Transcript Hardy Weinberg Equiibrium with more than 2 alleles

Chapter 9: Studying
Adaptation: Evolutionary
analysis of form and function
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.
Giraffe neck length
 Is
explanation for giraffes neck true?
 How
do we demonstrate a trait is an
adaptation?
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.
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.
Giraffe neck length
 Simmons
and Scheepers alternative
explanation: giraffes neck evolved as a
weapon.
 Bulls
use their necks as clubs in combat
over mates.
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.
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.
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?
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.
Caveats about adaptation
 Not
all differences among populations
are adaptive. Giraffe populations have
different coat patterns. May or may not be
adaptive.
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.
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.
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.
Tephritid fly
Jumping spider
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.
Tephritid fly displays
 Experimental
design tested hypotheses by
using flies capable of giving all or only part
of the display.
 Five groups of flies.
Tephritid fly displays
 Predictions
for how predators (both spider
and non-spider) will respond to display
clearly distinguished between competing
hypotheses.
Tephritid fly displays
 Experiment:
Flies from each treatment
group presented in random order to
starved predators in test arena.
 Recorded
minutes.
predators response for 5
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.
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.
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.
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.
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.
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.
Garter snake choices
 Snakes
spent most of time beneath rocks
or basking.
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.
Garter snake choices
 Medium
rocks have variation in
temperature and snake can move around
and stay within optimal temperature range.
Garter snake choices
 Huey
et al. (1989) predicted snakes would
preferentially choose medium rocks and
avoid thin rocks.
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.
Trade-offs and constraints in
selection
 Begonia
involucrata is monoecious. There
are separate male and female flowers on
same plant.
 Pollinated
 Male
by bees.
flowers offer bee a reward in form of
pollen. Female flowers offer no reward.
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.
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.
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.
Trade-offs and constraints in
selection
 Used
arrays of artificial flowers of 3
different sizes. Recorded frequency of
bee visits.
Trade-offs and constraints in
selection
 Larger
flowers attracted more bees.
Selection is directional
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.
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.
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.
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.
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.
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.
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.
Fuchsia flower color change
 Why
doesn’t tree just drop flowers. Why
change their color?
 Constraint:
Growth of pollen tubes is slow.
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.
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.
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?
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 sunflowerlike plants.
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.)
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.
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.
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.
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.
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.
Host plant shifts in beetles
 Skip
section 9.7.
 9.8 (short) worth reading.