THE EVOLUTION OF POPULATIONS

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THE EVOLUTION OF POPULATIONS
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The Origin of Species

• Most biologists were convinced that evolution
  occurs
• Many were still skeptical of natural selection
  as the mechanism of evolution
• Understanding of inheritance was missing
• How does variation arise?
• How are traits transmitted?
• Mendels discoveries were not well circulated

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MENDEL REDISCOVERED

• Mendels research rediscovered in early 1900s
• Many felt that Mendels Laws of Inheritance were
  contrary to Darwins natural selection
• Darwin focused mainly on quantitative characters
• Influenced by multiple gene loci
• Polygenic traits
• Mendel focused on discrete either-or traits
• e.g., Flowers are either purple or white

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POPULATION GENETICS

• Began in 1930s
• Recognizes and emphasizes
• Extensive variation within populations
• Importance of quantitative characters
• Mendelism and Darwinism were reconciled
• Genetic basis of variation and natural selection
  were worked out

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MODERN SYNTHESIS

• Comprehensive theory of evolution
• Began in early 1940s
• Integrated discoveries from
• Paleontology
• Taxonomy
• Biogeography
• Population genetics

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MODERN SYNTHESIS

• Theodosius Dobzhansky (geneticist)
• Sewall Wright (geneticist)
• Ernst Mayr (biogeographer/taxonomist)
• George Gaylord Simpson (paleontologist)
• G. Ledyard Stebbins (botanist)

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MODERN SYNTHESIS

• Key points of emphasis
• Importance of populations as units of evolution
• Populations evolve, individuals do not
• Central role of natural selection
• Most important mechanism of evolution
• NOT the only mechanism of evolution
• Idea of gradualism
• Changes occur over long periods of time
• Accumulation of small changes ? large changes

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POPULATION GENETICS

• Population
• A localized group of individuals belonging to the
  same species
• Species
• A group of populations whose individuals have to
  ability to interbreed and produce fertile
  offspring
• Gene pool
• All alleles of all genes of all individuals
  within a population

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ALLELE FREQUENCY

• Each locus (gene) is represented twice within
  diploid individuals
• If all members of a population are homozygous for
  a particular allele, the allele is fixed
• Generally, two or more alleles exist for a gene
• i.e., Most alleles are not fixed
• Each allele has a relative frequency in the gene
  pool

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ALLELE FREQUENCY

• Allele frequencies affect genotype frequencies
• For example
• If the frequency of A 0.4, and
• If the frequency of a 0.6
• Then
• The frequency of AA 0.4 0.4 0.16
• The frequency of Aa 2 0.4 0.6 0.48
• The frequency of aa 0.6 0.6 0.36
• The frequency of AA Aa aa 1.00 (100)

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HARDY-WIENBERG EQUATION

• Let p the frequency of A
• Let q the frequency of a
• p q 1 (assuming there are only two alleles)
• Then
• The frequency of AA p2
• The frequency of Aa 2pq
• The frequency of aa q2
• (p q)2 p2 2pq q2 1

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HARDY-WIENBERG EQUATION
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HARDY-WIENBERG EQUATION

• 1/10,000 babies born in the Unites States have
  the genetic disorder phenylketonuria (PKU)
• q2 1/10,000 0.0001
• q (0.0001)½ (square root)
• q 0.01
• p 1 - q 1 0.01 0.99
• p2 (0.99)2 0.09801 0.98
• 2pq 2(0.99)(0.01) 0.0198 0.02
• q2 (0.01)2 0.0001

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HARDY-WEINBERG THEOREM

• Allelic and genotypic frequencies will remain
  constant within a population over generations as
  long as the following conditions are met
• Very large population size
• No migration
• No mutations
• Random mating
• No natural selection
• How often are these conditions met?

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HARDY-WEINBERG THEOREM

• If the conditions of the Hardy-Weinberg
  equilibrium are essentially never met, then why
  bother with it?

?
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HARDY-WEINBERG THEOREM

• As long as the conditions of the Hardy-Weinberg
  Equilibrium are met
• Allelic and genotypic frequencies will not change
• The population will not evolve
• A change in allelic or genotypic frequencies
  within a population over time evolution
• The Hardy-Weinberg Theorem describes a
  non-evolving population

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HARDY-WEINBERG THEOREM

• If one or more of the conditions of the
  Hardy-Weinberg Equilibrium is not met
• Allelic and genotypic frequencies will change
• The population will evolve
• A change in allelic or genotypic frequencies
  within a population over time evolution
• If allelic and/or genotypic frequencies change,
  it is because one or more of these conditions is
  not being met

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HARDY-WEINBERG ASSUMTIONS

• Very large population size
• Random changes in allelic frequencies due to
  sampling error are termed genetic drift
• Random fluctuations in the gene pool are more
  pronounced in smaller populations
• What is the chance that you will get 70 heads
  when you flip a coin ten times?
• One hundred times?
• One thousand times?

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GENETIC DRIFT
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BOTTLENECK EFFECT

• Natural disasters or other factors may
  drastically reduce the size of a population
• Remaining population does not accurately
  represent the original populations gene pool
• Overall genetic variability is reduced
• Genetic drift will continue to affect the
  populations gene pool until the population size
  increases
• Minimizing genetic drift is very important in
  managing endagered species

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BOTTLENECK EFFECT

• Cheetahs once widespread in Africa and Asia
• Population bottleneck 10,000 years ago
• Last ice age
• Second bottleneck in 19th century
• Overhunted to near extinction
• Genetic variability is almost nonexistent
• Tissue rejection from organ transplants is
  essentially nonexistent

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FOUNDER EFFECT

• Genetic drift is significant whenever a small
  number of individuals from a larger population
  colonizes a new area
• Subpopulation does not represent parent
  population
• Founder effect
• Accounts for the high frequency of certain
  genetic disorders in human populations
  established by a small population of colonists

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HARDY-WEINBERG ASSUMTIONS

• No migration
• A population may gain or lose alleles as
  individuals (or gametes) enter or leave the
  population
• Gene flow

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HARDY-WEINBERG ASSUMTIONS

• No mutation
• Mutations are heritable changes in DNA
• Produce new alleles
• Ultimate source of genetic variation
• Relatively rare
• Frequency of new alleles can change if acted upon
  by natural selection

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HARDY-WEINBERG ASSUMTIONS

• Random mating
• Mating is random with respect to certain traits
• What blood type do you find attractive?
• Mating is not random for many traits
• Skin color, eye color, hair color, etc.
• Non-random mating will affect genotype
  frequencies, but will not affect allelic
  frequencies

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HARDY-WEINBERG ASSUMTIONS

• No natural selection
• Hardy-Weinberg equilibrium requires that all
  individuals are equally likely to survive and
  reproduce
• This condition is never met
• Natural selection is the primary mechanism of
  adaptive evolution

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HARDY-WEINBERG THEOREM

• Natural selection ? adaptive evolution
• Small population size ? genetic drift
• Migration ? gene flow
• Mutations ? new genetic variation
• Random mating ? altered genotype frequencies

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GENETIC VARIATION

• Exists within populations
• Exists between populations
• Some is heritable
• Some is not heritable

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GENETIC VARIATION

• Variation within populations
• Most consists of quantitative characters
• Vary along a continuum
• e.g., Skin color, height, etc.
• Some consists of discrete characters
• e.g., Mendels discrete either-or inheritance

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GENETIC VARIATION

• Variation within populations Polymorphism
• Applies only to discrete characters
• Two different forms of a character are called
  morphs
• Populations are polymorphic for a character if
  two or more forms are each represented in high
  enough frequencies to be readily noticeable

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GENETIC VARIATION

• Variation between populations
• Most species exhibit geographic variation
• Differences in gene pools between populations or
  subgroups of populations
• Can occur both between and within populations
• Natural selection can contribute to this
  variation
• Some environmental factors are likely to be
  different in different places

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GENETIC VARIATION

• Variation between populations
• Cline
• Graded change in some trait along a geographic
  axis
• e.g., Increase in body size in many North
  American birds and mammals with increasing
  latitude
• Decreased surface area-to-volume ratio
  decreases heat loss

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GENETIC VARIATION

• Genetic variation is generated through
• Mutation
• Sexual recombination

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GENETIC VARIATION

• Mutations
• Heritable change in DNA sequence
• Rare and random
• Ultimate source of new alleles
• The frequency of mutant alleles can be altered by
  genetic drift and natural selection
• e.g., Mutation rates in HIV are very high due to
  the inability of reverse transcriptase to correct
  errors
• HIV evolves very quickly

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GENETIC VARIATION

• Sexual recombination
• Recombines existing alleles in new combinations
• Independent segregation of chromosomes during
  meiosis
• Crossing over during meiosis
• Combination of alleles from two separate
  individuals

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GENETIC VARIATION

• Natural selection can reduce variation by
  removing unfavorable genotypes
• Variation is preserved by
• Diploidy
• Balanced polymorphisms

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GENETIC VARIATION

• Diploidy
• Much genetic variation is hidden from selection
• Recessive alleles in heterozygotes
• These alleles may not be suitable for present
  conditions, but may be beneficial if the
  environment changes

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GENETIC VARIATION

• Balanced polymorphisms
• Maintenance of stable frequencies of two or more
  phenotypic forms within a population
• Natural selection may preserve variation
• e.g., Heterozygote advantage
• e.g., Density-dependent selection

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HETEROZYGOTE ADVANTAGE

• Two b-globin alleles
• HbA HbS
• Three genotypes
• HbA/HbA
• Susceptible to malaria
• HbA/HbS
• Best genotype
• HbS/HbS
• Sickle-cell disease

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DENSITY-DEPENDENT SELECTION

• Survival and reproduction of any one morph
  declines if that phenotypic form becomes too
  common in the population
• e.g., Parasitic infection rates are higher in
  more common morphs of certain snails
• Parasitic infection involves binding to host
  receptors, which differ between morphs

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EVOLUTIONARY FITNESS

• a.k.a., Darwinian fitness or relative fitness
• Relative contribution an individual makes to the
  next generation relative to the contributions of
  other individuals
• Depends on phenotypic features of individual

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EFFECT OF SELECTION

• Natural selection can affect the frequency of a
  heritable trait in a population in three
  different ways
• Directional selection
• Diversifying selection
• Stabilizing selection

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EFFECT OF SELECTION

• Directional selection is most common in periods
  of environmental change or upon migration into a
  new habitat
• e.g., Evolution of beak size in medium ground
  finch of the Galapagos Islands
• Small seeds plentiful in wet years
• Large seeds more important in dry years (all
  seeds important)

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EFFECT OF SELECTION

• Diversifying selection occurs when environmental
  conditions favor individuals on both extremes of
  a phenotypic range over intermediate phenotypes
• e.g., Black-bellied seedcrackers (Cameroon, West
  Africa)
• Small-billed feed mainly on soft seeds
• Large-billed specialize in cracking large seeds
• Intermediate-billed inefficiently use both

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EFFECT OF SELECTION

• Stabilizing selection acts against extreme
  phenotypes and favors the more common
  intermediate variants
• Reduces variation and maintains status quo
• e.g., Most human birth weights between 3 4
  kilograms

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SEXUAL REPRODUCTION

• Why bother with sexual reproduction?
• Reduced reproductive output relative to asexual
  reproduction

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SEXUAL REPRODUCTION

• Why bother with sexual reproduction?
• Meiosis and fertilization generate genetic
  variation upon which natural selection can act

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SEXUAL SELECTION

• Males and females of many species differ in
  secondary sexual characteristics
• Not directly associated with reproduction
• Sexual dimorphism
• e.g., Males may be larger or more colorful, than
  females, possess elaborate manes, tails, or
  other structures, etc.
• Sexual dimorphism is a product of sexual
  selection

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SEXUAL DIMORPHISM
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SEXUAL SELECTION

• Intrasexual selection
• Direct competition between members of one sex for
  mates of the opposite sex
• In vertebrates, males usually compete for females

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SEXUAL SELECTION

• Intersexual selection
• A.k.a., Mate choice
• Individuals of one sex (usually females) are
  choosy in selecting mates of the other sex
• Males with certain features are more attractive

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PERFECT ORGANISMS

• Natural selection cannot fashion perfect
  organisms
• Evolution is limited by historical constraints
• Ancestral anatomy is modified, not replaced
• Adaptations are often compromises
• e.g., Flippers help a seal swim, but are not as
  efficient when it walks on land
• Not all evolution is adaptive
• Chance affects the genetic structure of
  populations to a great degree
• Not all fixed alleles are better than the alleles
  that were lost
• Selection can only edit existing variation
• New alleles do not arise on demand natural
  selection can only favor the fittest variations
  from available phenotypes