Transcript Studying Adaptation

Adaptation
• What is an adaptation?
• It is a genetically based trait, or integrated
suite of traits, that increases the fitness of its
possessor.
What is adaptation.
• Adaptations come about in response to a
problem. That is, it is achieved through
the process of natural selection.
• It is through the adaptations brought about
by natural selection that organisms appear
to be well designed.
Identifying Adaptive Traits
• Not all traits are adaptive
– Learned traits
– Traits that were originally adaptive for one
problem, now used to fix another.
– Traits that have no current benefit toward
fitness i.e. molecular genetic drift
• Traits must be rigorously tested to
determine whether they are adaptive
All hypothesis must be tested
•
Hypothesis are tested by
1. using the hypothesis to make predictions
2. Next, the predictions are tested by either
-
experimentation,
observation,
Or comparative studies
3. Then the test are analyzed to determine if
the predictions are correct.
The Giraffe’s Neck
The Awkwardness of Giraffes
• Why are Giraffes so
tall?
• Is it an adaptation?
• What is it an
adaptation for?
Giraffe’s Neck Reconsidered
• Simmons and Scheepers
conducted studies to
observe whether the
giraffe’s long neck was used
to gain an advantage in
forging.
• They found that giraffes
most often feed on foliage
that is at shoulder height.
• In times of drought when
foliage is scarce they feed
on low brush
• Giraffe adaptation
• The giraffe neck was
actually adapted as a
means of defeating other
males in a battle over
females.
• Bull giraffes employ their
heads and necks as clubs
and occasionally even kill
each other.
If the giraffe neck was actually selected for as a
means of defeating other males in a battle over
females. The neck is now coopted for use in
feeding higher in the trees than other organisms
can.
• Is this adaptation?
– Adaptation: a trait or integrated suite of traits that
has evolved in response to selection for the
function that it currently performs and that
increases the fitness of its possessor. (fighting)
• The giraffe’s neck is an exaptation when it is used to
eat leaves off tall trees.
Exaptation
• The term exaptation refers to situations in
which traits perform a certain function now
but either arose for some other function or
originally had no function at all.
• A type of exaptation in which there was no
original function is called spandrel
Example
• In male giraffes long necks were originally
adapted for fighting then their current advantage
for feeding would be an exaptation (arose for a
different adaptive reason)
• but in females long necks would be a spandrel
since they originally arose with no adaptive
value for females but now may impart a feeding
advantage.
Points to Consider
• Should differences
among populations
or species always
be considered
adaptive?
• Exaptive?
– Spot patterns on
giraffes
Masai giraffe
Reticulated giraffe
How do We determine whether a trait is an
adaptation?
• Three major approaches to determining
adaptive significance of traits
– Experiments
– Observational studies
– Comparative studies
Experimental example
Zonosemata (snowberry) flies and jumping
spiders
• What is being investigated?
Zebra Jumping spiders stalk their prey.
Warn others of their species off with leg waving behavior
A prey of the jumping spider, the snowberry fly, exhibits a
curious behavior that resembles the leg-waving of the
jumping spider.
QUESTION: Why do the flies wave their striped wings?
Experimental example
Zonosemata flies
• What are 3 hypotheses that might explain this
behavior?
Experimental example
Zonosemata flies
•What was the experimental set-up
• What were some of the controls used in
the experiment and why was each
important?
Experimental example
Zonosemata flies
• What predictions were made?
Experimental example
Zonosemata flies
• What were the
experimental results?
RESULTS
Observational Studies
• When are these type of studies done?
• When experiments are impractical or
inappropriate, observations can yield sufficient
information to evaluate a hypothesis
– Experimental study: Why giraffes have long necks?
Observational Studies
• Observational studies must employ the following
criteria:
1. Hypothesis must lead to observed
predictions.
2. Observed occurrence of the trait must be
shown to be non random in the population.
3. The observed trait is adaptive
Example – Garter Snake study
Show they are choosing a particular
temperature more often than would
happen by random movements
• Watched snakes, where they spent their time
and what their body temps were
• Found that they maintain their body
temperatures between 28 and 32 degrees
Celsius.
• Discovered options for thermoregulation
sun/shade
under rocks ( thin,medium, thick)
moving up or down in burrows
Found that of the 3, all could be used to effectively
maintain desired daytime temps but only rocks
could provide enough warmth at night
• Studied thin, medium and thick rocks.
• Predicted only medium rocks work for
the right temps both night and day.
• Most snakes found under rocks.
Now have to show that being under
medium rocks is not random behavior
• Compared availability of thin, medium
and thick rocks in the habitat to the
frequency that each was used by garter
snakes
• All rocks are equally represented in the
habitat so if random events, the snakes
should be found equally under each
type of rock.
• Results ….
Comparative Studies
• Compares traits across different species
lines
• Proper application of comparative
methods requires knowledge of the
evolutionary relationships among the
species under study.
Example of comparative study
Bats
• Question
– Why do some bats have bigger testes than
others?
• Hypothesis:
– David Hosken hypothesized that large testes
are an adaptation for sperm competition
• Prediction
– If you compared different species of bats,
those that form larger social groups will have
larger testes because there is more
competition for passing on their genes?
Studies initial showed correlation between
social group size and testes size.
However a closer look indicates that the data
could be skewed by evolutionary relationships.
• Perhaps the larger testes groups are simply from one
common ancestor and the smaller from another.
– Example
If we replace the
individual points
for A, B and C and
for D,E and F with
a single point
representing their
most recent
common ancestor
we get….
But two data
points is not very
reliable for
making extended
conclusions.
Plot sister species
independently
Felsenstein method
A better question: when
species diverge form a
common ancestor does the
species that evolves larger
group sizes also evolve
larger testes?
Drag point
closest to the
vertical axis
to the origin
Erase lines
Felsenstein’s method
• Each data point
represents the
divergence that arose
between a pair of
sister species as they
evolved away from
their common
ancestor.
Bat results: show that when a bat species evolved larger
group sizes than its sister species, it also tended to evolve
larger testes for its body size.
Phenotypic Plasticity
Section 9.5
Phenotypic Variation
• Phenotypes are influenced by the environment
• Total variation is called the phenotypic variation
and it has two components: VG and VE.
– contributions come from both the environment and
the genetic makeup
• Phenotypes may respond in different degrees to
different environmental situations
Phenotypic Plasticity
• When an individuals phenotype is
influence by its environment it is said to be
plastic.
• When phenotypes are plastic, individuals
with identical genotypes may have
different phenotypes, provided they live in
different environments.
Can plasticity be adaptive?
1. Show that the plastic trait is positively
selected for
2. show that individuals that are plastic for
the trait are more fit than those who are
not plastic
Daphnia magna: The water flea
• Tiny crustacean that
lives in freshwater
lakes.
• Daphnia reproduce
asexually (clones).
• Thus researchers are
able to grow
genetically identical
fleas in different
environments
The Water flea Experiment
• Researchers studied 10 clones (genotypes) from
each of three different lakes.
• They were grown in 2 different environments
– One where fish predators had lived and one where
fish had not lived.
• checked phototactic (to light) response in the
two different environments
Water Flea Results
• The Daphnia from Blandaart lake, where fish are present,
were much more plastic in their ability to respond to light
than Daphnia from lakes that had no fish
• Showed that selection for phenotypic plasticity has been
selected for in a lake where fish are present.
Phenotypic Plasticity
• The Phenotypic plasticity is the result of
differences in phenotypic expression (for a
given genotype) based on the interaction
of specific individuals with the environment
in which each lives.
• Phenotypic plasticity may evolve.
• Phenotypic plasticity may or may not be
adaptive.
Tradeoffs and constraints
Factors that limit adaptive
evolution
Trade-offs
• Two evolutionary forces may work on the
same part from different directions and the
resources devoted to one body part or
function may be stolen resources from
another part or function
• Giraffe’s long neck may allow to fight off
competition but it sure makes getting a drink
inconvenient, difficult and maybe even
dangerous.
A trade-off example
The Begonia
• In Begonias there is a trade-off between the size of
female flowers and the size of the inflorescence.
Even though larger female flowers attract more
pollinators, the female flowers remain smaller than
optimal for pollination because bees also visit larger
inflorescences and larger inflorescences can not
contain individual flowers as large as the optimal
flower size alone would dictate
• trade-off between the number of female flowers and
individual flower size may be dictated by two things.
1) more flowers, more seeds and 2) perhaps more
bees will be attracted to larger inflorescences
Constraints
• Constraints are factors that tend to slow
the rate of adaptive evolution.
– They prevent a population from evolving a
particular trait to its optimal value.
Two types of constraints
• Developmental – based on how an
organism develops in embryo or how
an organisms structure is related to
function.
• Phylogenetic – (Historical constraint)
based on inheriting the needed genetic
variation from its ancestors
Constraints
• Fuchsia excorticata
– Is pollinated by birds
– Flowers turn from
green to red
– Researchers realized
the color changed to
red after polination.
Constraints
(developmental example)
• Why does the Fuchsia retain its flowers
and turn them red for 5 days AFTER
pollination?
– Pollinators are no longer visiting, the flowers
are of no more use but are still tapping needed
resources.
• Investigations showed that it was a cue for
pollinators.
– telling pollinators which flowers not to visit.
Constraints
(developmental example)
• The pollen tubes need to
grow through the area of
the abscission layer.
– When the petals drop an
Abscission zone develops
and blocks the pollen
tubes.
• If petals are dropped too
soon the pollen tube
never makes it to the
ovules.
Genetic variation
• Constraint may also occur due to a lack of
the genetic variation needed to allow the
adaptation that appears to make so much
sense yet never occurs.
Question?
• What adaptive compromises has taken
place with regards to sickle cell and
Malaria
Adaptations work with what is
available
Many structures are far from
optimally designed
Every Adaptive Trait Evolves from
something else
• Because of Phylogenetic constraints
every adaptive trait evolves from
something that was already present.
• The fact that everything evolves from
something else is just one reason why
an organism’s traits, even when clearly
adaptive, are often imperfect
Contrivances
• Contrivances - less than optimally
designed structures (if interested visit
http://www.talkorigins.org/faqs/jury-rigged.html
• Stephen Gould gives many examples
The Panda’s thumb
Eye Development see the link at
http://www.pbs.org/wgbh/evolution/library/0
1/1/quicktime/l_011_01.html
• In order to show that a particular structure has
developed from similar ancestral structures, one
must be able to …
1. Establish the ancestral condition
2. Understand the transformational sequence,
how and why the characters changed through
time
• Mammalian Ear development is well
documented in this way in the fossil record
The Human Ear
• External ear: Hearing; terminates at eardrum
• Middle ear: Hearing; contains auditory ossicles
• Inner ear: Hearing and balance; interconnecting fluidfilled tunnels and chambers
15-62
• Karl Reichert:
– The Parts of the
ears of mammals
are the same thing
as parts of the jaws
of reptiles. (1837)
Ear evolution steps and
rationale
Large rod that connects
the upper jaw to the braincase
Ancestral history
Crossopterygians (Fish) had none
of the ear bones
Ear evolution
Acanthostega (an airbreathing swamp dweller)
has a stapes which
connects a hole over the
inner ear with a notch in
the skull called the
spiracle.
The hyomandibula functioned as
an exaptation (preadaptation) for hearing.
Mammal-like Reptiles
• At the end of the Permian fossils of mammal like reptiles
were abundant.
• The most reptilian of the mammal-like reptiles had only a
single bone in its middle ear.
• The successively more mammalian mammal-like reptiles
the bones of the reptilian jaw got smaller and smaller.
Embryonic Development of the
Middle Ear
Gives rise to the
Malleus and Incus
Gives rise to the Stapes
• From a developmental stand point, the stapes
bone develops from the Hyoid arch, while the
incus and malleus develop from the Meckel’s
cartilage.
Inner Ear
• Labyrinth
– Bony
• Cochlea: Hearing
• Vestibule: Balance
• Semicircular canals:
Balance
– Membranous
• -Lymphs
– Endolymph
• In membranous
labyrinth
– Perilymph
• Space between
membranous and bony
labyrinth
15-70
Inner ear working right and not
Inner Ear Hair
Cells
http://www.aecom.yu.edu/aif/gallery/haircells/haircell.gif
http://www.dana.org/uploadedImages/Images/Content_Images/SenBodyFunc1_PR2007_cont.jpg
Inner Ear Hairs
• The hairs are covered
with gel
• When the gel moves,
the hairs bend.
• As they bend, they send
messages to the brain.
• The brain interprets
messages as location
and/or sound.
http://openlearn.open.ac.uk/file.php/3373/SD329_1_017i.jpg
Neuromasts
• Fish have similar structures in their skin.
• It works the same way.
• They sense pressure changes in the water.
http://www.bio.miami.edu/~cmallery/150/neuro/c7.49.12.lateral.line.jpg
Constraints
• Evolution did not develop new bones for
the middle ear. The bones were already
there; they were repurposed to a different
function.