Lesson 3: Evolution and Behavior Flashcards
adaptation
An evolutionary process that results in a
population of individuals with traits best
suited to the current environment
fittness
- Survivorship and reproduction
- Works via Natural Selection and related
processes
Population analyses can reveal selection
- Often, we determine if natural selection is at work on a trait by looking at what’s going on with the trait at a population level
- If selection is occurring on the trait, then we
should be able to see shifts in a population
stabilizing selection
The highest fitness in
a population is
shown by individuals
with an intermediate
trait value in the
middle of the
spectrum
- This leads to a narrowing
of the variety of the
distribution of traits within
the population - and the trait becomes
stable in the population
over generations
stabilizing selection in cichlid fish
Compared growth rate of fish in
different-sized patches of food
*Used divided trays to create food
patches: dots represent food
*Put one patch in an aquarium with
one “focal fish” – the individual whose
growth will be measured
*Added 4 competitors to each tank so
the focal fish would have to try to
defend its territory (the food patch)
Stabilizing selection in cichlid fish (2)
Measured amount of
defensive behavior with
different-sized food
patches
Result was that defending
larger territories required
more work: more chases
per minute
Stabilizing selection in cichlid fish (3)
Measured growth rate of fish
defending different-sized
food patches
Result was that medium-
sized territories had the best
balance of cost and benefit.
SO – stabilizing selective pressure for intermediate territories
Directional Selection
The highest fitness in
a population is
shown by individuals
with an extreme trait
value at one end of
the spectrum
- This causes the average
distribution of the trait
within the population to
shift in the direction of
the highest fitness
Directional selection in Hawaiian crickets
Directional selection in Hawaiian crickets
Why? A parasitic fly from North America had invaded. This fly finds victims by listening for the cricket calls (so victims are male) and then lays its eggs on the back of the cricket.
The maggots then attack and eat the cricket
Drectional selection in Hawaiian crickets RESULT
Result? A serious downside to
being a calling male cricket
There is now Natural selection
against calling but females still
prefer calling (sexual selection)
But the crickets were not going extinct.
Instead there was rapid evolution -
directional selection – for the crickets
to be very very quiet.
what had occurred within the male population
A mutation caused males to lose the
sound-producing parts of their wings.
These males did not get attacked and
so survived better than normal males
Phenotypic Plasticity:
generally refers to seeing a change in
phenotype when the environment changes – without a change in genotype. (Plasticity refers to (changeability)
sexual selection for the females on Kauai
Females on Kauai have
evolved to be somewhat less
choosy than females from
normal populations - they no
longer require males to chirp.
This is sexual selection on
the females.
Convergent Evolution for the crickets
Silent males are also spreading in
the population on Oahu because of
the parasitic fly
This is due to a different mutation in
the wing that produces the same
result.
Disruptive Selection
The highest fitness in
a population is shown
by individuals with an
extreme trait value at
both ends of the
spectrum
This leads to a split in
the population – with
some having one
extreme trait and
some having the other
Disruptive selection in spadefoot toad
tadpoles
Spadefoot toads come in two
main varieties
DETRITIVORES consume detritus
and small invertebrates
CARNIVORES consume
invertebrates
A third group: INTERMEDIATES
are rarer but are generalists
Is this due to disruptive selection
against the intermediates? (spadefoot toad tadpoles)
If so, would predict that
carnivores and detritovores have
higher fitness than intermediates
Might see this in terms of feeding
– perhaps intermediates less fit
because they cannot feed as
efficiently
Disruptive selection in spadefoot toad
tadpoles—-Growth of the different
morphs when given
detritus as food source:
Detritovores did better
than intermediates which
did better than carnivores
Disruptive selection in spadefoot toad
tadpoles —- Ability to catch shrimp as a
function of morphology:
Carnivores did better than
intermediates which did better
than Detritovores
Did a “mark-recapture” study to look at
growth & survivorship
Conclusion:
Individuals with specialized traits on
either end of the spectrum have higher
fitness than individuals with
intermediate traits
Evolutionarily Stable Strategies I
–We can think of a ‘strategy’ as a mechanism
that an animal has for achieving some goal –
like finding food, or winning mates
–Some strategies will probably work better
than others, so we expect those to be
favored by natural selection
Evolutionarily Stable Strategies II
- Strategies that are selected against should
disappear from the population - Strategies that are selected for should remain in the population from generation to generation – thus, in evolutionary terms, they are stable
Evolutionarily Stable Strategies III
In the event that one single strategy is favored
above all others (It’s OPTIMAL), then the
population should display just one stable strategy (for a particular task) – this is called a pure ESS
- If multiple strategies are equally favored, then they
should all be stable – this is called a mixed ESS
and is a source of variation in the population
Digger Wasps - a mixed ESS
Digger wasps use two strategies to
obtain burrows
- “Diggers” dig their own burrows
- “Borrowers” use abandoned burrows
so don’t have to dig their own
- Which strategy a wasp uses is
genetic. Diggers always dig and
borrowers always borrow
How can two strategies be maintained in the
population? —- Frequency-dependent selection
- Advantage to being a borrower is zero energy
spent digging - But this advantage goes away if diggers are
rare because then abandoned burrows are
rare. - If diggers are rare, they are selected for. If
borrowers are rare, then they are selected for
-Populations can reach equilibrium
why do sexes very? sexual selection
Sexual Selection occurs on traits that exist to enhance
1.) the chance of getting a mate
or
2.) of choosing a good one
sexual dimorphism
Morphological and behavioral differences between the sexes that arise because males and females have different selection
pressures when it comes to finding and choosing a mate
Sexual selection in house finches: Key info
Males are red,
but females are grayish-
brown – probably so they
can hide more easily from
predators
Key Info: Coloration of male
house finches ranges from red to
orange to yellowish mostly due to
health & diet
Sexual selection in house finches: hypothesis
Red is a sexually selected trait in
males because females
prefer red males
Testable Prediction
TESTABLE PREDICTION: If
females actually prefer to mate
with red males, then red ones
should get mates more easily
than yellowish ones
Experiment 1: House Finches
- Quantified plumage color intensity of
four males - Allowed females to choose among
males (measured how much time out of
two hours the females spent with each
of the four males) - Result: females mostly preferred the
reddest male (p<.05). No difference
between the middle males
Experiment 2: House Finches
Male feathers were dyed to be redder or
less red (plumage index in parentheses is
the original score)
- Allowed females to choose among males
- Result:
- Females preferred the reddest males
Conclusion:
- Red plumage is a sexually selected trait in
house finches
In kin selection, individuals can
increase their fitness by helping kin T/F
True
individual selection
Natural selection at the level of individuals
group selection
-Selection that favors particular groups of individuals over other such groups of the same species
- Idea that an individual might behave in a way that is good for the species even if the individual suffers
- Behavior of Lemmings often portrayed as example of group
selection
Do lemmings commit suicide?
- Lemmings portrayed as
“committing suicide” by diving
off cliffs into water during times
when food is scarce - The idea is that they sacrifice
themselves for the greater
good of saving everyone else.
This is “group selection”
Do lemmings commit suicide pt.2
- Because natural selection
works on individual genes,
group selection can only work if
the sacrifice increases the level
of the sacrifice gene in the
remaining population
Difficulties with Group Selection
- The problem is that cheaters would
win. - Cheaters would invade and
because they survived better, the
‘suicidal’ genes would be lost from
the population - In actuality, when population
numbers are high, lemmings
disperse to reduce intraspecific
competition (natural selection)
