This presentation was originally prepared by

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This presentation was originally prepared by C.
William Birky, Jr. Department of Ecology and
Evolutionary Biology The University of
Arizona It may be used with or without
modification for educational purposes but not
commercially or for profit. The author does not
guarantee accuracy and will not update the
lectures, which were written when the course was
given during the Spring 2007 semester.
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Population and Evolutionary Genetics The Nature
and Origin of Biological Diversity
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Homo sapiens?
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AdhF/AdhF AdhF/AdhS
S F
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Now look at the DNA sequence The bp difference
at site 74, C vs. A, is the one that causes the
difference between threonine and lysine at amino
acid 25. This is a change in the second codon
position. There are a number of other
differences between the three Adh genes from D.
melanogaster, and also between the three species.
Most of these are synonymous differences
differences that change from one codon to a
synonymous codon and hence dont change an amino
acid. E.g. site 15 codon difference ATC vs. ATT,
both code for isoleucine. Change in third codon
position, where most changes are
synonymous/silent.
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• Hypothetical example
• Population of Drosophila melanogaster is examined
  for genetic variation at the Adh locus.
• Sample of 100 flies subjected to electrophoresis.
• Calculate genotype and gene frequencies
• Numbers of genotypes 40 FF 40 FS 20 SS total 100
• Genotype frequencies 0.40 0.40 0.20
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• Numbers of genes 80 F 40 F
• 40S 40S 200
• Allele frequencies 0.4F 0.2F 0.6 F
• 0.2S 0.2S 0.4 S
• 1

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The trouble with the gene pool is that there is
no life guard.
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Inbreeding Produces Homozygotes e.g. selfing
Aa
1/4 1/2 1/4
1/4 AA
1/2 Aa
1/4 aa
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all Aa
1/4 1/2 1/4
1/4 AA
1/2 Aa
1/4 aa
1/4 1/2 1/4
3/8 AA
1/4 Aa
3/8 aa
1/4 1/2 1/4
7/16 AA
1/8 Aa
7/16 aa
How many heterozygotes after n generations?
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all Aa
1/4 1/2 1/4
1/4 AA
1/2 Aa
1/4 aa
1/4 1/2 1/4
3/8 AA
1/4 Aa
3/8 aa
1/4 1/2 1/4
7/16 AA
1/8 Aa
7/16 aa
How many heterozygotes after n generations? (1/2)
n
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Calculation of p is tedious by hand for large
samples. Best to use calculator with statistical
functions or use Excel spreadsheet. Example of
data from small sample of freshwater invertebrate
Keratella cochlearis. This is the nucleotide
diversity of 590 bp of the mitochondrial cox1
gene.
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