Transcript Stabilizing Selection

Cross-Dressing Salmon
By Tom Horvath (adapted with changes by Julie Korb)
SUNY College at Oneonta
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CQ#1: Which statement best describes
natural selection?
A. Survival of the fittest.
B. The strongest individuals get to reproduce.
C. The best adapted individuals survive and
reproduce.
D. Survival and reproduction is a matter of luck.
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http://www.youtube.com/watch?v=5DqjsWsY8-g
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CQ#2: Out of all the eggs a female lays,
how many hatchlings (or smolts) do you
think will survive to spawn?
A.
B.
C.
D.
E.
50% (50 out of 100)
10% (10 out of 100)
1% (1 out of 100)
0.1% (1 out of 1000)
Less than 0.1 %
Mass of fertilized salmon eggs
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About 0.03% survive & reproduce themselves
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Which are the lucky few who
make it to reproduce?
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Charles Darwin’s Theory of Natural
Selection
It is not just random luck.
Some individuals have a better
chance of surviving and
reproducing than others.
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CQ#3: Are all these smolts ABSOLUTELY identical?
A: Yes
B: No
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Variation in salmon translates into variations
in survival and reproductive success.
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Theory of Evolution
by Natural Selection
• More offspring produced than survive to
adulthood (OVERPRODUCTION).
• Variation among individuals of a species.
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What good is variation?
Talk to your neighbor and list as many characteristics a
salmon may possess that helps it survive.
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ADAPTATION
• Any characteristic that improves the survival
or reproductive success of an organism.
• Often the result of natural selection.
• Organisms match closely with their
environment.
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Survival is only half the story –
you have to reproduce to pass
on those good traits that aided
survival to this point.
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Theory of Evolution
by Natural Selection
• More offspring produced than survive to adulthood
(OVERPRODUCTION).
• Inheritable variation among individuals of a species.
• Best adapted individuals survive and reproduce
(UNEQUAL REPRODUCTIVE SUCCESS).
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Typical Dominant Male Phenotype
http://www.arkive.org/atlantic-salmon/salmo-salar/video-09a.html?offset=0pt
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The most dominant males (usually largest and most
aggressive) successfully defend their redds and
inseminate the eggs. Smaller males that also have
the kype lose out in head-to-head competition with
larger males.
CQ#4: Are we seeing “survival of the fittest” in
action here?
A. Yes
B. No
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A Different Type of Male
Female Mimic
•Lack dominant male characteristics (e.g., no kype).
•Appear more female-like.
Disadvantage: They can’t compete head-to-head with
dominant males either, but….
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Why don’t female mimics get
weeded out through natural
selection if they are not fit?
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CQ#5: Why are female mimics still found
in each generation?
A. Some outcompete the dominant males in the redds.
B. They appear from mutations randomly each
generation.
C. They have a different reproductive strategy from
dominant males and it works.
D. Some females change into males under stressful
environmental conditions.
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Cross-Dressing Salmon
Female mimics can stay near the redds because dominant
males don’t see them as competition; they think they are
females.
These fish get a brief chance to inseminate a few eggs before
being ultimately bumped out by dominant males.
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CQ#6: Even though female mimics are not
dominant, are they fit?
A. Yes
B. No
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Survival of the Fittest?
Darwinian fitness: contribution an individual makes to the
gene pool of the next generation relative to the contribution
of others.
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Fitness can be compared with relative
values (values from 1 to 0)
• A fitness value of 1 is assigned to the
phenotype with the highest representation.
• All other phenotypes are assigned based on
their reproductive success relative to the
dominant type.
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Fitness Value Example
• Dominant male phenotype produces 300
male offspring in the next generation.
• Female mimic phenotype produces 50 male
offspring in the next generation.
• Small male phenotype produce only 5 male
offspring.
Dominant Male Phenotype – Fitness = 300/300 = 1.0
Female Mimic Phenotype – Fitness = 50/300 = 0.17
Small Male Phenotype – Fitness = 5/300 = 0.02
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Natural Selection
• Analyzing change in frequency of traits in a
population:
• Three patterns emerge:
– Directional selection
– Stabilizing selection
– Diversifying selection
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Natural Selection
Directional Selection Individuals of one
extreme phenotype
favored.
Diversifying Selection Both extreme phenotypes
favored; intermediate
phenotypes selected
against.
Stabilizing Selection Individuals with
intermediate phenotype
favored;
Extreme phenotypes
selected against.
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Directional Selection
Individuals of one extreme phenotype favored
Frequency
Population with
no natural selection
Population under
directional selection
Body Size of Salmon
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Stabilizing Selection
Individuals with intermediate phenotype favored; extreme
phenotypes selected against
Frequency
Population under
stabilizing selection
Population with
no natural selection
Body Size of Salmon
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Diversifying Selection
Both extreme phenotypes favored; intermediate
phenotypes selected against
Frequency
Population with
no natural selection
Population under
disruptive selection
Body Size of Salmon
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CQ#7: Given our fitness value from the
example, which pattern of selection would
we expect in the salmon population?
A.
B.
C.
D.
Directional selection
Stabilizing selection
Diversifying selection
Unnatural selection
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CQ#8: Do individuals have to be the
“strongest” to be fit?
A. Yes
B. No
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CQ#9: Which of these traits might confer
fitness upon an individual?
A. Being sneaky and tricking the dominant males into
letting you hang around the spawning females.
B. Having an awesome hook on your jaw and large
body size.
C. Having sperm that are extra good at finding and
fertilizing eggs.
D. Being extra good at storing fat, which will fuel all
that waterfall jumping while migrating upstream.
E. Any of the above will increase fitness.
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