Evolution of Populations

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Chapter 23

• Evolution of Populations

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Population Genetics

• Darwins natural selection was not widely
  accepted because he could not explain the basis
  of inheritance
• Mendels work went unappreciated until the early
  20th century, and when it was discovered the idea
  of natural selection took off

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Population Genetics

• Terms to know
• Population localized group of individuals
  belonging to the same species
• Species group of populations whose individuals
  have the potential to interbreed
• Individuals near a population center are on
  average more closely related to each other than
  they are to members of other populations

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Gene Pool

• Total aggregate of genes in a population at any
  one time
• All alleles at all loci in all individuals
• If all members of a population are homozygous for
  an allele that allele is said to be fixed
• Most are not fixed and therefore each allele has
  a relative frequency

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Allele Frequency

• Wildflowers R red flowers r white flowers
• 500 individuals
• 20 rr
• 160- Rr
• 320 RR
• There are 1000 copies of the gene for flower
  color
• R accounts for 800 of these genes (320 x 2
  640 for RR plants plus 160 x 1 160 for Rr
  plants) 800
• 800/1000 0.8 or 80 of the alleles are R and
  therefore 20 are r.

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Hardy-Weinberg Theorem

• Describes a population that does not evolve
• Theorem states that frequencies of alleles and
  genotypes in a populations gene pool remain
  constant over the generations unless acted upon
  by agents other than Mendelian segregation and
  recombination of alleles
• Or shuffling of alleles due to meiosis and random
  fertilization has no effect on the overall gene
  pool of a population

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Hardy-Weinberg Theorem

• Wildflower example
• 500 plants (0.8 R and 0.2 r)
• Assume the union of sperm and egg are random, so
  each gamete has an equal chance of fertilizaion
• Every time a gamete is drawn from the population
  it has a 0.8 chance of being R and 0.2 chance of
  being r

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Cont.

• So now lets use the rule of multiplication to
  determine the genotypes of individuals in the
  next generation
• Chance of being RR 0.8 x 0.8 or 0.64
• Chance of being rr 0.2 x 0.2 or 0.04
• Chance of being Rr 0.8 x 0.2 plus 0.2 x 0.8 or
  0.32

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Hardy Weinberg Equation

• For a gene where only two alleles occur we use
  the letter p to represent frequency of one
  allele and q to represent the other
• p q 1 (p0.8 and q0.2)
• So therefore p1-q and q1-p
• When gametes combine their alleles to form
  zygotes the probability of generating an RR
  individual genotype is p2 (p2 0.64 for our
  example

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Hardy-Weinberg Equation

• Probability of getting a rr individual is q2
  0.04 for our example
• There are two ways of getting a Rr individual
  depending on which parent supplies the egg
  therefore the frequency of heterozygous
  individuals is 2pq (2 x 0.8 x 0.2) or 0.32
• And finally p2 2pq q2 1

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Population Genetics and Health Science

• We can use the equation to determine allele
  frequencies for an allele
• Example 1 out of 10,000 babies in the US has PKU
  (homozygous recessive disorder) so therefore q2
  0.0001 (1/10000)
• To find the allele frequency we use square root
  of 0.0001 to find that q0.01 and p0.99. Now we
  can find the proportion of individuals who do not
  carry the allele and people who are carriers
• P2 .9801 (homozygous dominant
• 2pq 0.0198 (heterozygous) and q2 0.0001

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Hardy-Weinberg Assumptions

• For a population NOT to evolve it has to meet the
  following requirements
• Very large population size
• No migration
• No mutations
• Random mating
• No natural selection

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Causes of Microevolution

• Evolution generation to generation change in a
  populations frequencies of alleles
• Two main causes of microevolution are
• Genetic Drift
• Natural Selection

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Genetic Drift

• Toss a coin 1000 times and get 700 heads and 300
  tails and you would be suspicious about that coin
• Flip it 10 times and get 7 heads and 3 tails and
  that would not be out of reason
• This result is called sampling error because the
  sample is so small
• This logic applies to alleles
• Remember that a zygote receives an allele from
  both parents, and the larger the population the
  more closely the new generation will resemble the
  parent generation

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Genetic Drift
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Genetic DriftBottleneck Effect

• Chance disastrous events lead to a reduction in
  alleles for a population
• Reduces variability

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Genetic DriftFounder Effect

• Genetic drift is also likely when a few
  individuals colonize an isolated island lake or
  other new habitat.
• The smaller the sample size, the less genetic
  makeup of the colonists will represent the gene
  pool of the large population they left

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Natural Selection

• Hardy-Weinberg equilibrium states that all
  individuals have an equal chance at reproduction
  and survival
• Never completely met
• Wildflower example white flowers may be more
  visible to herbivores and therefore leave fewer
  offspring and so the r allele is not passed along
  as successfully as the R allele

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Gene Flow

• Migration is gene flow
• Gene flow reduces differences between populations

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Mutation

• Mutation change in an organisms DNA
• New mutation that is transmitted in gametes can
  immediately change the gene pool by replacing one
  allele for another
• Original source of genetic variation upon which
  natural selection acts

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Genetic Variation

• Individuals vary within and among populations
• Variation within populations
• Quantitative characters heritable variations
  consist of characters that vary along a continuum
  (short to tall and everything in between)
• Discrete characters either or characters (red
  flowers or white flowers)

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Genetic Variation

• Variation between populations
• Most species exhibit geographic variation

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Mutation and Sexual Recombination

• These two things generate Genetic Variation
• New alleles arise only by mutation
• Most mutations are not passed on to offspring
• Even ones that get passed on are usually unseen
• Very small percentage get passed on to offspring
  and provide some benefit

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Closer look at Natural Selection

• Survival of the fittest not really
• The genotypes of individuals who reproduce more
  successfully will be passed on
• Relative fitness contribution of a genotype to
  the next generation compared to the contribution
  of alternative genotypes for the same locus
• If red flowers are the most successful at
  reproducing then their fitness is set at 1
• If white flowers produce only 80 as many
  offspring then their fitness is 0.8

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Effects of Natural Selection
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Natural Selection maintains Sexual Reproduction

• Asexual reproduction is far more efficient and
  produces more individuals

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Sexual Selection and Sexual Dimorphism

• Males and females of animal species are usually
  quite different
• Male cardinals are red while females are drab
  brown
• Male peacocks have large feather and females
  dont
• Intrasexual selection characters arise due to
  competition among males for a resource
• Intersexual selection characters arise due to
  female choosiness

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Natural Selection Cannot Produce Perfection

• Evolution is limited by historical constraints
• Adaptations are often compromises
• Not all evolution is adaptive
• Selection can only edit existing variations