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EVOLUTION
Inheritable Variation
Where does variation come from?
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Remember that inheritable variation comes
from mutations and gene shuffling
Inheritable variation is the raw material for
natural selection and fuels evolution
Natural selection acts on variations in
phenotype only
Traits Controlled by a Single Gene
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Traits controlled by a single gene with only 2
alleles have 3 possible genotypes
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This results in at most 3 possible phenotypes
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Traits determine fitness
Traits Controlled by Several Genes
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Many traits are
polygenic and/or
multiple alleles
This leads to a wide
range of phenotypes
(think human traits like
height, skin color, etc)
http://www.algebralab.org/img/49ab8f77-f675-423a-b8af-d46874987ab3.gif
Genes, Fitness, and Adaptation
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A species is defined as a group of similarlooking organisms that can breed with each
other to produce fertile offspring.
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Reproductive isolation is the idea that
individuals of different species cannot
interbreed.
Genes, Fitness, and Adaptation
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Individuals within a species share the same
gene pool – group of alleles that are shared
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Change in one member of a species can
spread through the population, but not to
other species
Genes, Fitness, and Adaptation
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When there are no selection pressures, each
allele in the gene pool occurs in a particular
frequency
Sexual reproduction itself does not change
the relative frequency of an allele
Think of a deck of cards
Evolution – Genetically Defined
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Using genetics, evolution is defined as
any change in the relative frequencies of
alleles in the gene pool of a species.
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Evolution is a natural consequence of the
nature of genes and DNA and the interaction
between organisms and their environment.
Natural Selection – Single-Gene Traits
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Let’s assume that there is a mutation in one
copy of one gene carried by one individual
If the mutation is harmful, it decreases that
organism’s fitness and fewer copies of that
allele will be passed on to future generations.
The mutated allele will become less common
in the population.
Natural Selection – Single-Gene Traits
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If the mutation is helpful, it increases that
organism’s fitness and over time carriers of
the mutated allele will produce more
offspring.
After a few generations, relative frequency of
the allele will increase.
Natural Selection – Polygenic Traits
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Polygenic traits have a wide range of
phenotypes (bell curve)
Fitness throughout the curve varies, and
natural selection can act on these traits in
different ways