Download Evolution of Populations Summary of Natural Selection

Evolution of Populations
Summary of Natural
Selection
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Individual organisms differ, and some
of this variation is inheritable
Organisms produce more offspring
than can survive thus creating a
struggle for survival
Organisms that are better suited
survive and reproduce more than
those that less suited to their
environment
Summary Cont…
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Those individuals that are better able to
survive are more likely to pass down the
beneficial traits to their offspring
Over LONG periods of time the beneficial
traits become prevalent throughout the
population
All species alive today are descended with
modifications from ancestral species thus
uniting all living things in a tree of life
Genes and Genetic
Variation
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Remember when Darwin came up with
his theory, we did not yet know what
the chemical factors of inheritance was
We now know that it is ______
Variation can also be “carried” by
individuals but not expressed
(heterozygous)
Sources of Genetic
Variation - Mutations
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Mutations are the changes in DNA sequence
due to mistakes or exposure to chemicals or
energy
They may be beneficial, detrimental, or
have no effect at all
Mutations that improve fitness tend to stay
in the population due to natural selection,
those that are harmful tend to disappear
Sources of Genetic Variation
– Gene Shuffling
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Most heritable differences are caused by the
random shuffling of genes that occur during
gamete production
Sexual reproduction changes the
phenotypes of a population but not the
relative frequencies of genes
Much like the odds of drawing an ace from a
deck of cards is always 4/52 (1/13) no
matter how many times you shuffle
Single Gene Traits
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A trait that is controlled by a single
gene usually has only two alleles.
Ex…
– Widows peak in humans
W = widows peak
 w = no widows peak
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Polygenic Traits
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Traits controlled by many genes
This produces many different
phenotypes
Ex…
– Height in humans
Natural Selection on
Single-Gene Traits
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Natural selection on a single-gene trait
can lead to changes in allele
frequencies and thus to evolution
– P. 397
– Peppered Moth
Natural Selection on
Polygenic Traits
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When a trait is controlled by two or
more genes natural selection can
affect a population in one of three
ways.
– Directional Selection
– Stabilizing Selection
– Disruptive Selection
Directional Selection
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When individuals at
one end of the
curve (one extreme
OR the other) have
the advantage and
the population
shifts in one
direction.
Stabilizing Selection
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When individuals
near the center of
the curve are
selected for
(average), keeps
most individuals in
a population near
the average
Disruptive Selection
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When individuals
near the middle are
selected against,
and the population
splits so that each
extreme grows
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Genetic Drift
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In small populations if an individual that
carries a particular trait and by chance has
more offspring, then the trait or allele can
become more common (by chance, not
selection)
If this happens when a small group of
individuals break away from the main group
and colonize a new habitat it is called the
founder effect
Evolution vs. Genetic
Equilibrium
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The Hardy-Weinberg Principle states
that allele frequencies will remain
constant unless one or more factors
cause those frequencies to change
When the allele frequencies remain
constant it is called genetic equilibrium
and the population will not evolve
Genetic Equilibrium
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In order for a population to be in
genetic equilibrium five conditions
have to be met
– Random mating, very large population,
no movement in or out, no mutations, no
natural selection
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If one of these conditions are not met
the pop. will evolve