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Where we’ve been:• Charles Darwin and his travels (Darwin Notes 1)• Natural Selection (Notes 2)– Moths and Natural Selection Spoons
• Evidence for Evolution (Notes 3)– Comparative Anatomy, Biogeography, DNA– Packet, Video, Webquest, Biochemical Comparisions
• Mechanisms of Evolution (webquest and this)– Genetic Drift, Sexual Selection, Mutation, Migration,
Natural Selection
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16.1 GENES AND VARIATION
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I. “Gaps” in Darwin’s Theory 1. How do VARIATIONS arise?
-Variation was one of the key “ingredients” of NATURAL SELECTION
- Remember: variation, selection, time- Some variations are better than others, the
environment SELECTS those. 2. How are FAVORABLE variations (traits) passed on to
offspring?
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• What scientist’s work could have help Darwin with these questions?
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• MENDEL!!! • (Remember, he described INHERITANCE of
“factors” in pea plants).
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II. Putting it all together! By the 1930s, Mendel and Darwin’s work were
combined. Molecular biologists had also discovered that:
1) DNA was the molecule of heredity. DNA
determines phenotype. 2) Sources of variation include MUTATION and
GENE SHUFFLING
+
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• We can now describe evolution in GENETIC terms. We can be more specific than “change over time.”
• Evolve: change over time
• Evolution (in genetic terms) is any CHANGE in the relative FREQUENCY of ALLELES in a population.
ORAKA: biological evolution or microevolution
A change in genetic composition within a population over generations.
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Genetic Vocabulary Review
• Gene• Trait• Genotype• Allele• Phenotype
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Vocab: 1. A POPULATION is a group of individuals of the
same species that interbreed and reproduce. 2. A GENE POOL is all genes, including different
alleles, that are in a population.
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• 3. RELATIVE FREQUENCY is the # of times an allele appears in a population.
General Equation:Relative frequency of an allele=
# of the certain allele in the population # of TOTAL alleles in the population
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MAIN IDEA!
• Allele Frequency is about finding
(# of A’s)(total # of A’s + a’s)
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Application
What is the allele frequency in themice population
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Which mice genotypes contain the dominant allele for fur color (B)?
48% heterozygous
black
16% homozygous
black36% homozygous
brown
allele for brown fur
allele for black fur
• 2, the BB and Bb mice How many dominant alleles are in the homozygous dominant black mouse? The heterozygous mouse?
• 2, 1
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***Relative frequency of a DOMINANT allele= (# homozygous dominant x 2) + (# heterozygous x 1)
(# individuals in population x 2) Why multiply the # of individuals by 2?
48% heterozygous
black
16% homozygous
black36% homozygous
brown
allele for brown fur
allele for black fur
Each individual has two alleles for a trait, one from mom, one from dad.
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Which mice genotypes contain the recessive allele (b)? How many recessive alleles are in the heterozygous
mouse? The homozygous recessive mouse?
48% heterozygous
black
16% homozygous
black36% homozygous
brown
allele for brown fur
allele for black fur
2, the Bb and bb mice
1, 2
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****Frequency of a RECESSIVE allele= (# homozygous recessive x 2) + (# heterozygous x 1)(# individuals in population x 2)
48% heterozygous
black
16% homozygous
black36% homozygous
brown
allele for brown fur
allele for black fur
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III. Example: Calculating the relative frequencyExample 1: Let's consider a gene with only two alleles. In mice,
Black fur color (BB or Bb) is dominant to brown fur color (bb).
In a population of 100 mice, 36 mice are homozygous dominant (BB), 48 mice are heterozygous (Bb) and 16 are brown (bb).
Relative frequency of B= # of B alleles in the population
# of TOTAL alleles in the population
Relative frequency of a dominant allele:
= (# homozygous dominant x 2) + (# heterozygous x 1) (# individuals in population x 2)
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Relative frequency of B
= (36 x 2) + (48 x 1)= .60 = 60% (100 x 2) = 200
or 36+36+48 = 120 = 60% 100+100 200
How could we figure out the frequency of “b” without doing the big equation?
0.6 + 0.4 =1.0
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Note that the allele frequencies add up to 1. *This is a law of population genetics:
The sum of all allele frequencies will always be equal to 1. This is because 1 represents the frequency of all possible alleles within the population.
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Frequency of b?
Frequency of b = # of b alleles in the population # of TOTAL alleles in the population
Frequency of a recessive allele:
= (# homozygous recessive x 2) + (# heterozygous x 1)(# individuals in population x 2)
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Frequency of b=
(16 x 2) + (48 x 1) = 80 = 0.4 = 40% (100 x 2) = 200 200
or
(16 + 16 + 48) = 80 = 0.4 = 40% (100 +100) = 200
Note that the two allele frequencies add up to ONE.0.6 + 0.4 =1.0
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• Let’s say that these mice have several generations of offspring. We want to know if EVOLUTION has occurred. How would we know?
• If the relative frequencies of the alleles CHANGE, then evolution has occurred!
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IV. 2 Sources of Variation: A Review
1.Mutations are any change in the DNA sequence (AATAC AATAT)
May be caused by:Mistakes during replicationRadiation or chemicals in the environment
Effects: INCREASE Fitness, DECREASE Fitness, or NO Effect
When would a mutation have no effect?
Ex: Silent mutation, no change in amino acid sequence
Clip: Why don't horses have wheels?
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• When would a mutation that caused a phenotypic change have no effect on fitness?
If it were something that did not effect SURVIVAL and REPRODUCTION. Ex. extra finger or extra teeth.
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2. Sexual Reproduction (Gene Shuffling)
Q: Why don’t you look exactly like your parents? (what were the sources of genetic variation you learned in meiosis?)
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Meiosis:• Random assortment of
genes, random separation of chromosomes
(the Law of Independent Assortment).
• Crossing Over: exchanging parts of homologous chromosomes
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• REMEMBER! Some variations are better than others, the environment SELECTS those.
• The source of variation is on the DNA level!