Natural Selection
1. Do we see the same variation within different wild species
that we do in domesticated ones like dogs, pigeons and horses?
Lemurs of Madagascar
Amazonian Frogs
Natural Selection
2. If humans select traits for dogs, pigeons and the five major
mammals, who selects the traits of all those Lemurs, Frogs, and
other wild animals?
NOBODY
WHICHEVER TRAITS WORK BEST FOR THE
ENVIRONMENT THEY LIVE IN ARE THE
ONES THAT ARE PASSED ON THROUGH THE GENES
Animals not possessing the beneficial traits don’t do as well, or just die
Example #1:
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The neck of the
Giraffe
And Lamarck
The slow
hypothetical
evolution: Response
to Acacia Trees
The Red Queen
Hypothesis...
Example #2:
Versicolor Moth
Early trees had light-colored bark
 Only the light-colored moths survived.
Selection was for less melanin.
 After industrialization, the tree bark was
darker.
 Only the darker colored moths now
survived. Selection was for more
melanin.
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Example #3
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The leaf bug
What serve as pressures?
Weather:
changes in local weather (e.g. when an Ice Age comes on),
migration to a new area (e.g. Alaska down into California)
Change in surroundings/cover:
change in density of vegetation (e.g. scrub to forest)
change in color of vegetation (e.g. versicolor moth)
Change in food or predators:
food grows defenses (e.g. Acacia Trees and Giraffes)
only fastest predators survive (e.g. speed of Cheetahs)
The Main Types of Selection
Pressures
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Directional Selection
– Easiest to identify
– Camouflage and mimicry usually fall here
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Disruptive Selection
– Causes species to diverge
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Stabilizing Selection
– Drives a population to specialize
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Sexual Selection
Directional Selection
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Most quantifiable
changes in a
population are
directional, for
example...
Neck of Giraffe
Moth color (melanin)
Camouflage/Mimics
Many sexually
selected traits
Directional Selection: Mimicry
(mimic environment)
Directional Selection: Mimicry
(mimic other animal)
Other Mimicry Examples
(from video clips)
1. Viceroy and Monarch butterflies
2. Orchid and the Echnumatid wasp
3. Firefly mimic
4. Cobra mimic
5. Angler Fish with lure
Stabilizing Selection
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When the extremes
of the trait aren’t as
well suited
Causes
specialization
E.g. bird clutch size
Disruptive Selection
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Causes divergence
within the species
Results in
specialization for
each branched
group
E.g. Darwin’s
Finches
Sexual Selection: Females
Females must behave in a way that ensures that their offspring
survive and mate, and that they have as many offspring as possible
If they behave this way, the genes for this behavior are passed on.
If they don’t behave this way, the genes are not passed on.
(remember, over millions of years all of your ancestors have
successfully mated…)
Behavior #1: Pick a mate who will help you raise the young
(if young have long infancy period)
Behavior #2: Pick a mate who has the “fittest genes” for your
environment and mate with them
(if young have short infancy period)
Sexual Selection: Males
Males also behave in a way that maximizes the propogation of
their genes. The ones who possess traits that allow them to mate
and have many surviving offspring pass on those “prolific” genes.
Behavior #1: Choose one female to raise the young with
(if young have long infancy period)
Behavior #2: Convince the females that they have the “fittest genes”
for their environment
(if young have short infancy period)
Behavior #1: male and female
identical and best adapted
(Gibbons, Albatrosses, many seals)
Behavior #2: Males compete
so that females choose them
(Elephant Seals, Elk, Lions, Gorillas, Widowbird)
These “Tournament Species”
tend to be Sexually Dimorphic
Males and females of the same species have different forms
and are easily distinguishible
The male sexual characteristics identify which males are dominant
Often the superficial sexual characteristics imply more essential ones
- the probiscus, size and deepness of voice of
elephant seals are probably all controlled by the same gene
Some of these characteristics allow males to fight each other more
effectively. But what about the ones that don’t help them physically
compete against each other?
Males and females of the same species have
different forms and are easily distinguishable
The male sexual characteristics identify which
males are dominant
Often the superficial sexual characteristics imply more
essential ones- the probiscus, size and deepness of voice of
elephant seals are probably all controlled by the same gene
Some of these characteristics allow males to fight each other
more effectively. But what about the ones that don’t help
them physically compete against each other?
Do the females really go for it?
1. Catch long-tailed birds and cut off tails.
2. Paste these tails onto short-tailed widowbirds.
3. Catch some long-tailed birds, cut tails and paste back on.
Findings:
The long-tailed widowbirds were more successful at mating
and had more offspring
(whether their tails were long naturally or pasted on)
How did these characteristics come
about if they advertise to predators?
Theory #1: A bottleneck occurred in the population.
An outward, superficial gene was linked on the chromosome
near a more essential one.
The females began choosing the best mate on superficial
cues.
The superficial cues have been undergoing selection ever
since.
Theory #2: That dumb characteristic risks the life of the male.
If he can survive with all those colors and obvious
feathers, he must be some kind of stud.