Title: Chapter 9: Studying Adaptation: Evolutionary analysis of form and function
1Chapter 9 Studying Adaptation Evolutionary
analysis of form and function
2Giraffe 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.
3Giraffe neck length
- Is explanation for giraffes neck true?
- How do we demonstrate a trait is an adaptation?
4Giraffe 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.
5Giraffe 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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7Giraffe neck length
- Simmons and Scheepers alternative explanation
giraffes neck evolved as a weapon. - Bulls use their necks as clubs in combat over
mates.
8Giraffe 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.
9Giraffe neck length
- Males with heavier necks consistently win in
interactions with other males. - Females also more likely to mate with males with
larger necks.
10Giraffe 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?
11Giraffe neck length
- Cannot uncritically accept hypotheses about
adaptive significance of traits. Must be tested
rigorously. - Also should bear in mind certain caveats about
adaptation.
12Caveats about adaptation
- Not all differences among populations are
adaptive. Giraffe populations have different coat
patterns. May or may not be adaptive.
13Caveats 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.
14Why 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.
15Tephritid 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.
16Tephritid fly Jumping spider
17Tephritid 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.
18Tephritid 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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20Tephritid fly displays
- Predictions for how predators (both spider and
non-spider) will respond to display clearly
distinguished between competing hypotheses.
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22Tephritid fly displays
- Experiment Flies from each treatment group
presented in random order to starved predators in
test arena. - Recorded predators response for 5 minutes.
23Tephritid 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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25Tephritid 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.
26Tephritid 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.
27Advantages 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.
28Observational 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.
29Do 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.
30Garter snake choices
- Snakes spent most of time beneath rocks or
basking.
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32Garter 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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35Garter snake choices
- Medium rocks have variation in temperature and
snake can move around and stay within optimal
temperature range.
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37Garter snake choices
- Huey et al. (1989) predicted snakes would
preferentially choose medium rocks and avoid thin
rocks.
38Garter 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.
39Trade-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.
40Trade-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.
41Trade-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.
42Trade-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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44Trade-offs and constraints in selection
- Used arrays of artificial flowers of 3 different
sizes. Recorded frequency of bee visits.
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46Trade-offs and constraints in selection
- Larger flowers attracted more bees. Selection is
directional
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48Trade-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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50Trade-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.
51Trade-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.
52Trade-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.
53Flower 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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56Fuchsia 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.
57Fuchsia 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.
58Fuchsia flower color change
- Why doesnt tree just drop flowers. Why change
their color? - Constraint Growth of pollen tubes is slow.
59Fuchsia 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.
60Fuchsia 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.
61Does 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?
62Host 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.
63Host 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.)
64Host 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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66Host 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.
67Host 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.
68Host 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.
69Host 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.
70Host plant shifts in beetles
- Skip section 9.7.
- 9.8 (short) worth reading.