Chapter 9: Studying Adaptation: Evolutionary analysis of form and function

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Chapter 9 Studying Adaptation Evolutionary
analysis of form and function
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Giraffe neck length

• Giraffes famous for their long necks. Classical
  explanation is that long necks evolved to enable
  giraffes to reach higher browse.
• Long neck is an adaptation a trait or set of
  traits that increase the fitness of an organism.

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Giraffe neck length

• Is explanation for giraffes neck true?
• How do we demonstrate a trait is an adaptation?

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Giraffe neck length

• To demonstrate that a trait is an adaptation
  must
• determine what trait is for
• show that individuals with trait contribute more
  genes to next generation than those without it.

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Giraffe neck length

• Simmons and Scheepers (1996) questioned
  conventional explanation for giraffe neck length.
• Observations of giraffes feeding showed they
  spend most time in dry season feeding at heights
  well below maximum neck length.

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Giraffe neck length

• Simmons and Scheepers alternative explanation
  giraffes neck evolved as a weapon.
• Bulls use their necks as clubs in combat over
  mates.

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Giraffe neck length

• Males have necks 30-40cm longer and 1.7 times
  heavier than females of same age.
• Males skulls are armored and 3.5 times heavier
  than females.

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Giraffe neck length

• Males with heavier necks consistently win in
  interactions with other males.
• Females also more likely to mate with males with
  larger necks.

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Giraffe neck length

• Long and heavier-necked males intimidate other
  males and obtain more matings. Thus, trait
  increases reproductive success of possessor.
• But why do females have long necks?

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Giraffe neck length

• Cannot uncritically accept hypotheses about
  adaptive significance of traits. Must be tested
  rigorously.
• Also should bear in mind certain caveats about
  adaptation.

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Caveats about adaptation

• Not all differences among populations are
  adaptive. Giraffe populations have different coat
  patterns. May or may not be adaptive.

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Caveats about adaptation

• Not every trait is an adaptation. Giraffes can
  feed high in trees, but does not necessarily mean
  that this is why they have long necks.
• Not all adaptations are perfect. Long neck makes
  drinking very difficult.

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Why do tephritid flies wave their wings?

• Testing adaptive explanations with experiments.
• Tephritid fly Zonosemata vittigera has
  distinctive dark bands on its wings. When
  disturbed holds wings straight up and waves them
  up and down.

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Tephritid fly displays

• Display appears to mimic threat display of
  jumping spiders.
• Suggested (i) mimicking jumping spider may deter
  other predators (ii) mimicry may deter jumping
  spiders.

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Tephritid fly Jumping spider
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Tephritid fly displays

• Greene et al. (1987) set out to test ideas.
• Hypotheses
• 1. Flies do not mimic spiders. Display has other
  function.
• 2. Flies mimic spiders to deter non-spider
  predators.
• 3. Flies mimic spiders to deter spiders.

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Tephritid fly displays

• Experimental design tested hypotheses by using
  flies capable of giving all or only part of the
  display.
• Five groups of flies.

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Tephritid fly displays

• Predictions for how predators (both spider and
  non-spider) will respond to display clearly
  distinguished between competing hypotheses.

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Tephritid fly displays

• Experiment Flies from each treatment group
  presented in random order to starved predators in
  test arena.
• Recorded predators response for 5 minutes.

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Tephritid fly displays

• Results clear cut.
• Non-spider predators ignored display and captured
  flies of all 5 groups with equal probability.
• Spiders generally retreated from flies with
  barred wings that gave wing waving display.

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Tephritid fly displays

• Greene at al. (1987) experiment well designed.
• 1. There were effective controls. Cutting and
  gluing control (B) ensures that group C flies
  failure to deter attack not due to gluing.
• 2. All treatments handled alike. One arena used.

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Tephritid fly displays

• 3. Randomization of presentation of flies
  eliminated any effects of presenting flies in a
  set order.
• 4. Experiment replicated with multiple
  individual predators used.

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Advantages of replicated experiments

• Advantage of replicated experiments.
• Reduce effects of chance events.
• Allows researchers to estimate how precise their
  estimates are by measuring amount of variation in
  data.
• Can apply statistical analysis to results.

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Observational studies

• Not all hypotheses about adaptation can be easily
  tested experimentally.
• Behavioral thermoregulation Most animals are
  ectothermic and depend on external sources of
  heat. Try to maintain body temperature within
  narrow limits by behavioral means.

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Do garter snakes make adaptive choices in burrow
selection

• Huey et al. (1989) studied thermoregulation of
  garter snakes.
• Snakes prefer to maintain body temperature
  between 28 and 32 degrees C.
• Monitored snakes temperatures using implanted
  transmitters.

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Garter snake choices

• Snakes spent most of time beneath rocks or
  basking.

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Garter snake choices

• Size of rock important to thermoregulatory
  strategy.
• Snakes under thin rocks would get too cold at
  night and too hot during day.
• Thick rocks would offer protection, but generally
  are a bit too cool.

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Garter snake choices

• Medium rocks have variation in temperature and
  snake can move around and stay within optimal
  temperature range.

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Garter snake choices

• Huey et al. (1989) predicted snakes would
  preferentially choose medium rocks and avoid thin
  rocks.

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Garter snake choices

• All three rock sizes equally common. Snakes
  avoided thin rocks choosing medium or thick ones
  to spend the night beneath.
• Medium rocks used twice as often as thick rocks
  and about nine times as often as thin rocks.

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Trade-offs and constraints in selection

• Begonia involucrata is monoecious. There are
  separate male and female flowers on same plant.
• Pollinated by bees.
• Male flowers offer bee a reward in form of
  pollen. Female flowers offer no reward.

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Trade-offs and constraints in selection

• Bees make more and longer visits to male flowers.
• Female flowers closely resemble male flowers.
  Rate at which female flowers attract males
  determines fitness.
• Fitness depends on close resemblance to males.

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Trade-offs and constraints in selection

• Agren and Schemske (1991) examined two hypotheses
  about mode of selection in these begonias.
• 1. Bees visit female flowers that most resemble
  male flowers. Selection is stabilizing best
  phenotype for females is mean male phenotype.

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Trade-offs and constraints in selection

• 2. Females that look like most rewarding male
  flowers will be visited more often. If bees
  prefer larger male flowers then selection is
  directional with larger female flowers favored.

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Trade-offs and constraints in selection

• Used arrays of artificial flowers of 3 different
  sizes. Recorded frequency of bee visits.

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Trade-offs and constraints in selection

• Larger flowers attracted more bees. Selection is
  directional

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Trade-offs and constraints in selection

• Given that larger flowers attract more bees close
  resemblance in size of female to male flowers
  appears maladaptive. Why are they not larger?
• Trade-off between number and size of flowers in
  infloresences. The larger the flowers, the fewer
  there are.

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Trade-offs and constraints in selection

• There is a limited amount of energy that can be
  devoted to flower production. Plants can produce
  many small flowers or fewer large ones.

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Trade-offs and constraints in selection

• Infloresences with more flowers possibly favored
  for two reasons
• Bees prefer infloresences with more flowers.
• More flowers means greater potential seed
  production.

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Trade-offs and constraints in selection

• Female flower size thus shaped by directional
  selection for larger flowers and trade-off
  between number and size of flowers.

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Flower color change in fuchsia a constraint

• Fuchsia excortica bird pollinated tree.
• For first 5.5 days flowers are green then they
  turn red. Transition from green to red takes
  about 1.5 days.
• Red flowers remain on tree about 5 days.

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Fuchsia flower color change

• Flowers produce nectar only on days 1-7. Most
  pollen exported by then. Flower remains
  receptive to pollen but rarely receives any after
  day 7.
• Avian pollinators ignore red flowers.

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Fuchsia flower color change

• Why do these fuchsia flowers change color?
• Signalling that flower in unreceptive means that
  pollinators do not waste viable pollen on
  non-receptive stigmas. Instead deliver it to
  other flowers on the tree.

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Fuchsia flower color change

• Why doesnt tree just drop flowers. Why change
  their color?
• Constraint Growth of pollen tubes is slow.

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Fuchsia flower color change

• Pollen grain must grow a tube from tip of stigma
  to reach ovary and fertilize egg.
• Takes 3 days for pollen tube to reach ovary and
  1.5 days to develop abscission layer to cut
  flower off. Explains 5 day period for red
  flowers.

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Fuchsia flower color change

• Because flowers must be retained 5 days selection
  favored plants that altered flower color.
• These were able to make better use of pollinators.

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Does lack of genetic variation constrain
evolution?

• Genetic variation is raw material for evolution
  from which adaptations are developed.
• Can populations be constrained from evolving by a
  lack of genetic variation?

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Host plant shifts in beetles

• Host plant shifts in beetles.
• Futuyma et al. studied herbivorous leaf beetles
  (genus Ophraella) and their use of host plants.
• Each species feeds as larvae and adults on one or
  a few closely related sunflower-like plants.

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Host plant shifts in beetles

• Each plant species makes a unique combination of
  defensive chemicals to deter herbivores.
• Beetles have complex set of adaptations to live
  on host plant (ability to recognize plant,
  ability to detoxify chemicals, etc.)

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Host plant shifts in beetles

• Evolutionary history of beetle shows that several
  host plant shifts have occurred.
• Observed shifts are only a subset of potentially
  possible shifts.
• Futuyma et al. tried to explain why some shifts
  have occurred , but others have not.

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Host plant shifts in beetles

• Two main hypotheses
• 1. All host shifts genetically possible. If all
  shifts are genetically possible then ecological
  factors or chance may explain observed pattern.
• 2. Most host shifts genetically impossible.
  Most beetles lack genetic variation to enable
  them to use more than a few hosts.

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Host plant shifts in beetles

• Hypotheses not mutually exclusive. Futuyma et
  al. were looking to see if genetic constraints
  were at least partially responsible for observed
  pattern.

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Host plant shifts in beetles

• Tested 4 beetle species on six possible host
  plants.
• In most cases beetles showed no genetic variation
  for ability to recognize offered plant as food or
  to survive by eating it.
• Hypothesis 2 thus partially supported.

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Host plant shifts in beetles

• Also tested to see if beetles did best on host
  plants that were close relatives of own host
  plant and to see whether beetles did best on host
  plants that were the hosts of close beetle
  relatives.
• Beetles did so. This is further evidence
  consistent with hypothesis 2 that genetic
  variation has constrained host choice.

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Host plant shifts in beetles

• Skip section 9.7.
• 9.8 (short) worth reading.