Evolution as Genetic Change

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Evolution as Genetic Change
evolutionary fitness an organisms success at
passing on its genes to offspring
evolutionary adaptation any genetically
controlled trait that increases fitness
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Evolution as Genetic Change
Natural selection acts on traits, not genes.
The birds are selecting the trait of body color,
not the genes that cause the body color.
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Evolution as Genetic Change
But, the result is that the relative frequencies
of alleles that increase fitness tend to increase
in a population.
the frequency of the allele for being light has
increased
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Evolution as Genetic Change
Therefore, populations evolve, not individuals!
Some individuals in both populations are light
and some are dark, but the has changed.
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Natural Selection on Single-Gene Traits
A mutation in a single gene may be enough to
cause the population to change.
Mutation in color allele in egg.
parent
offspring
Light coloration starts increasing in population.
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Natural Selection on Polygenic Traits
Polygenic traits produce ranges of phenotypes
that fit a bell curve.
Depending on which variation is most fit, the
population will change in some way.
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Natural Selection on Polygenic Traits
Situation 1 no form of the trait is more fit
No Selection
The population is said to be in equilibrium
with respect to the trait.
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Natural Selection on Polygenic Traits
Situation 2 one end of the range is more fit
Directional Selection
normal distribution
one end is more fit than the other
The population shifts in the direction that is
more fit.
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Natural Selection on Polygenic Traits
Situation 3 the center of the range is more fit
Stabilizing Selection
both ends disappear
normal distribution
In this case, the center of the range is
considered to be stable.
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Natural Selection on Polygenic Traits
Situation 4 the center of the range is less fit
Disruptive Selection
the center disappears
normal distribution
In this case, the center of the range is
unstable. Two species may result.
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Genetic Drift
a random change in allele frequency
- population is usually small
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Genetic Drift
Founder Effect
- often happens when a small population colonizes
a new area
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Evolution vs. Genetic Equilibrium
This principle states that a population will
remain stable unless one of 5 conditions occurs.
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Evolution vs. Genetic Equilibrium
Conditions that can affect equilibrium
1. random mating
2. size of the population
3. migration
4. mutations
5. natural selection
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Evolution vs. Genetic Equilibrium
1. random mating
If random mating occurs, individuals are not
picking their own mates based on some
characteristic (fitness). The less fit have an
equal chance of having offspring.
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Evolution vs. Genetic Equilibrium
2. size of the population
Allele frequencies are more stable in larger
populations than smaller ones (less genetic
drift).
Losing 1 doesnt have much effect on allele
frequencies.
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Evolution vs. Genetic Equilibrium
3. migration
Movement into or out of a population cannot occur
if equilibrium is to be maintained.
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Evolution vs. Genetic Equilibrium
4. mutations
Mutations cannot occur if the population is to
remain stable.
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Evolution vs. Genetic Equilibrium
5. natural selection
Natural selection cannot occur if equilibrium is
to be maintained.