Allometry and Isometry
• y changes as a function of x
• Allometric equation  y = bxa
– (where y= one char, x=another, a=coeff. of
allometry, and b=constant proportion relating y
and x)
• if “a” = 1 then b = y/x which means
that y changes in direct proportion to x
• a<1  y increases less rapidly than x
• a>1  y increases more rapidly than x
Allometry and Isometry
• Many times it is easiest to express this
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equation like this:
log y = log b + a log x
This gives a straight line with slope = a and
an intercept = log y
Most morphological evolution can be
described in terms of Allometric
relationships.
Allometric relationships with body mass are
often the consequence of adaptation
• Structures that support an organism
must change disproportionately in
shape as weight increases.
• Tree trunk mass to cross sectional is
3/2 power of height
Evolution of Tolerance
• In animals, a series of responses occur
sequentially in response to stress
• Lets examine these steps...
Allometry and Isometry
• Example:
• In cold environments, large size is
advantageous in birds and mammals
because they lose heat more slowly,
Thus requiring less food to maintain
constant body temp.
• Bergmann’s Rule  “birds and
mammals larger in colder climates than
same/related species in warmer
climates
Evolution of Tolerance
1. Changes in behavior
2. Hormone-modulated biochemical and
3.
4.
5.
physiological functions
Slower, longer lasting changes in
physiology (“acclimation”)
In some instances, developmental changes
in morphology
At population level, genetic changes due to
differences among genotypes in survival
and reproduction rates caused by the
stress
Evolution of Tolerance
• If the responses of individual
organisms cannot fully compensate for
the stress, fitness is reduced
• This may lead to genetic changes
• Some changes entail developmental
responses and these are reversible
– e.g., Seasonal Responses
What Limits Geographical
Ranges of Species?
• Some ranges are set by biotic factors,
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interspecific competition & predation, or by
abiotic factors such as temperature and
water availability
This question is thus complex and difficult to
answer
The simplest hypothesis is the lack of
genetic variation for tolerance of
physiological stress
– However, in general this is not likely...
What Limits Geographical
Ranges of Species?
• Successful colonization of sites may
require numerous coincident adaptive
changes
• This suite of adaptations may be an
improbable concatenation of genetic
variants for many characteristics
– e.g. Seasonal timing of reproduction &
Growth...
What Limits Geographical
Ranges of Species?
• Trade-offs exist between adaptation to
conditions within and beyond the
margin of the range
• Trade-offs limit adaptation to a new
environment due to gene flow from
old  new
(center of range  periphery)
What Limits Geographical
Ranges of Species?
• The explanation put fourth by Mayr
• Gene flow from the main range of a species into the
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marginal populations prevents them from further
adapting by breaking down adaptive combinations of
interacting genes
So, a marginal population may be better if able to
adapt & expand range if it could not exchange genes
with interior populations
Perhaps species have evolved broader ranges then
we give them credit  because the adapted
extralimital population we call different species
Adaptation
• Let’s examine some methods used by
evolutionary biologists to test
hypotheses about adaptations
– Experiments
– Observational studies
– Comparative Method
All Hypotheses Must be
Tested: the Giraffe’s Neck
• Everyone knows that the giraffe
evolved a long neck to be able to eat
the tallest leaves, thereby escaping
from competition with other herbivores
• Simmons and Scheepers challenged
this notion and offered an alternative
explanation for the giraffe’s long neck
All Hypotheses Must be
Tested: the Giraffe’s Neck
• They found that giraffe’s most often ate
leaves at shoulder height, not from the tops
of trees
All Hypotheses Must be
Tested: the Giraffe’s Neck
• They also found that males with the
longest necks have the largest, hardest
skulls
• Maybe long necks evolved for
competition for females
– Female necks became longer because of
selection for longer male necks
• Neck-as-a-weapon hypothesis
All Hypotheses Must be
Tested: the Giraffe’s Neck
• Pratt and Anderson classified social
status of males
– Class C were young adults
– Class A were large adults
– Class B were small adults
• Class A males had wider, stronger
heads
• Studied displacement by classes and
receptivity of females of classes
All Hypotheses Must be
Tested: the Giraffe’s Neck
• There is evidence for selection on
longer necks for reaching high and
male-male competition
• When studying adaptation remember
that:
– Differences among populations or species
are not always adaptive
– Not every trait is an adaptation
– Not every adaptation is perfect
Function of Wing Markings
and Wavings of Zonosemata
• Tephritid fly that has distinct dark bands on
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wings
Holds wings up and waves them
Display seems to mimic threat display of
jumping spiders
Perhaps flies mimic jumping spiders to avoid
predation
– Avoid predation by other predators
– Or mimic jumping spiders to avoid predation by
jumping spiders
Function of Wing Markings
and Wavings of Zonosemata
• Phrase a precise question
– Do wing markings and waving behavior of
Zonosemata mimic threat displays of jumping
spiders and deter predation?
• List three alternative hypotheses
– Flies do not mimic jumping spiders
• Display may be used in courtship
– Flies mimic jumping spiders to deter non-spider
predators
– Flies mimic jumping spiders to deter jumping
spiders
Function of Wing Markings
and Wavings of Zonosemata
• Experimental procedure
– Clipped wings of Zonosemata and house
flies, exchanged wings, and glued them on
opposite fly
• Clipping and gluing did not affect flying or
displaying
– Created five experimental groups to test
hypotheses
Function of Wing Markings
and Wavings of Zonosemata
• Jumping spiders retreated from flies
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displaying with marked wings
Other predators killed and ate test flies
Function of Wing Markings
and Wavings of Zonosemata
• Results consistent with hypothesis 3 but not
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1 or 2
Support for hypothesis that Zonosemata
deters its predators by acting like one
Important experimental design
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Testing control groups
All treatments handled identically
Randomization of order of treatments
Replication of treatments
Function of Wing Markings
and Wavings of Zonosemata
• Why was replication important?
– Reduced distortion of results by unusual
individuals or conditions
– Can estimate precision of results
• Study successful because many
variables were tested, but each was
tested independently
Observational Studies
• Experimental studies are preferred but it is
often not feasible to experiment
– e.g., cannot exchange giraffe’s necks with
other animal
• Behavior is hard to experiment with because
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the experiment often alters the natural
behavior
Must use observational studies sometimes
– Often they are nearly as powerful as
experimental studies
Behavioral Thermoregulation
• Desert iguanas (Dipsosaurus dorsalis) are
ectothermic
– Must regulate body temperature behaviorally
• Can only function between 15° and 45°C
• Examine thermal performance curve to see
adaptation to particular temperature
• Body temperature affects physiological
performance
• Keep body temperature close to 38°C
Desert iguanas
(Dipsosaurus dorsalis)
Night Retreats of Garter
Snakes
• Do snakes make adaptive choices of
where to sleep at night?
• Ray Huey implanted garter snakes with
radio transmitters with thermometers
• Preferred body temperature is 28– 32°C
• Keep body temperature near preferred
during day
– Exposed or under rocks
Night Retreats of Garter
Snakes
• How do they choose good retreats at
night?
• Thickness of rock determines
microhabitat temperature
– Thin rocks heat a lot during day and cool a
lot during night
– Thick rocks heat and cool slowly
– Medium rocks heat and cool just enough
• Garter snakes should choose medium
rocks
Night Retreats of Garter
Snakes
• Huey placed snake models under
different rocks, in burrows, and on
surface
– Tested temperature fluctuations
• Found that snakes choose medium
rocks to heat and cool near preferred
temperature range
The Comparative Method
• Purpose of the comparative method is
to remove the effects of evolutionary
history from an analysis
• The reasons why you need to remove
effects of phylogeny from ecological or
behavioral analyses are best
demonstrated through examples
The Comparative Method
• Why do some bat species have bigger testes?
• Some bats have larger testes for their body size
than others
• Hosken hypothesized that bigger testes evolved
for sperm competition
• Female bats may mate with more than one male
so the more sperm deposited by a male, the
better chance he has of fertilizing the eggs
– Bigger testes mean more sperm
The Comparative Method
• Hosken reasoned that bat species that
live in larger groups would have greater
sperm competition
• Therefore, they should evolve larger
testes
• Hosken collected data on roost group
size and testes size and found a
significant correlation
The Comparative Method
• Hosken realized
that this
correlation may
be misleading
The Comparative Method
• Joe Felsenstein developed a way to evaluate
cross-species correlation among traits
– Start with a phylogeny
– Look at where sister species diverge
– Does the species that evolves larger group sizes
also evolve larger testes?
– Plot pairs of sister species connected
– Drag closest point to origin
– Erase origin points and examine independent
contrasts
The Comparative Method
• Hosken repeated bat analysis with
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Felsenstein’s Phylogenetically Independent
Contrasts method
Significant positive correlation
Complex Adaptations in
Current Research
• Will now examine how researchers use
the methods mentioned above to
investigate hypotheses about complex
topics
– Experiments
– Observational studies
– Comparative Method
Evolution of Adaptive Traits
• Every adaptive trait evolves from
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something else
How did the mammalian ear evolve?
Mammalian ear has three bones (ossicles)
– Malleus, incus, and stapes
• Other vertebrates do not have all three
• Ear bones transmit energy from tympanic
membrane to oval window in inner ear
Evolution of Adaptive Traits
• Why do we have three bones instead of
one?
• Increases sensitivity of hearing
• To figure out where the bones came
from we must:
– Establish the ancestral condition
– Understand the transformational sequence
• How and why they changed over time
Evolution of Adaptive Traits
• Acanthostega gunnari, one of the
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oldest tetrapods (360 My old)
One of the first animals to walk on land
and have to listen to airborne sounds
Descended from rhipidistian
crossopterygian fish
Fish have no ossicles but
Acanthostega had a stapes
Did the stapes help it hear?
Evolution of Adaptive Traits
• Acanthostega’s stapes fit into a hole in
the side of the braincase that connects
the inner ear and a notch near the
spiracle
• In later tetrapods, notch holds
tympanum
• Stapes of Acanthostega is homologous
with later groups
– Its function is probably homologous as
well
Evolution of Adaptive Traits
• Acanthostega’s stapes could not have appeared out
of nowhere
– Remember that the panda’s thumb was an exapted carpal
bone
• Stapes of Acanthostega is homologous to
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crossopterygian hyomandibula bone
Hyomandibula acts as a brace between the jaw and
braincase
Muscles attached to hyomandibula pump the jaws to
open and close the spiracle
– Muscles attached to stapes in Acanthostega probably had
same function
Evolution of Adaptive Traits
• Acanthostega was a transitional form
• Hyomandibula was an exaptation for
hearing
• Hyomandibula and stapes are also
developmentally homologous
– Both form from second gill arch
• What about malleus and incus?
– Only mammals have them
– First appeared in fossil mammals
Evolution of Adaptive Traits
• In position, malleus and incus are
homologous with two jaw bones in
reptiles, amphibians, and early
mammals
– Articular and quadrate
• Malleus, incus, articular, and quadrate
develop from first gill arch
– Are developmentally homologous
Evolution of Adaptive Traits
• Ancestor of mammals, the cynodonts, jaw
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joint is formed of quadrate and articular
Stapes is only bone of hearing
Examine fossils to see transition sequence
Later mammals, upper and lower jaws
articulate without quadrate and articular
These bones free to evolve new function
More recent mammals, quadrate and articular
articulate with stapes
– Function only in conduction of sound
Evolution of Adaptive Traits
• Natural selection caused adaptation for
better airborne hearing
• If ossicles are detached from jaw
hearing is better
• In mammal evolution the three bones
reduced in size, moved away from jaw,
and changed function