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Natural Selection and evolution
Evolution is: a change in gene frequencies over time. We’ll get back to this.
Natural selection is a mechanism that can lead to evolution.
Natural selection:
Jointly described by Charles Darwin and Alfred Russell Wallace
"On the Tendency of Varieties to Depart Indefinitely from the Original Type." --Wallace
On the origin of the origin of the species by means of natural selection-- Darwin
Darwin’s four postulates (note slightly different from Alcock):
1) Individuals within species are variable
2) Some of these variations are passed on to offspring
3) In every generation more offspring are produced than can survive
4) The survival and reproduction of individuals are not random: some variations cause
individuals to leave more offspring than others, and to the extent that the variation is
heritable, it is selected for.
Another way to say it: Heritable variation leads to differential reproduction.
Nothing magic about natural selection– some animals (and plants and bacteria…)
reproduce more than others.
Their offspring are somewhat like them. If they also reproduce more than others, we
will eventually see more and more of the population taking on the traits that lead to
increased reproduction.
BASIS FOR MOST OF OUR ANALSES OF BEHAVIOR: GETTING GENES INTO THE
NEXT GENERATION.
Shorthand: reproductive success, fitness.
Are Darwin’s four postulates met?
An example: Blackcap warblers (European bird species)
Variation:
--Birds take different routes to
wintering grounds (in red)
--Recently some birds noticed
overwintering in England.
(how? bands; Why? feeders,
global warming)
Were British birds Scandinavian
stoppers, or pioneers of a new
route from Germany?
Hypothesis 1: From Scandinavia
Hypothesis 2: From Germany
If variation is heritable, what
predictions would each hypothesis
make about the migration direction
of OFFSPRING of birds that
overwinter in Britain?
How could we test these
hypotheses?
Emlen Funnel:
Heritability– the experiment:
Take adults that overwinter in Britain back to the lab. Let them breed, and raise their
chicks. Test the adults, the offspring from the British adults, and birds from SW
Germany in the Emlen funnels.
Where do adults captured
from Britain go?
Where do their kids go?
Where do birds from SW
Germany go?
Which hypothesis does this
support?
Does variation appear to be
heritable?
Heritability, part two:
Experiment: Breed birds from Austria/Hungary region
with birds from SW Germany.
If variation is heritable, what might you expect?
What would happen to the offspring birds in the wild?
What does this imply about general variation in the population?
What about the other postulates (more are born than can survive, variation leads to
increase in reproductive success)?
We don’t know about blackcaps, but all species have the capability to expand their
population much more rapidly than they actually do.
Esceriscia coli: gut bacteria, can divide every 20 min. 72 generations per day, or 2 72
bacteria in one day. About 5,000,000,000,000,000,000,000 per day (would cover earth
out to the moon in a few days).
A slow breeder, from Darwin’s Origin:
The elephant is reckoned to be the slowest breeder of all known animals, and I have
taken some pains to estimate its probable minimum rate of natural increase: it will be
under the mark to assume that it breeds when thirty years old, and goes on breeding till
ninety years old, bringing forth three pairs of young in this interval; if this be so, at the
end of the fifth century there would be alive fifteen million elephants, descended from the
first pair."
The point? Animals DO NOT reproduce at full capacity– populations are being held in
check by something.
(Except for one species….)
Last Postulate: does trait increase reproductive success?
… Under study now. But British birders (a crazy lot) report increasing numbers in winter.
Also, number of feeders and average winter temperature in Britain are both rising.
Another line of evidence for how selection can act on behavior: Artificial selection.
What is artificial selection? Same process as natural selection, but WE are providing the
selective force (by choosing breeders).
--used for thousands of years by farmers, breeders, etc. The original genetic
engineering.
“If making a Chihuahua from a wolf is not playing God, I don’t know what is” -Toby
Bradshaw, UW professor.
-- Just as we have selected for corn with lots of oil, chicken with big breasts,
cattle with marbled meat, etc…, we have also selected for BEHAVIOR.
Example: dog breeds. Whereas other plant and animals have been selected to
eat, most dogs have been selected to DO things that we find useful.
(see next slide: Doberman- guard, Pointer and Retriever- both
hunting, but different behaviors, border collie- herding. The wolf is the
ancestor of all these breeds)
Dogs as example of selection for behavioral traits:
Do wolves do all of these things?
(no!)
Where did the variation that leads to these novel traits come from? (from wolves?)
So???
Does this variation affect reproductive success? Yes! (because we control reproduction)
Natural selection works with variation present in population (plus new mutations), but
can select for new combinations– new traits. In the next generation, novel combinations
may again be selected for, and so on…
This implies a historical process: selection is operating on the results of past selection,
and so on…..
(end 6/22)
Adaptation: adaptations are the result of natural selection. Red color is an adaptation
for candy survival against a red background.
A short trip to England is an adaptation for overwinter survival and quick summer returns
to the breeding grounds in blackcap warblers.
Adaptations are traits that result from natural selection and increase their bearer’s
reproductive success.
Galapagos finch
Descent with modification:
-- understanding how PAST natural selection and adaptation explains PRESENT
traits.
-- all lineages can be traced back to a common ancestor.
--species branch off from other species, and so on. Imagine a family tree of
species. In fact, we will be using this– called Phylogenetic tree
Phylogenetic tree: we will use this
throughout the course.
-- how created?
-- useful way to understand the
influence of evolutionary history on
behavior.
-- If species B performs a certain
behavior, species X and Y probably will
too.
A more formal statement:
Any 2 species can be traced back
ultimately to a common ancestor. The
differences between them are the result
of the history of the adaptations of their
respective lineages since they diverged
from their common ancestor. Thus the
more recent their common ancestor, the
more similar two species should be.
Which two species should be most
similar here?
Which species are alive now? (hint:
see time axis).
However, we already learned that natural selection can lead to differences in migration
behavior and candy coloration, among other things.
--if several lineages of candy (skittles, starburst, gummi bears) were subjected to
predation against a red background, what might happen?
--other species of birds also live in England during the summer. Might they also stay to
enjoy the winter feeders along with the blackcap warblers? Need their response have
anything to do with their genetic relationship to warblers?
To say the same thing more formally: The more similar the selection pressures
affecting species are, the more similar we expect them to be.
How does this fit with the statement from the last slide?
Species can be similar due to shared evolutionary history, this is called homology.
And species can be similar due to similar adaptations, this is called analogy.
• Homologous traits in different
animals are similar because they are
derived from a
common ancestor.
• Example of homology = bones of
mammalian forelimb – forelimbs of
human, horse, seal, bat, kangaroo,
mole etc. have different functions but
have the same identifiable sequence
and arrangement of bones:
• Upper arm single bone (humerus)
• Forearm has 2 bones (radius & ulna)
• Small bones in wrist (carpals)
• Bones in hand (metacarpals & phalanges)
Can you think of behavioral
examples?
• Analogous traits in different animals are similar because they are adaptations to
a similar ecological niche. If you traced back to the common ancestor of the
animals, it would not have that trait.
• Example of analogy – seal vs. penguin flippers and body shape – designed for
locomotion through water, but underlying structure in two animals (one a mammal,
one a
bird) are very different.
• Divergence: two evolving groups of recent common ancestry becoming more
dissimilar.
• Convergence: two evolving groups of distant common ancestry becoming more
similar.
• Example:
Analogy
Homology
Three concepts:
Natural selection
Descent with modification
Adaptation
How are they related to each other? To
understanding Animal behavior?
If you don’t know, ask now!!!
A further extension: the group selection fallacy
Natural selection acts on INDIVIDUALS (they
either survive and reproduce, or do not).
Thus, a behavior cannot evolve for “the good of
the species”.
Avoid group selection thinking! Always ask
yourself, could a cheater invade the system?