The Evolution of Populations Chapter 23

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The Evolution of PopulationsChapter 23

• Associate Professor Pamela L. Pannozzo
• Principles of Biology I BSC 1010

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Closer Examination of the Mechanism of Evolution

• Microevolution change in the genetic makeup of
  a population from generation to generation
• Macroevolution large-scale evolutionary changes
  between species

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Some bent grass individuals (Agrostis tenuis)
tolerate heavy metals
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Population Genetics

• Study of how populations change genetically over
  time
• Populations evolveindividuals do not!
• Mendelian genetics Darwinian theory of
  evolution by natural selection
• Population localized group of individuals
  capable of interbreeding and producing fertile
  offspring
• Types of isolation

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MAP AREA
CANADA
ALASKA
Beaufort Sea
Porcupine herd range
NORTHWEST TERRITORIES
Fairbanks
Fortymile herd range
Whitehorse
ALASKA
YUKON
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Gene Pools and Allele Frequencies

• Gene pool all alleles in a population at any
  one time
• Allele frequency proportion of particular
  allele in a population

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Allele Frequency

• Hypothetical population consists of 500
  individuals, 2 alleles CR CW for flower pigment,
  incomplete dominance
• 320 red flowers (64)(CR CR), 160 pink flowers
  (32)(CR CW), 20 white flowers (4)(CW CW)
• ?
• 320 x 2 640
• 160 x 1 160
• Total CR alleles in the population 800
• 500 individuals x 2 alleles each 1000 alleles
  in the population
• ?
• 800 CR alleles / 1000 total alleles .80 80
  CR allele frequency
• 200 CW alleles / 1000 total alleles .20 20
  CW allele frequency

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What will be the frequency of the possible
genotypes in the next generation?

• p2 2pq q2 1
• p frequency of CR allele
• q frequency of CW allele
• p2 probability of two CR gametes frequency of
  homogenous CRCR genotype
• q2 probability of two CW gametes frequency of
  homogenous CWCW genotype
• 2pq probability of CR CW gametes frequency
  of heterogenous CRCW genotype
• (0.8)(0.8) 2(0.8)(0.2) (0.2)(0.2) 1
• 0.64 0.32 0.4 1
• The next generation will have 64 red flowers (CR
  CR) 32 pink flowers (CR CW) 4 white flowers
  (CW CW)

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The Hardy-Weinberg Theorem

• p2 2pq q2 1
• Frequencies of alleles and genotypes in a
  populations gene pool remain constant from
  generation to generation, provided that only
  Mendelian segregation and recombination of
  alleles are at work
• Describes a hypothetical population that is not
  evolving
• Used as a benchmark to measure changes
• Based on several important assumptions

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Hardy Weinberg Assumptions

• Random mating
• No mutatiions
• Extremely large population size
• No natural selection
• No gene flow

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Real Populations

• The five conditions for non-evolving populations
  are rarely met in nature
• Allele and genotype frequencies do change over
  time
• Caused by departures from any of the Hardy
  Weinberg conditions

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Key Agents of Evolutionary Change

• Mutation
• Origination of new alleles in germ-line cells
• Rate
• Plants/animals 1 mutation/100,000
  genes/generation
• Viruses/bacteria much faster, even daily
• Types
• Point mutations
• Chromosomal mutations
• Rearrangement
• Gene duplication
• Sexual recombination

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Evolutionary Change Caused by Changes in Alleleic
Frequencies

• Three major factors alter allele frequencies
• Genetic drift
• Gene flow
• Natural Selection

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Genetic Drift

• Changes in allele frequencies due to chance
  events
• Bottleneck effect sudden change in the
  environment that may drastically reduce the size
  of a population
• Founder effect a few individuals become
  isolated from a larger population

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CRCR
CWCW
CRCR
CRCR
CRCR
CRCW
CRCW
CRCR
CRCR
Only 5 of 10 plants leave offspring
Only 2 of 10 plants leave offspring
CWCW
CRCR
CRCR
CRCR
CWCW
CRCR
CRCW
CRCW
CRCR
CRCR
CRCR
CWCW
CRCR
CRCW
CRCR
CRCW
CRCR
CRCR
CRCW
CRCW
CRCR
Generation 3 p 1.0 q 0.0
Generation 1 p (frequency of CR) 0.7 q
(frequency of CW) 0.3
Generation 2 p 0.5 q 0.5
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Original population
Bottlenecking event
Surviving population
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Gene Flow

• Immigration or emigration, resulting from
  movement of fertile individuals or gametes
• Population gains or loses alleles
• Homogenizes populations over time

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Natural Selection

• Differential reproductive success passes
  favorable alleles to the next generation in
  greater proportions
• Fitness, relative fitness
• Primary mechanism for adapting a population to
  its environment
• Operates on inheritable genetic variation

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Ways Natural Selection Alters Allele Frequency

• Directional Selection
• Disruptive Selection
• Stabilizing Selection

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Original population
Frequency of individuals
Original population
Evolved population
Phenotypes (fur color)
Directional selection
Disruptive selection
Stabilizing selection
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Mechanisms that Preserve of Genetic Variation

• Recessive alleles
• Balancing Selection
• Heterozygote advantage
• Frequency-dependent selection
• Neutral Variation

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LE 23-13
Frequencies of the sickle-cell allele
02.5
2.55.0
5.07.5
Distribution of malaria caused by Plasmodium
falciparum (a protozoan)
7.510.0
10.012.5
gt12.5
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Sexual Selection

• Intrasexual selection competition among
  individuals of one sex for mates of the opposite
  sex
• Intersexual selection occurs when individuals
  of one sex (usually females) are choosy in
  selecting their mates

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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

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Genetic Variation Within and Among Populations

• Polymorphism differences in discrete
  characteristics within populations
• Geographic isolation results in variation among
  populations
• Clines graded change in a trait along a
  geographic axis

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LE 23-10
1
2.4
3.14
5.18
6
7.15
8.11
9.12
10.16
13.17
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XX
1
2.19
3.8
4.16
5.14
6.7
9.10
11.12
13.17
15.18
XX
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LE 23-11
Heights of yarrow plants grown in common garden
100
Mean height (cm)
50
0
3,000
Altitude (m)
2,000
Sierra Nevada Range
Great Basin Plateau
1,000
0
Seed collection sites