Ch. 22/23 Warm-up

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Ch. 22/23 Warm-up

• What is the evidence for evolution?
• (Review) What are the 3 ways that sexual
  reproduction produces genetic diversity?
• What is 1 thing you are grateful for today?

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Ch. 23 Warm-up

• In a population of 200 mice, 98 are homozygous
  dominant for brown coat color (BB), 84 are
  heterozygous (Bb), and 18 are homozygous (bb).
• The allele frequencies of this population are
• B allele ___ b allele ___
• The genotype frequencies are
• BB ___ Bb ___ bb ___
• Use the above info to determine the genotype
  frequencies of the next generation
• B (p) ___ b (q) ___
• BB (p2) ___ Bb (2pq) ___
• bb (q2) ___

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Ch. 23 warm-up

• Use the following information to help you answer
  the question below
• Population 1000 people
• AA 160 Aa 480 aa 360
• What are the genotype ratios? Allele
  frequencies?
• Use directional, stabilizing or disruptive
  selection to answer the following
• The mice in the Arizona desert have either dark
  or light fur.
• Birds produce 4-5 eggs per clutch
• Average human baby weighs 7 lbs.
• Darwin's finches and beak size during drought

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The Evolution of Populations

• Chapter 23

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What you must know

• How mutation and sexual reproduction each produce
  genetic variation.
• The conditions for Hardy-Weinberg equilibrium.
• How to use the Hardy-Weinburg equation to
  calculate allelic frequencies and to test whether
  a population is evolving.

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Smallest unit of evolution

• Microevolution change in the allele frequencies
  of a population over generations

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• Darwin did not know how organisms passed traits
  to offspring
• 1866 - Mendel published his paper on genetics
• Mendelian genetics supports Darwins theory ?
  Evolution is based on genetic variation

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Sources of Genetic Variation

• Point mutations changes in one base (eg. sickle
  cell)
• Chromosomal mutations delete, duplicate,
  disrupt, rearrange ? usually harmful
• Sexual recombination contributes to most of
  genetic variation in a population
• Crossing Over (Meiosis Prophase I)
• Independent Assortment of Chromosomes (during
  meiosis)
• Random Fertilization (sperm egg)

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• Gene pool all of the alleles for all genes in
  all the members of the population
• Diploid species 2 alleles for a gene
  (homozygous/heterozygous)
• Fixed allele all members of a population only
  have 1 allele for a particular trait
• The more fixed alleles a population has, the
  LOWER the species diversity

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Hardy-Weinberg Theorm

• Hardy-Weinberg Theorm The allele and genotype
  frequencies of a population will remain constant
  from generation to generation
• UNLESS they are acted upon by forces other than
  Mendelian segregation and recombination of
  alleles
• Equilibrium allele and genotype frequencies
  remain constant

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Conditions for Hardy-Weinberg equilibrium

• No mutations.
• Random mating.
• No natural selection.
• Extremely large population size.
• No gene flow.
• If at least one of these conditions is NOT met,
  then the population is EVOLVING!

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• Allele Frequencies
• Gene with 2 alleles p, q
• p frequency of dominant allele (A) q
  frequency of recessive allele (a)

Note 1 p q 1 q p
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• Genotypic Frequencies
• 3 genotypes (AA, Aa, aa)
• p2 AA (homozygous dominant)
• 2pq Aa (heterozygous)
• q2 aa (homozygous recessive)

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Allele frequencies
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Genotypic frequencies
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Strategies for solving H-W Problems

• If you are given the genotypes (AA, Aa, aa),
  calculate p and q by adding up the total of A
  and a alleles.
• If you know phenotypes, then use aa to find q2,
  and then q. (p 1-q)
• To find out if population is evolving, calculate
  p2 2pq q2.
• If in equilibrium, it should 1.
• If it DOES NOT 1, then the population is
  evolving!

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Hardy-weinberg practice problem 1

• The scarlet tiger moth has the following
  genotypes. Calculate the allele and genotype
  frequencies () for a population of 1612 moths.
• AA 1469 Aa 138 aa 5
• Allele Frequencies
• A a
• Genotypic Frequencies
• AA
• Aa
• aa

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Hardy-weinberg practice problem 2PTC Tasters

• Taster AA or Aa Nontaster aa
• Tasters ____ Nontasters ___
• q2 q
• p q 1 p 1 q
• p2 2pq q2 1

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Causes of evolution
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Conditions for Hardy-Weinberg equilibrium

• No mutations.
• Random mating.
• No natural selection.
• Extremely large population size.
• No gene flow.
• If at least one of these conditions is NOT met,
  then the population is EVOLVING!

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• Minor Causes of Evolution
• 1 - Mutations
• Rare, very small changes in allele frequencies
• 2 - Nonrandom mating
• Affect genotypes, but not allele frequencies
• Major Causes of Evolution
• Natural selection, genetic drift, gene flow (3-5)

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Major Causes of Evolution

• 3 Natural Selection
• Individuals with variations better suited to
  environment pass more alleles to next generation

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Major Causes of Evolution

• 4 Genetic Drift
• Small populations have greater chance of
  fluctuations in allele frequencies from one
  generation to another
• Examples
• Founder Effect
• Bottleneck Effect

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• Genetic Drift

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Founder Effect

• A few individuals isolated from larger population
• Certain alleles under/over represented

Polydactyly in Amish population
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Bottleneck Effect

• Sudden change in environment drastically reduces
  population size

Northern elephant seals hunted nearly to
extinction in California
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Major Causes of Evolution

• 5 Gene Flow
• Movement of fertile individuals between
  populations
• Gain/lose alleles
• Reduce genetic differences between populations

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How does natural selection bring about adaptive
evolution?
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• Natural selection can alter frequency
  distribution of heritable traits in 3 ways
• Directional selection
• Disruptive (diversifying) selection
• Stabilizing selection

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Disruptive Selection eg. small beaks for small
seeds large beaks for large seeds
Stabilizing Selection eg. narrow range of human
birth weight
Directional Selection eg. larger black bears
survive extreme cold better than small ones
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Sexual selection

• Form of natural selection certain individuals
  more likely to obtain mates
• Sexual dimorphism difference between 2 sexes
• Size, color, ornamentation, behavior

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Sexual selection

• Intrasexual selection within same sex (eg. M
  compete with other M)
• Intersexual mate choice (eg. F choose showy M)

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Preserving genetic variation

• Diploidy hide recessive alleles that are less
  favorable
• Heterozygote advantage greater fitness than
  homozygotes
• eg. Sickle cell disease

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Natural selection cannot fashion perfect
organisms.

• Selection can act only on existing variations.
• Evolution is limited by historical constraints.
• Adaptations are often compromises.
• Chance, natural selection, and the environment
  interact.

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Sample Problem

• Define the following examples as directional,
  disruptive, or stabilizing selection
• Tiger cubs usually weigh 2-3 lbs. at birth
• Butterflies in 2 different colors each represent
  a species distasteful to birds
• Brightly colored birds mate more frequently than
  drab birds of same species
• Fossil evidence of horse size increasing over
  time