adaptive stories is as wide
as our minds are fertile, new stories can always be postulated.”
• Moreover, confirmation of the ‘story’ is loosely based on
evidence that would be consistent with natural selection as the
mechanism.
• But natural selection is quite frequently not observed.
• Excellent example with mountain bluebird aggression (p 588).
The Spandrels of San Marco
 The alternatives? (p 590)
• The null hypothesis: No adaptation and no selection at all.
Are the presence of polymorphisms the result of selection or
mutation and genetic drift?
• No adaptation and no selection on the trait in question, but a
change in the trait is a consequence for selection occurring on
another trait elsewhere, hence the correlation between
selection and the trait in question. – this is called?
The Spandrels of San Marco
 Decoupling selection and adaptation. (p592)
• Selection without adaptation – selection may occur to favor
individuals with high fecundity, but it will not have adaptive
value for the population – not enough food, or the high
population numbers attract predators. – but high fecundity
will almost always be selected for.
The Spandrels of San Marco
 Decoupling selection and adaptation. (p592)
• Adaptation without selection - may reflect phenotypic plasticity
to the environment, and hence may not be selected for (since it
is variable).
• Adaptation and selection occur, but there is no selective basis
for differences among adaptations. Related species may
develop different solutions to the same problem in different
areas. One solution is not necessarily better than another –
ergo, there is not a perfect solution. (p 593)
• Adaptation and selection occur, but the adaptation is a
secondary utilization. E.g. the spandrels of san Marco. (p 593)
The Spandrels of San Marco
 Summarizing
• The adaptationist programme breaks organisms down into
parts and genes.
• If selection acts only on the parts, then that diminishes the
value of the integrated whole. That is, the integration of traits
to produce an organism is less than or only equal to the sum of
its parts.
• G. & L. argue for a more holistic perspective.
Figure 10-22