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Walters-BSC2010-Chapter23-10 - Chapter 23 The Evolution of...

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Chapter 23: The Evolution of Populations
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The Origin of Species Convinced biologists evolution occurring Dubious: natural selection Mendel contemporary, ignored until 1900s Mendel worked with discrete characteristics (purple flower, white flower = 1 locus), Darwin suggested quantitative characters vary along a continuum (beak length, multiple loci)
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We now study microevolution : smallest scale of change in populations over generations Population : group of individuals of a species sharing common geographic area Species : group of populations that have potential to interbreed in nature
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How do we get variation within/between populations? 1) Sexual reproduction (crossing over, independent assortment of chromosomes, fertilization) 2) Mutations (point mutation, large gene sequences, gene numbers) 3) Geographic variation: differences in genetic composition of separate populations May be isolated May be a cline (graded change in character along a geographic axis) Isolated Cline
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Measuring Variation Within/Between Populations Gene variability (whole genes) Nucleotide variability (nucleotide sequences of DNA) Gene variability quantified as average heterozygosity: average percent of loci that are heterozygous (fruit flies heterozygous for 14% loci, homozygous for rest. Therefore: average heterozygosity = 14%). Nucleotide variability : compare DNA sequences of individuals in population. Fruit flies: 1% of nucleotides differ. Therefore: 1% nucleotide variability. Humans: 0.1% nucleotide variability
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Hardy-Weinberg (HW) Principle: When studying evolution in a real population, helpful to compare to a NON-EVOLVING population (HW Principle) If no differences, then real population is not evolving (H-W equilibrium). If different, then real population is evolving. If evolving, then ask – WHY?
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Gene Pool All genes in population at any one time Includes all alleles at all gene loci in all individuals in a population
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Gene Pool If diploid: 2 alleles for trait Homozygous or Heterozygous Fixed : all members of population identical homozygotes for trait Not fixed : each allele has relative frequency in gene pool
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Start to think in Hardy-Weinberg terms
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Hardy-Weinberg Formulas: For simple dominant/recessive traits with AA, Aa, aa, you need to know 2 formulas to answer any question: 1) p + q = 1 Alleles: p = A, q = a 2) p 2 + 2pq + q 2 = 1 Genotypes: p 2 = AA, 2pq = Aa, q 2 = aa
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