chapter 9: studying adaptation: evolutionary analysis of form and function

Chapter 9: Studying Chapter 9: Studying Adaptation: Evolutionary Adaptation: Evolutionary

analysis of form and functionanalysis of form and function

Giraffe neck lengthGiraffe neck length

Giraffes famous for their long necks. Giraffes famous for their long necks. Classical explanation is that long necks Classical explanation is that long necks evolved to enable giraffes to reach higher evolved to enable giraffes to reach higher browse.browse.

Long neck is an adaptation: a trait or set of Long neck is an adaptation: a trait or set of traits that increase the fitness of an traits that increase the fitness of an organism.organism.


Is explanation for giraffes neck true?Is explanation for giraffes neck true?

How do we demonstrate a trait is an How do we demonstrate a trait is an adaptation?adaptation?


To demonstrate that a trait is an To demonstrate that a trait is an adaptation must:adaptation must:

determine what trait is fordetermine what trait is for show that individuals with trait contribute show that individuals with trait contribute

more genes to next generation than those more genes to next generation than those without it.without it.


Simmons and Scheepers (1996) Simmons and Scheepers (1996) questioned conventional explanation for questioned conventional explanation for giraffe neck length.giraffe neck length.

Observations of giraffes feeding showed Observations of giraffes feeding showed they spend most time in dry season they spend most time in dry season feeding at heights well below maximum feeding at heights well below maximum neck length.neck length.


Simmons and Scheepers alternative Simmons and Scheepers alternative explanation: giraffes neck evolved as a explanation: giraffes neck evolved as a weapon.weapon.

Bulls use their necks as clubs in combat Bulls use their necks as clubs in combat over mates. over mates.


Males have necks 30-40cm longer and 1.7 Males have necks 30-40cm longer and 1.7 times heavier than females of same age.times heavier than females of same age.

Males skulls are armored and 3.5 times Males skulls are armored and 3.5 times heavier than females.heavier than females.


Males with heavier necks consistently win Males with heavier necks consistently win in interactions with other males.in interactions with other males.

Females also more likely to mate with Females also more likely to mate with males with larger necks.males with larger necks.


Long and heavier-necked males intimidate Long and heavier-necked males intimidate other males and obtain more matings. other males and obtain more matings. Thus, trait increases reproductive success Thus, trait increases reproductive success of possessor.of possessor.

But why do females have long necks?But why do females have long necks?


Cannot uncritically accept hypotheses Cannot uncritically accept hypotheses about adaptive significance of traits. Must about adaptive significance of traits. Must be tested rigorously.be tested rigorously.

Also should bear in mind certain caveats Also should bear in mind certain caveats about adaptation.about adaptation.

Caveats about adaptationCaveats about adaptation

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

Caveats about adaptationCaveats about adaptation

Not every trait is an adaptation.Not every trait is an adaptation. Giraffes Giraffes can feed high in trees, but does not can feed high in trees, but does not necessarily mean that this is why they necessarily mean that this is why they have long necks. have long necks.

Not all adaptations are perfect.Not all adaptations are perfect. Long Long neck makes drinking very difficult.neck makes drinking very difficult.

Why do tephritid flies wave their Why do tephritid flies wave their wings?wings?

Testing adaptive explanations with Testing adaptive explanations with experiments.experiments.

Tephritid fly Zonosemata vittigera has Tephritid fly Zonosemata vittigera has distinctive dark bands on its wings. When distinctive dark bands on its wings. When disturbed holds wings straight up and disturbed holds wings straight up and waves them up and down.waves them up and down.

Tephritid fly displaysTephritid fly displays

Display appears to mimic threat display of Display appears to mimic threat display of jumping spiders.jumping spiders.

Suggested (i) mimicking jumping spider Suggested (i) mimicking jumping spider may deter other predators (ii) mimicry may may deter other predators (ii) mimicry may deter jumping spiders.deter jumping spiders.

Tephritid fly Jumping spider


Greene et al. (1987) set out to test ideas.Greene et al. (1987) set out to test ideas. Hypotheses:Hypotheses: 1. Flies do not mimic spiders. Display has 1. Flies do not mimic spiders. Display has

other function.other function. 2. Flies mimic spiders to deter non-spider 2. Flies mimic spiders to deter non-spider

predators.predators. 3. Flies mimic spiders to deter spiders.3. Flies mimic spiders to deter spiders.


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

Five groups of flies. Five groups of flies.


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


Experiment: Flies from each treatment Experiment: Flies from each treatment group presented in random order to group presented in random order to starved predators in test arena.starved predators in test arena.

Recorded predators response for 5 Recorded predators response for 5 minutes.minutes.


Results clear cut.Results clear cut. Non-spider predators ignored display and Non-spider predators ignored display and

captured flies of all 5 groups with equal captured flies of all 5 groups with equal probability.probability.

Spiders generally retreated from flies with Spiders generally retreated from flies with barred wings that gave wing waving barred wings that gave wing waving display.display.


Greene at al. (1987) experiment well Greene at al. (1987) experiment well designed.designed.

1. There were effective controls. Cutting 1. There were effective controls. Cutting and gluing control (B) ensures that group and gluing control (B) ensures that group C flies failure to deter attack not due to C flies failure to deter attack not due to gluing.gluing.

2. All treatments handled alike. One 2. All treatments handled alike. One arena used.arena used.


3. Randomization of presentation of flies 3. Randomization of presentation of flies eliminated any effects of presenting flies in eliminated any effects of presenting flies in a set order.a set order.

4. Experiment replicated with multiple 4. Experiment replicated with multiple individual predators used.individual predators used.

Advantages of replicated Advantages of replicated experimentsexperiments

Advantage of replicated experiments.Advantage of replicated experiments. Reduce effects of chance events.Reduce effects of chance events. Allows researchers to estimate how Allows researchers to estimate how

precise their estimates are by measuring precise their estimates are by measuring amount of variation in data.amount of variation in data.

Can apply statistical analysis to results.Can apply statistical analysis to results.

Observational studiesObservational studies

Not all hypotheses about adaptation can Not all hypotheses about adaptation can be easily tested experimentally.be easily tested experimentally.

Behavioral thermoregulation: Most animals Behavioral thermoregulation: Most animals are ectothermic and depend on external are ectothermic and depend on external sources of heat. Try to maintain body sources of heat. Try to maintain body temperature within narrow limits by temperature within narrow limits by behavioral means.behavioral means.

Do garter snakes make Do garter snakes make adaptive choices in burrow adaptive choices in burrow

selectionselection Huey et al. (1989) studied Huey et al. (1989) studied

thermoregulation of garter snakes.thermoregulation of garter snakes. Snakes prefer to maintain body Snakes prefer to maintain body

temperature between 28 and 32 degrees temperature between 28 and 32 degrees C.C.

Monitored snakes’ temperatures using Monitored snakes’ temperatures using implanted transmitters.implanted transmitters.

Garter snake choicesGarter snake choices

Snakes spent most of time beneath rocks Snakes spent most of time beneath rocks or basking. or basking.


Size of rock important to thermoregulatory Size of rock important to thermoregulatory strategy. strategy.

Snakes under thin rocks would get too Snakes under thin rocks would get too cold at night and too hot during day.cold at night and too hot during day.

Thick rocks would offer protection, but Thick rocks would offer protection, but generally are a bit too cool.generally are a bit too cool.


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


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


All three rock sizes equally common. All three rock sizes equally common. Snakes avoided thin rocks choosing Snakes avoided thin rocks choosing medium or thick ones to spend the night medium or thick ones to spend the night beneath. beneath.

Medium rocks used twice as often as thick Medium rocks used twice as often as thick rocks and about nine times as often as rocks and about nine times as often as thin rocks.thin rocks.

Trade-offs and constraints in Trade-offs and constraints in selectionselection

BegoniaBegonia involucratainvolucrata is monoecious. There is monoecious. There are separate male and female flowers on are separate male and female flowers on same plant.same plant.

Pollinated by bees.Pollinated by bees.

Male flowers offer bee a reward in form of Male flowers offer bee a reward in form of pollen. Female flowers offer no reward. pollen. Female flowers offer no reward.


Bees make more and longer visits to male Bees make more and longer visits to male flowers.flowers.

Female flowers closely resemble male Female flowers closely resemble male flowers. Rate at which female flowers flowers. Rate at which female flowers attract males determines fitness. attract males determines fitness.

Fitness depends on close resemblance to Fitness depends on close resemblance to males.males.


Agren and Schemske (1991) examined Agren and Schemske (1991) examined two hypotheses about mode of selection in two hypotheses about mode of selection in these begonias.these begonias.

1. Bees visit female flowers that most 1. Bees visit female flowers that most resemble male flowers. Selection is resemble male flowers. Selection is stabilizing: best phenotype for females is stabilizing: best phenotype for females is mean male phenotype.mean male phenotype.


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


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


Larger flowers attracted more bees. Larger flowers attracted more bees. Selection is directional Selection is directional


Given that larger flowers attract more bees Given that larger flowers attract more bees close resemblance in size of female to close resemblance in size of female to male flowers appears maladaptive. Why male flowers appears maladaptive. Why are they not larger?are they not larger?

Trade-off between number and size of Trade-off between number and size of flowers in infloresences. The larger the flowers in infloresences. The larger the flowers, the fewer there are.flowers, the fewer there are.


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


Infloresences with more flowers possibly Infloresences with more flowers possibly favored for two reasons: favored for two reasons:

Bees prefer infloresences with more Bees prefer infloresences with more flowers.flowers.

More flowers means greater potential seed More flowers means greater potential seed production.production.


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

Flower color change in fuchsia: Flower color change in fuchsia: a constrainta constraint

FuchsiaFuchsia excorticaexcortica bird pollinated tree. bird pollinated tree.

For first 5.5 days flowers are green then For first 5.5 days flowers are green then they turn red. Transition from green to red they turn red. Transition from green to red takes about 1.5 days.takes about 1.5 days.

Red flowers remain on tree about 5 days.Red flowers remain on tree about 5 days.

Fuchsia flower color changeFuchsia flower color change

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

Avian pollinators ignore red flowers.Avian pollinators ignore red flowers.


Why do these fuchsia flowers change Why do these fuchsia flowers change color?color?

Signalling that flower in unreceptive Signalling that flower in unreceptive means that pollinators do not waste viable means that pollinators do not waste viable pollen on non-receptive stigmas. Instead pollen on non-receptive stigmas. Instead deliver it to other flowers on the tree.deliver it to other flowers on the tree.


Why doesn’t tree just drop flowers. Why Why doesn’t tree just drop flowers. Why change their color?change their color?

Constraint: Growth of pollen tubes is slow.Constraint: Growth of pollen tubes is slow.


Pollen grain must grow a tube from tip of Pollen grain must grow a tube from tip of stigma to reach ovary and fertilize egg. stigma to reach ovary and fertilize egg.

Takes 3 days for pollen tube to reach Takes 3 days for pollen tube to reach ovary and 1.5 days to develop abscission ovary and 1.5 days to develop abscission layer to cut flower off. Explains 5 day layer to cut flower off. Explains 5 day period for red flowers.period for red flowers.


Because flowers must be retained 5 days Because flowers must be retained 5 days selection favored plants that altered flower selection favored plants that altered flower color. color.

These were able to make better use of These were able to make better use of pollinators.pollinators.

Does lack of genetic variation Does lack of genetic variation constrain evolution?constrain evolution?

Genetic variation is raw material for Genetic variation is raw material for evolution from which adaptations are evolution from which adaptations are developed.developed.

Can populations be constrained from Can populations be constrained from evolving by a lack of genetic variation?evolving by a lack of genetic variation?

Host plant shifts in beetlesHost plant shifts in beetles

Host plant shifts in beetles.Host plant shifts in beetles. Futuyma et al. studied herbivorous leaf Futuyma et al. studied herbivorous leaf

beetles (genus beetles (genus OphraellaOphraella) and their use of ) and their use of host plants.host plants.

Each species feeds as larvae and adults Each species feeds as larvae and adults on one or a few closely related sunflower-on one or a few closely related sunflower-like plants.like plants.


Each plant species makes a unique Each plant species makes a unique combination of defensive chemicals to combination of defensive chemicals to deter herbivores.deter herbivores.

Beetles have complex set of adaptations Beetles have complex set of adaptations to live on host plant (ability to recognize to live on host plant (ability to recognize plant, ability to detoxify chemicals, etc.) plant, ability to detoxify chemicals, etc.)


Evolutionary history of beetle shows that Evolutionary history of beetle shows that several host plant shifts have occurred.several host plant shifts have occurred.

Observed shifts are only a subset of Observed shifts are only a subset of potentially possible shifts. potentially possible shifts.

Futuyma et al. tried to explain why some Futuyma et al. tried to explain why some shifts have occurred , but others have not.shifts have occurred , but others have not.


Two main hypotheses:Two main hypotheses: 1. All host shifts genetically possible. If all 1. All host shifts genetically possible. If all

shifts are genetically possible then shifts are genetically possible then ecological factors or chance may explain ecological factors or chance may explain observed pattern.observed pattern.

2. Most host shifts genetically impossible. 2. Most host shifts genetically impossible. Most beetles lack genetic variation to Most beetles lack genetic variation to enable them to use more than a few hosts.enable them to use more than a few hosts.


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


Tested 4 beetle species on six possible Tested 4 beetle species on six possible host plants.host plants.

In most cases beetles showed no genetic In most cases beetles showed no genetic variation for ability to recognize offered variation for ability to recognize offered plant as food or to survive by eating it.plant as food or to survive by eating it.

Hypothesis 2 thus partially supported.Hypothesis 2 thus partially supported.


Also tested to see if beetles did best on Also tested to see if beetles did best on host plants that were close relatives of host plants that were close relatives of own host plant and to see whether beetles own host plant and to see whether beetles did best on host plants that were the hosts did best on host plants that were the hosts of close beetle relatives.of close beetle relatives.

Beetles did so. This is further evidence Beetles did so. This is further evidence consistent with hypothesis 2 that genetic consistent with hypothesis 2 that genetic variation has constrained host choice.variation has constrained host choice.


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

chapter 9: studying adaptation: evolutionary analysis of form and function

Documents

giraffe neck length

function slide

giraffes neck

giraffe neck length

maximum neck length

long necks

giraffe populations

tephritid flies