Chapter 23

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Chapter 23
2012-09-16 13:48:58

Exam 1
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  1. Microevolution or population genetics
    Smallest unit of evolution

    Natural selection acts on physical variation

    The variation of physical features or behavior is a result of underlying genetic variation

    Natural selection alters the frequency of alleles in a population

    Alleles are different forms of a gene
  2. Sources of Variation
    • Changes in DNA nucleotide sequence
    • Can affect single nucleotide, many nucleotides 
  3. Sources of variation 
    Sexual reproduction
    • Meiosis leads to variation in gametes
    • • Independent assortment, crossing over

    • – Randomness of fertilization
    • • Which sperm fertilizes egg?

    – Mate selection 
  4. Hardy-Weinberg Theorem: Describes why a population isn't evolving.
    Equilibrium and Conditions
    • A population where allele % do not change
    • from one generation to the next

    – No change in allele %, no microevolution

    • • If equilibrium broken, evolution is occurring
    • – Then can try to explain why 
  5. Hardy-Weinberg Theorem: Describes why a population isn't evolving.

    Equation and Calculations
    HW equation shows allele frequency p2 +2pq+q2 =1 (p+q=1alsohelpful)

    • p and q represent the relative frequencies of two possible alleles for a gene (ex. A, a)

    • p2 and q2 represent the frequencies of the homozygous genotypes (ex. AA, aa)

    • 2pq represents the frequency of the heterozygous genotype (ex. Aa)
  6. Hardy-Weinberg Conditions
    • 1. No Mutations
    • 2. Random Mating
    • 3. No Natural Selection
    • 4. Large Population Size
    •           Genetic drift less significant
    • 5. No gene flow
    •            Populations are isolated
  7. Genetic Drift: Founder Effect
    • When a few individuals become isolated from a larger population
    •     Typically results in altered allele frequency

    • Small population can have less variation
    • Small population subject to genetic drift
  8. Genetic Drift: Bottleneck Effect
    • Occurs when population size is decimated
    •      Environmental change, natural disaster etc.
    •      Typically results in altered allele frequency

    • New population has less variation
    • Small population subject to genetic drift 
  9. Genetic Drift
    • Can lead to a loss of genetic variation
    • Certain alleles can be lost from population
    • Certain alleles can be fixed in population

    Fixed alleles can sometimes be harmful

    Can explain why some genetic disorders become prevalent in isolated populations 
  10. Gene Flow
    • Transfer of alleles in & out of a population
    • – Movement of fertile individuals or gametes
    • – Can alter original allele frequencies

    • Extensive gene flow combines neighboring populations into a single population
    •     1 large population with a common gene pool
    •     Ex. Global human population
    •     Previously separated populations coming together 
  11. Modes of Selection
    • Act on phenotypes in a normal distribution
    • – Can change original allele frequency

    • • Directional selection
    • – Favors one extreme trait

    • • Disruptive selection
    • – Favor both extreme trait

    • • Stabilizing selection
    • – Favors intermediate trait