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I.
Microevolution
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Evolution occurs when populations
don’t meet all the H-W assumptions
Process by which a population’s
genetic structure changes =
microevolution
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Changes in allele frequencies result from five
evolutionary processes
1)
2)
3)
4)
5)
Mutation
Nonrandom mating
Natural selection
Genetic drift
Gene flow
I.
Microevolution
A.
Mutation
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Unpredictable change in nucleotide sequence of DNA
Spontaneous, unpredictable, permanent
Somatic mutations seldom passed to next generation
Most mutations harmless
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Neutral variation
Not reflected in phenotype
May be passed to next generation
Expressed mutations tend to be harmful
May increase genetic variability and influence alleles
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Mutation rates low (1 in every 100,000 genes per
generation)
Minor impact compared to recombination
I.
Microevolution
B.
Nonrandom Mating
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Occurs when individuals select mates on the
basis of phenotype
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Choice
Proximity
Individuals living closer tend to be more closely
related (genetically similar) than individuals
farther away
I.
Microevolution
B.
Nonrandom Mating
1.
Inbreeding
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2.
Increases homozygosity
Ultimate: Self-fertilization (e.g. in plants)
May lead to inbreeding depression and reduced fitness
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Declines in fertility, increased juvenile mortality
Ex: White-footed mice brought into captivity and inbred had
significantly lower survivorship when released vs. noninbred mice
Assortative Mating
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Mates selected based on phenotype
Ex: Fruit flies with more bristles prefer other bristly flies and
vice-versa
Increases homozygosity
May lead to shifts in genotype frequencies but doesn’t add
variation
I.
Microevolution
C.
Natural Selection
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Alters allele frequencies to increase adaptation
to environmental conditions
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Allele frequencies tend to shift toward most favorable
alleles
Individuals that survive and produce fertile
offspring have a selective advantage
I.
Microevolution
D.
Genetic Drift
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Results from random events that change allele
frequencies within a population
Small populations more prone to substantial
changes, including reduced variation and
loss of rare alleles
May lead to increased frequency or fixing of
harmful alleles
Fig. 23.9
I.
Microevolution
D.
Genetic Drift
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Random process; alleles may be lost or
preserved independently of benefit
Typically leads to loss of alleles  decrease of
genetic diversity in population
If population decreases in size and loses
diversity, then increases in size, resulting large
population may display influence of genetic drift
when population was small
I.
Microevolution
D.
Genetic Drift
1.
Bottleneck effect
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Usually due to rapid, severe decline in population size
followed by increase in population
May produce allele frequencies very different from
pre-bottleneck conditions
Fig. 23.10
Fig. 23.11
I.
Microevolution
D.
Genetic Drift
1.
Bottleneck effect
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2.
Ex: Elevated frequency of Tay-Sachs Disease in
Ashkenazi Jews
Ex: Genetic homogeneity in populations of African
cheetahs
Founder effect
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Allele frequencies in small populations may reflect
genotypes of founding individuals
Common in isolated populations
Ex: Finns descended from small group of people
~4000 years ago; genetically distinct from other
Europeans