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Testing adaptive hypotheses
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What is (an) adaptation?
What is (an) adaptation?
Generally speaking, adaptations are traits or
characters that appear to be too well-fitted to
their environment to have arisen by chance.
That is, they must be the result of selection.
Adaptations may involve morphological,
physiological or behavioural traits. They arise
through the accumulation of a series of small
improvements over time.
A trait, or integrated set of traits, that increases
the fitness of an organism.
The process of improving the fit of phenotype to
environment through natural selection
Testing adaptive hypotheses
What is (an) adaptation?
"If it could be demonstrated that
any complex organ existed
which could not possibly have
been formed by numerous
successive slight modifications,
my theory would absolutely
break down." — Darwin
Testing adaptive hypotheses
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Testing adaptive hypotheses
Examples of adaptations
The eye
Bird wings
The human brain
Homeothermic temperature regulation
Human language
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Testing adaptive hypotheses
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Identifying adaptations
Testing adaptive hypotheses
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Identifying adaptations – an incomplete example
Why do polar bears have white coats?
• Adaptive hypothesis:
In order to identify a trait as an adaptation, we
must first hypothesize its use or function, and
then test that hypothesis.
As we saw in the Gould & Lewontin article, it is
important to test the hypothesis of adaptation
against a variety of null and alternative
hypotheses.
camouflage
white coat is an adaptation for
• Test:
observe hunting behaviour and assay use of
camouflage
• Result:
camouflage not usually important in hunting
• New adaptive hypothesis:
trapping solar heat
white coat is an adaptation for
• Test: hairs are actually clear and translucent, and trap 16%
of incident light energy – better than most hair types.
Results don’t support our first adaptive hypothesis.
Testing adaptive hypotheses
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Testing adaptive hypotheses
Identifying adaptations – an incomplete example
Identifying adaptations – an incomplete example
Why do polar bears have white coats?
Why do polar bears have white coats?
• Adaptive hypothesis:
camouflage
white coat is an adaptation for
• Test:
observe hunting behaviour and assay use of
camouflage
• Result:
camouflage not usually important in hunting
• New adaptive hypothesis:
trapping solar heat
white coat is an adaptation for
• Adaptive hypothesis:
camouflage
white coat is an adaptation for
• Test:
observe hunting behaviour and assay use of
camouflage
• Result:
camouflage not usually important in hunting
• New adaptive hypothesis:
trapping solar heat
white coat is an adaptation for
• Test: hairs are actually clear and translucent, and trap 16%
• Test: hairs are actually clear and translucent, and trap 16%
Results are consistent with our new adaptive hypothesis.
What’s missing?
of incident light energy – better than most hair types.
of incident light energy – better than most hair types.
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Testing adaptive hypotheses
Identifying adaptations
Testing adaptive hypotheses
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Approaches to testing adaptive hypotheses
The polar bear example shows that ecological and
physiological patterns are consistent with one particular
adaptive hypothesis – but it doesn’t show that fur color
evolved via a process of adaptation.
We’ve compared alternative hypotheses of adaptation –
but we haven’t tested the biological null hypothesis:
that no adaptation has occurred.
The primary null hypothesis is that traits have evolved
due to drift (according to the neutral model).
Ex per im ent al st udies
There are various experimental and theoretical ways to
Ex . test
Wing
marks &ofwing
waving
inhas
Tephritid
flies.
hypotheses
adaptation.
Each
its benefits
and
drawbacks:
• Observational studies (e.g., the polar bear study)
Tephritid flies have dark bands on their wings and
• Experiments
wave their
wings when disturbed in a manner that is
Theoretical
models
•
reminiscent of their
major predator's territorial
Comparative
method
display• -e.g., jumping
spiders' leg waving.
• Molecular evidence
Do flies mimic their predators?
Testing adaptive hypotheses
Experimental approaches: example
Tephritid flies have dark bands on their wings. When
disturbed, they wave their wings in a manner
reminiscent of the territorial behavior of their predator
(leg waving in jumping spiders).
Do the flies mimic their predators?
Does this mimicry deter predators?
Does it deter jumping spiders in particular?
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If so, is this mimicry to deter any predator, or is it
specifically to deter jumping spiders?
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Testing adaptive hypotheses
Experimental approaches: example
Testing Adaptive Hypotheses
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Testing adaptive hypotheses
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Experimental approaches: example
The evidence suggests that the morphology and
behavior of the flies increases their fitness in the face of
their primary predator.
Text
Conclusion:
Unlike the observational approach, this gives direct
evidence of a fitness advantage.
However, experiments like this still don’t directly test
the hypothesis that the traits evolved due to the
process of adaptation.
Testing adaptive hypotheses
Theoretical approaches
Two classes of models predict how a trait should evolve
under a specific set of environmental circumstances
(usually ignoring genetics altogether).
• Optimality models assume that a trait will evolve
to impart the highest possible fitness.
• Evolutionarily Stable Strategy (ESS) models assume
that the fitness of a phenotype depends on what
other phenotypes are present. This doesn’t always
lead to maximal fitness.
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Testing adaptive hypotheses
Theoretical approaches – example
We might do an ESS model to explore the possibility
that lekking behavior in birds is an adaptation to
predation risk (because the risk of predation gets
spread out among large numbers of males).
The model would predict the number of males we
should see on a lek as a function of the risk of
predation and the likelihood of mating.
We could then measure whether the number of males
we observe in nature is consistent with the predictions
of the model, given our observations of predation and
mating probabilities.
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Testing adaptive hypotheses
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Theoretical approaches – example
Testing adaptive hypotheses
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The comparative method
These models offer quantitative predictions for
observational or experimental studies.
If we have an adaptive hypothesis for a trait, then we
might expect to see a correlation between the
explanatory variable (X) and the trait itself (Y).
However, they seldom compare predictions to
alternative hypotheses (either adaptive or non-adaptive)
Also, the quantitative predictions might not take into
account constraints on adaptation – any failure of
observations to match predictions might be due to such
constraints.
However, another possible explanation for such a
correlation is the process of evolution itself: organisms
whose common ancestors had both X and Y are also
Thelikely
Comparative
Method
to have X and
Y.
Ex . Testis size in bat species.
Male bats vary in the size of testes. One hypothesis
for this variation is that large testes produce more
sperm, an advantage in sperm competition.
Testing adaptive hypotheses
The comparative method – Example
Testis size in bats
Male bats vary from species to species in the size of
their testes.
Adaptive hypothesis: Larger testes produce more
sperm, which provides an advantage if sperm from
multiple males competes for fertilization in a female.
Prediction: Species with larger social groups should
have males with larger testes (because more males are
competing for reproductive access to females).
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Sperm competition might be more intense in larger
social groups, where more males compete for
reproductive access to females.
Testing adaptive hypotheses
The comparative method – Example
Prediction:
Species
groups
Hypot hesis:
Testis with
size islarger
largersocial
in species
withshould
larger
social
groups.
have
males
with
larger testes (because more males are
competing for reproductive access to females).
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1. Need phylogeny
2. Calculate contrasts between sister taxa
Closely related3.species
Evaluate relationship with phylogeneticallymight have similarcorrected
group
values.
size
and
testis
size
NOT
But not all data points are
because of sperm
independent.
The comparative method – Example
The comparative method – Example
competition, but because
they share an ancestor
Null hypothesis: Closely related species might have
who had large testes and
similar group size and testis size simply because they
Closely related species lived in large groups.
share a common ancestor.might have similar group
size and testis size NOT
But not all data pointsbecause
are
of sperm
independent.
competition, but because
How do we control for
they share an ancestor the effects of shared
who had large testes and
history?
lived in large groups.
Closely related species
might have similar group
size and testis size NOT
How do we control for
because of sperm
the effects of shared
competition, but because
history?
they share an ancestor
who had large testes and
lived in large groups.
Testing adaptive hypotheses
Testing adaptive hypotheses
The comparative method – Example
How do we control for
the effects
of shared
Controlling for common
ancestry,
males in species
history?
larger groups sizes
still have larger testes.
with
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Testing adaptive hypotheses
Testing adaptive hypotheses
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The comparative method
Evidence that phylogenetically independent contrasts of
a trait (e.g., testis size) are correlated with a
hypothesized explanatory variable (e.g., group size)
suggests that:
• The trait has evolved in the absence of (or despite)
phylogenetic constraint
• Not all evolution of the trait has been neutral
(because there is evidence of directionality)
The comparative method explicitly tests adaptive
hypotheses against a null hypothesis.
Yes, males in species with larger group sizes also
Testing adaptive hypotheses
The comparative method – caveats
“Correlation is not causation.”
Still not a direct rejection of the neutral model.
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