Studying adaptation
The evolutionary analysis of form
and function in natural populations
Fundamental questions
• Is a trait an adaptation?
– Not all traits increase fitness (e.g.
hemoglobin makes blood red...),
although complexity is a key
– Chucks by male túngara frogs are
not favored by natural selection
from D.J. Futuyma, Evolutionary Biology
Fundamental questions
• What is the adaptive function (often
called adaptive significance) of a trait?
– Sometimes this is obvious, other times no
– The obvious can be wrong
– Finding the truth requires careful
evolutionary analysis
Example 1:Why are polar
bears white?
• Obvious answer: camouflage
when hunting seals
• Problems with this:
– Observations of hunting behavior
suggest camouflage is not often used
– Most hunts involve ambush at
breathing holes (Stirling 1974,
Anderson and Guravich 1992)
Alternative hypothesis: a solar
heat collector
• Photographed under UV, polar
bears are black (they absorb UV)
• Grojean et al (1980) examined
optical properties of polar bear fur
– hair is like a “drawn quartz tube with
irregular inner surface”
– reflects incident UV towards the skin
– light scattering and reflection make
the fur look white
Testing hypotheses for adaptations
giraffes have long necks to reach leaves
on tall trees, right?
Wrong.
An alternative:
neck length
evolved through
sexual selection
• Males have
necks that are
30-40 cm longer
and 1.7 times
heavier than
females
An alternative:
neck length
evolved through
sexual selection
• Longer-necked males
win contests with
shorter-necked males
and are preferred by
females (Pratt and
Andersen 1985)
Studying adaptation:
experimental approaches
Are wing markings and waving of
Zonosemata adaptations that mimic
jumping spider threat display?
Phidippus, the predator
Zonosemata, the prey species
The hypotheses
(1) Zonosemata is not a mimic. Wing markings
and display used for courtship?
(2) The flies mimic jumping spiders to deter
other predators.
(3) The flies mimic jumping spider threat
displays to deter jumping spiders
themselves.
Results show markings and wing-waving together
deter jumping spider predation
Mimicry does not deter
other predators
• Tests A, C and E on other spiders, mantises,
assassin bugs and whiptail lizards showed no
difference in attack times among treatments
A sheep in wolf’s clothing!
Observational studies:
behavioral
thermoregulation in
reptiles
• Like all ectotherms, desert iguana internal
temperature conforms to the environment
• They function optimally in a much narrower
range of temperatures (35 - 43C) than found
in their environment (15 - 47  C)
Behavioral thermoregulation appears to be
adaptive.
body temperature
“chosen” by
iguanas in lab
distribution of
body
temperature in
field-caught
iguanas
from Huey and Kingsolver (1989)
Adaptive thermoregulatory
behavior in garter snakes
• In the lab, snakes stay at 28 - 32 C
• Can they do this in the field, and if
so, how?
• Huey et al. (1989) implanted
transmitters to locate snakes and
remotely monitor body temperature
Western terrestrial form of
Thamnophis elegans
critical thermal tolerance limits
preferred temperature range
Snakes can
thermoregulate
within a narrow
temperature range in
the field.
Moving in a burrow is OK but
night temperatures are cold
Exposed snakes could move from
shade to sun to thermoregulate,
but are nighttime popsicles!
Snakes have options
during daytime, but
overnight the optimal
place is under a mediumsized rock
Active choice of rocks for
nightime cover appears to be
adaptive
Thin
Medium
Thick
(< 20 cm) (20 - 40 cm) (> 40 cm)
Rocks
32.4%
available
34.6%
33%
Rocks
chosen
61.5%
30.8%
7.7%
P < 0.05 by chi-square
The comparative method for
studying adaptation
In fruit bats and flying foxes
(Megachiroptera), is testis
size an adaptation to sperm
competition?
Australian grey-headed flying
fox Pteropus poliocephalus
Testis size as an adaptation
• In bats that roost in larger groups,
females mate with more males than in
bats with smaller roost group size
• Sperm competition should increase with
group size
• This may select for larger testes in bats
with larger group size
Testis size increases with roost group
size—appears to support the sperm
competition hypothesis.
data from Hosken (1998)
However, a problem with making
comparisons across species is
phylogenetic non-independence.
Species D,E,F may have larger
testes and larger groups because
their common ancestor did
To proceed, first we need a phylogeny
Then, we want to know: when species diverge from
a common ancestor, does the species that evolves a
larger group size also evolve larger testes?
The method of phylogenetically
independent contrasts (Felsenstein 1985)
Independent contrasts applied
to the bat data
from Hosken (1998)
Uncorrected
correlation
Independent contrasts show that
testes mass and group size
evolve together
Phenotypic plasticity
• Adaptations are nearly always a product of
genotype + environment
• Often, individuals with identical genotypes
are phenotypically plastic—they show
different phenotypes in different
environments
• Plasticity itself may be adaptive
Phenotypic plasticity: phototactic
behavior in Daphnia
• Daphnia is an excellent candidate because
females are parthenogenic
• Identical clones can be tested in different
environments
• De Meester (1996) placed 10 clones in a
cylinder and tested phototactic behavior
Phenotypic plasticity: phototactic
behavior in Daphnia
• A value of +1 means all ten swam
towards light, -1 if all swam away
• De Meester tested clones from each of 3
lakes
• He also tested whether they changed
their behavior in response to fish “scent”
(1) Phototactic behavior varies genetically—
clones vary in their response
(2) Behavior is plastic, particularly in Lake
Blankaart
(3) Plasticity itself can evolve: clones differ in
the extent to which fish induction modifies
their behavior
(4) Plasticity is adaptive: pronounced in Lake
Blankaart (many planktivorous fishes) and
absent in Citadelpark (fishless)
A straightforward hypothesis: fish
predation selects for plastic,
phototactic behavior
• To test: De Meester et al. (2001)
took sediment cores from 3 depths:
representing before, during and
after years of intense fish stocking
in a manmade pond
• They hatched resting eggs and
scored phototactic behavior of
adults
Daphnia life history
from the Daphnia Genomics Consortium
Clones hatching from eggs produced
during the years of heaviest fish
stocking are the most plastic!!
An evolutionary trade-off:
flower size in Begonia
• Begonia is monoecious: hermaphrodites
with male and female flowers
Male flowers (L):pollen reward;
Female flowers (R): no nectar
Inflorescence of many
flowers of both genders
Female flowers “mimic” male flowers
in color, shape and size: how and
why?
Schemske and Ågren (1995)
• H1: selection favors
intermediate-sized female
flowers resembling the
average male flower
• H2: selection favors larger
female flowers resembling
the largest (most attractive)
male flowers
Their experiment...
Equal numbers of small, medium
and large artificial flowers
arrayed in a natural forest
population
And results...
Approaches
Visits (pollinator lands)
H2 is correct: there is directional
selection for larger male flowers
So why are female flowers
intermediate in size?
An evolutionary tradeoff: the larger the flower,
the smaller the number
of flowers that can be
maintained in a cluster
(inflorescence)
A trade-off hypothesis:
opposing forces of
selection for flower size
and inflorescence size
Evolutionary constraints on
adaptation: flower color change in
Fuchsia
• F. excorticata: a flowering birdpollinated tree, endemic to
New Zealand
• Has showy flowers that are
green when exporting and
receiving pollen, then turn and
stay red for ~ 5 days
exporting pollen
pollination occurs before
day 8, but red flowers
receiving pollen remain for ~5 days more
from Delph and Lively (1989)
Why do flowers turn red?
• Color change is a cue to pollinators
to avoid flowers with no nectar
reward
• Pollinator “efficiency” benefits
flowers
– inviable pollen is not transferred
– nonreceptive stigmas do not receive
viable pollen
But why does Fuchsia retain
red flowers so long?
• Dropping flowers would be
a better signal to
pollinators
• It would be energetically
cheaper
An adaptive
hypothesis
• H1: red flowers attract birds and receptive
green flowers on the same plant receive
more pollen than without color change
– Delph and Lively removed red flowers from
some trees and not from others
– Trees with and without red flowers: no
difference in pollen deposited on green flowers
A “constraint”
hypothesis
• H2: Fuchsia is physiologically
constrained to retain red flowers long
after pollination is complete
– Delph and Lively removed hand-pollinated
flowers
– They dissected 10 flowers at a time to see
whether pollen tubes had reached the ovary
Their results
Days since
pollination
1
Per cent of flowers 0
with pollen tubes in
ovary
2
3
20%
100% 100%
4
Fuschia is constrained to retain its flowers. It was selected
to change flower color to provide a reliable pollinator cue.
exporting pollen
receiving pollen