The Evolution Of Populations

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The Evolution Of Populations
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Evolution and Variation

• Microevolution- small scale evolution change in
  allele frequencies in a population over
  generations.
• Discrete Characters- classified on an either-or
  basis
• Quantitative Characters- vary along a continuum
• Average Heterozygosity- (gene variability) the
  average percent of loci that are heterozygous.
• Nucleotide Variability- comparing DNA sequences
  of two individuals
• Geographic Variation- differences in genetic
  composition of separate populations.

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Mutation

• Mutation- the ultimate source of new alleles
• Point mutations- a change in one base in a gene
• Neutral and Beneficial Mutations
• Mutations Rates
• Plants/Animals- 1/100,000 genes per generation
• Prokaryotes- fewer mutations, shorter generation
  span, more genetic variation
• Viruses- more mutations, shorter generation span,
  RNA genome with fewer repair mechanisms

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Gene Pools and Allele Frequency

• Population- a group of individuals of the same
  species that live in the same area and
  interbreed, producing fertile offspring.
• Gene pool- all of the alleles for all the loci in
  all individuals of the population.
• Fixed- only one allele exists for a particular
  locus and all individuals are homozygous for that
  allele

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Hardy-Weinberg Principle

• H-W Equilibrium describes a constant frequency of
  alleles within a gene pool.
• p2 2pq q2 1
• where p2 and q2 represent the frequencies of the
  homozygous genotypes and 2pq represents the
  frequency of the heterozygous genotype

Alleles in the population
Frequencies of alleles
Gametes produced
p frequency of
Each egg
Each sperm
CR allele 0.8
q frequency of
80 chance
80 chance
20 chance
20 chance
CW allele 0.2
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Hardy-Weinberg Assumptions

• No mutations
• Random mating
• No natural selection
• Extremely large population size
• No gene flow
• Departure from any of these conditions usually
  results in evolutionary change.

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Practice Hardy-Weinberg Problem

• For a locus with two alleles (A and a) in a
  population at risk from an infections
  neurodegenerative disease, 16 people had genotype
  AA, 92 had genotype Aa, and 12 had genotype aa.
  Use the Hardy-Weinberg equation to determine
  whether this population appears to be evolving.

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mechanisms that alter allele frequency

• Natural selection
• Leads to adaptive radiation
• Genetic drift
• Founder Effect- a few individuals become isolated
  from the larger population- establish new
  population with different allele frequencies.
• Bottleneck Effect- a sudden change in the
  environment causing reduction in population size
  and altering allele frequency. Ex fires, flood
• Gene Flow- the transfer of alleles into or out of
  a population due to the movement of fertile
  individuals or their gametes.

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Founder Effect
10
B
Fig. 23-9
Original population
Bottlenecking event
Surviving population
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Effects of Genetic Drift

1. Genetic drift is significant in small populations
2. Genetic drift causes allele frequencies to change
   at random
3. Genetic drift can lead to a loss of genetic
   variation within populations
4. Genetic drift can cause harmful alleles to become
   fixed

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Natural Selection and Adaptive Evolution

• Relative Fitness- the contribution an individual
  makes to the gene pool of the next generation,
  relative to the contributions of other
  individuals.

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Directional Selection
Original population
Frequency of individuals

• Occurs when conditions favor individuals
  exhibiting one extreme of a phenotypic range.

Phenotypes (fur color)
Original population
Evolved population
(a) Directional selection
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Disruptive Selection
Fig. 23-13b
Original population
Frequency of individuals

• Occurs when conditions favor individuals at both
  extremes of a phenotypic range over individuals
  with intermediate phenotypes.

Phenotypes (fur color)
Evolved population
(b) Disruptive selection
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StabilizingSelection
Fig. 23-13c
Original population
Frequency of individuals

• Acts against both extreme phenotypes and favors
  intermediate variants.

Phenotypes (fur color)
Evolved population
(c) Stabilizing selection
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• Sexual Selection
• a form of natural selection in which individuals
  with certain inherited characteristics are more
  likely than other individuals to obtain mates.
• Sexual Dimorphism
• marked differences between the two sexes in
  secondary sexual characteristics, which are not
  directly associated with reproduction or
  survival.
• Intrasexual Selection
• Selection within the same sex. Individuals of
  one sex compete directly for mates of the
  opposite sex.
• Intersexual Selection
• mate choice- individuals of one sex (usually
  females) are choosy in selecting their mates from
  the other sex.

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Preservation of Genetic Variation

• Diploidy
• Hides genetic variation from selection in the
  form of recessive alleles
• Balancing Selection
• Occurs when natural selection maintains two or
  more forms in a population.
• Heterozygote Advantage
• Individuals who are heterozygous at a particular
  locus have greater fitness than do both kinds of
  homozygotes
• Frequency-Dependent Selection
• The fitness of a phenotype declines if it becomes
  too common in the population
• Neutral Variation
• Has no selective advantage or disadvantage

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Why Natural Selection Cannot Fashion Perfect
Organisms

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

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Exit Slip

• Of all the mutations that occur in a population,
  why do only a small fraction become widespread
  among the populations members?
• If a population stopped reproducing sexually (but
  still reproduced asexually), how would its
  genetic variation be affected over time? Explain.