The Evolution of Populations

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Chapter 23

• The Evolution of Populations

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Populations evolve

• Natural selection acts on individuals
• differential survival
• survival of the fittest
• differential reproductive success
• who bears more offspring
• Populations evolve
• genetic makeup of population changes over time
• favorable traits (greater fitness) become more
  common

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Populations gene pools

• Concepts
• a population is a localized group of
  interbreeding individuals
• population genetics if the study of how
  populations change over time
• gene pool is collection of alleles in the
  population
• remember difference between alleles genes!
• allele frequency is how common is that allele in
  the population
• how many A vs. a in whole population

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Hardy-Weinberg equilibrium

• Hypothetical, non-evolving population
• Frequencies of alleles and genotypes in a
  populations gene pool remain constant from
  generation to generation
• Only Mendelian segregation, recombination, and
  random mating are at work
• Serves as a model
• natural populations rarely in H-W equilibrium
• useful model to estimate the percentage of a
  population carrying an allele for an inherited
  disease

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Evolution of populations

• Evolution change in allele frequencies in a
  population
• hypothetical what conditions would cause allele
  frequencies to not change?
• non-evolving population
• REMOVE all agents of evolutionary change
• very large population size (no genetic drift)
• no migration (no gene flow in or out)
• no mutation (no genetic change)
• random mating (no sexual selection)
• no natural selection (everyone is equally fit)

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Hardy-Weinberg theorem

• Counting Alleles
• assume 2 alleles B, b
• frequency of dominant allele (B) p
• frequency of recessive allele (b) q
• frequencies must add to 1 (100), so
• p q 1

bb
Bb
BB
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Hardy-Weinberg theorem

• Counting Individuals
• frequency of homozygous dominant p2
• frequency of homozygous recessive q2
• frequency of heterozygotes 2pq
• frequencies of all individuals must add to 1
  (100), so
• p2 2pq q2 1

bb
Bb
BB
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H-W formulas

• Alleles p q 1
• Individuals p2 2pq q2 1

bb
Bb
BB
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Using Hardy-Weinberg equation
population 100 cats 84 black, 16 white How many
of each genotype?
q2 (bb) 16/100 .16 q (b) v.16 0.4 p (B) 1
- 0.4 0.6
p2.36
2pq.48
q2.16
bb
Bb
BB
What are the genotype frequencies?
Must assume population is in H-W equilibrium!
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Using Hardy-Weinberg equation
p2.36
2pq.48
q2.16
Assuming H-W equilibrium
bb
Bb
BB
Null hypothesis
p2.74
2pq.10
q2.16
p2.20
2pq.64
q2.16
Sampled data
How do you explain the data?
How do you explain the data?
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Variation natural selection

• Variation is the raw material for natural
  selection
• there have to be differences within population
• some individuals must be more fit than others

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Where does Variation come from?

• Mutation
• random changes to DNA
• errors in mitosis meiosis
• environmental damage
• point mutations, duplications
• Sex
• mixing of alleles
• recombination of alleles
• new arrangements in every offspring
• new combinations new phenotypes
• spreads variation
• offspring inherit traits from parent

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5 Agents of evolutionary change
Mutation
Gene Flow
Non-random mating
Genetic Drift
Selection
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1. Mutation Variation

• Mutation creates variation
• new mutations are constantly appearing
• 1 mutation in every 100,000 genes every
  generation
• Mutation changes DNA sequence
• changes amino acid sequence?
• changes protein?
• changes structure?
• changes function?
• changes in protein may change phenotype
  therefore change fitness

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2. Gene Flow

• Movement of individuals alleles in out of
  populations
• seed pollen distribution by wind insect
• migration of animals
• sub-populations may have different allele
  frequencies
• causes genetic mixing across regions
• reduce differences between populations

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3. Non-random mating

• Sexual selection

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4. Genetic drift

• Explains how allele frequencies can fluctuate
  unpredictably
• founder effect
• small group splinters off starts a new colony
• bottleneck
• some factor (disaster) reduces population to
  small number then population recovers
  expands again

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Founder effect

• When a new population is started by only a few
  individuals
• some rare alleles may be at high frequency
  others may be missing
• skew the gene pool of new population
• human populations that started from small group
  of colonists

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Bottleneck effect

• When large population is drastically reduced by a
  disaster
• famine, natural disaster, loss of habitat
• loss of variation by chance event
• alleles lost from gene pool
• not due to fitness
• narrows the gene pool

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Cheetahs

• All cheetahs share a small number of alleles
• less than 1 diversity
• as if all cheetahs are identical twins
• 2 bottlenecks
• 10,000 years ago
• Ice Age
• last 100 years
• poaching loss of habitat

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5. Natural selection

• Differential survival reproduction due to
  changing environmental conditions
• climate change
• food source availability
• predators, parasites, diseases
• toxins
• combinations of alleles that provide fitness
  increase in the population
• adaptive evolutionary change

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• Natural selection is the primary mechanism of
  adaptive evolution
• Natural selection
• Accumulates and maintains favorable genotypes in
  a population
• Genetic variation occurs in individuals in
  populations
• is not always heritable

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Variation Between Populations

• Most species exhibit geographic variation
• Differences between gene pools of separate
  populations or population subgroups

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• Some examples of geographic variation occur as a
  cline, which is a graded change in a trait along
  a geographic axis

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Evolutionary Fitness

• The phrases struggle for existence and
  survival of the fittest
• Are commonly used to describe natural selection
  can be misleading
• Reproductive success
• Is generally more subtle and depends on many
  factors
• Fitness individual contribution to gene pool of
  next generation (ex wild flowers attract
  pollinators)
• Relative Fitness- genotype contribution to next
  generation compared to other genotypes (ex red
  wildflowers produce less offspring than white or
  pink)

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Types of Selection

• Selection
• Favors certain genotypes by acting on the
  phenotypes of certain organisms
• Three modes of selection are
• Directional Favors individuals at one end of the
  phenotypic range
• Disruptive Favors individuals at both extremes
  of the phenotypic range
• Stabilizing Favors intermediate variants and
  acts against extreme phenotypes

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• The three modes of selection

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The Preservation of Genetic Variation

• Various mechanisms help to preserve genetic
  variation in a population
• Diploidy
• Maintains genetic variation in the form of hidden
  recessive alleles
• Balancing selection
• Occurs when natural selection maintains stable
  frequencies of two or more phenotypic forms in a
  population
• Leads to a state called balanced polymorphism

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Application of H-W principle

• Sickle cell anemia
• inherit a mutation in gene coding for hemoglobin
• oxygen-carrying blood protein
• recessive allele HsHs
• normal allele Hb
• low oxygen levels causes RBC to sickle
• breakdown of RBC
• clogging small blood vessels
• damage to organs
• often lethal

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Sickle cell frequency

• High frequency of heterozygotes
• 1 in 5 in Central Africans HbHs
• unusual for allele with severe detrimental
  effects in homozygotes
• 1 in 100 HsHs
• usually die before reproductive age

Why is the Hs allele maintained at such high
levels in African populations?
Suggests some selective advantage of being
heterozygous
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Malaria
Single-celled eukaryote parasite (Plasmodium)
spends part of its life cycle in red blood cells
1
2
3
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Heterozygote Advantage

• In tropical Africa, where malaria is common
• homozygous dominant (normal)
• die or reduced reproduction from malaria HbHb
• homozygous recessive
• die or reduced reproduction from sickle cell
  anemia HsHs
• heterozygote carriers are relatively free of
  both HbHs
• survive reproduce more, more common in
  population

Hypothesis In malaria-infected cells, the O2
level is lowered enough to cause sickling which
kills the cell destroys the parasite.
Frequency of sickle cell allele distribution of
malaria
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Frequency dependent selection

• In frequency-dependent selection
• The fitness of any morph declines if it becomes
  too common in the population

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Other forms of selection

• Neutral Variation
• Genetic variation that appears to confer no
  selective advantage
• UTR regions of eukaryotic genome
• Sexual selection
• Is natural selection for mating success
• Can result in sexual dimorphism, marked
  differences between the sexes in secondary sexual
  characteristics
• Intrasexual selection
• Is a direct competition among individuals of one
  sex for mates of the opposite sex

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• Intersexual selection
• Occurs when individuals of one sex (usually
  females) are choosy in selecting their mates from
  individuals of the other sex
• May depend on the showiness of the males
  appearance

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Why Natural Selection Cannot Fashion Perfect
Organisms

• Evolution is limited by historical constraints
• Adaptations are often compromises
• Chance and natural selection interact
• Selection can only edit existing variations