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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3http://www.youtube.com/watch?v=5DqjsWsY8-g

CQ#2: Out of all the eggs a female lays, how many hatchlings (or smolts) do you think will survive to spawn?

A. 50% (50 out of 100)B. 10% (10 out of 100)C. 1% (1 out of 100)D. 0.1% (1 out of 1000)E. Less than 0.1 %

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Mass of fertilized salmon eggs

About 0.03% survive & reproduce themselves5

Which are the lucky few who make it to reproduce?

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Charles Darwin’s Theory of Natural Selection

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It is not just random luck.

Some individuals have a better chance of surviving and reproducing than others.

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 Evolutionby 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. 11

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 Evolutionby 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

15http://www.arkive.org/atlantic-salmon/salmo-salar/video-09a.html?offset=0pt

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. YesB. No

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•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….

A Different Type of MaleFemale Mimic

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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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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.

Cross-Dressing Salmon

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CQ#6: Even though female mimics are not dominant, are they fit?

A. YesB. 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.

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Dominant Male Phenotype – Fitness = 300/300 = 1.0Female Mimic Phenotype – Fitness = 50/300 = 0.17Small Male Phenotype – Fitness = 5/300 = 0.02

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.

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Stabilizing Selection - Individuals with intermediate phenotype favored;Extreme phenotypes selected against.Diversifying Selection -

Both extreme phenotypes favored; intermediate phenotypes selected against.

Directional Selection

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Population withno natural selection

Population underdirectional selection

Body Size of Salmon

Fre

qu

ency

Individuals of one extreme phenotype favored

Stabilizing Selection

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Population withno natural selection

Population understabilizing selection

Body Size of Salmon

Fre

qu

ency

Individuals with intermediate phenotype favored; extreme phenotypes selected against

Diversifying Selection

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Population withno natural selection

Population underdisruptive selection

Body Size of Salmon

Fre

qu

ency

Both extreme phenotypes favored; intermediate phenotypes selected against

CQ#7: Given our fitness value from the example, which pattern of selection would we expect in the salmon population?

A. Directional selectionB. Stabilizing selectionC. Diversifying selectionD. Unnatural selection

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CQ#8: Do individuals have to be the “strongest” to be fit?

A. YesB. 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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