Download Population Genetics

Microevolution
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PACKET #59
CHAPTER #23
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Microevolution
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 Change in a population’s allele, or genotype,
frequencies over successive generations
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Occurs when a population does not meet all of the assumptions
of the Hardy-Weinberg principle
The small changes are referred to as microevolution.
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Microevolution II
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 There are five micro-evolutionary forces:  Non-random mating
 Mutations
 Genetic drift
Bottleneck effect
 Founder effect
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Gene flow
Natural selection
Stabilizing selection
 Directional selection
 Disruptive selection
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Microevolution
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NON-RANDOM MATING
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Non-random Mating
 Assortative Mating
 Individuals select mates on
the basis of phenotype—
indirectly selecting a
corresponding genotype.
 This may lead to
interbreeding
 Leads to an increased
homozygous allele
composition
 May lead to interbreeding
depression and lowered
“fitness” in the population
 Commonly seen in plants
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• Fitness
 Ability to pass on
genes to the next
generation
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Microevolution
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MUTATIONS
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Mutations
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 Unpredictable changes in DNA resulting in the
production of new alleles
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Introduces variation
Cause small deviations from Hardy-Weinberg equilibrium
Mutations in somatic cells are not heritable
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Mutations in alleles found in sex cells allow those changes to be
passed to the offspring
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Microevolution
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GENETIC DRIFT
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Genetic Drift
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 Random events that
change allele frequencies
in small populations
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Small populations are
more prone to lose alleles
present in low frequencies
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Bottleneck Effect
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 Rapid and severe
declines in population
size due to an adverse
environmental factor
 Results in an increase in
different allele
frequencies
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Bottleneck Effect
Example
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 Examples
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Northern elephant seals
Cheetah
American bison
Wollemi Pine
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Founder Effect
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 Occurs when a small population colonizes a new
area.
 Common in island populations
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Finnish population exhibits much less allelic variation than the
general European population
Amish population in Pennsylvania has a significant number of
individuals with the allele composition for a form of dwarfism.
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Microevolution
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GENE FLOW
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Gene Flow
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 The movement of alleles caused by a migration of
individuals between populations
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Migration of breeding individuals introduces new allelic
frequencies to a population
Tends to counteract natural selection and genetic drift
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Causes populations to become more genetically similar
Humans have experienced an increase in gene flow in the last
few hundred years.
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Microevolution
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NATURAL SELECTION
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Natural Selection
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 Natural selection results in changes in allele
composition that lead to adaptation and is based on
differential reproduction
 Natural selection does not act directly on an
organism’s genotype but acts on the phenotype.
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The phenotype represents an interaction between the
environment and all the alleles in the organism’s genotype.
Natural selection weeds out those individuals whose
phenotypes are less adapted to environmental changes
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Allowing the better adapted organisms to survive and pass their
alleles to future generations
 Natural selection acts indirectly on the genotype.
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Natural Selection
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 There are three types of natural selection:  Stabilizing selection
 Directional selection
 Disruptive selection
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Natural Selection
Stabilizing Selection
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 Favors intermediate
phenotypes
 Results in reduced
variation in a population
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Classic example is human
birth weight
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Natural Selection
Directional Selection
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 Favors one phenotype
over another
 Favors one extreme of
the normal distribution
over the other
phenotypes.
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Natural Selection
Disruptive Selection
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 Favors phenotypic
extremes
 Selects for two or more
different phenotypes
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May result in splitting of a
population into 2 or more
separate species.
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Necessities for Natural Selection
 Populations contain genetic
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variation, a necessity for natural
selection, introduced because of
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Mutations
Crossing Over
Independent Assortment
Random Fertilization
 In order to investigate genetic
variation scientists examine
genetic polymorphism
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The presence of two or more alleles at
a given locus.
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Locus is the location of a gene on the
chromosome.
Scientists compares different forms of
a protein using gel electrophoresis
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Resulting from slight changes in the
gene’s DNA nucleotide sequence
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