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Microbial Evolution

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Title: Microbial Evolution


1
Microbial Evolution
  • Ecology and Evolution are inextricably connected

2
  • Ecology the study of interactions between
    organisms and their environment (physical,
    chemical and biological conditions)
  •  
  • Evolution changes in the genetic composition of
    a population with the passage of each generation
  • change in allelic frequency in populations over
    time (alleles are different versions of the same
    gene)

3
Consider how the amount of genetic divergence
(change) forms a continuum
  • Microevolution Macroevolution small
    changes large changes

Microevolution adaptation Macroevolution
speciation
4
Four distinct mechanisms generate evolution
(change in allelic frequency in populations over
time)
  • 1. mutation
  • 2. gene flow
  • 3. genetic drift
  • 4. selection (natural and artificial)

5
  • 1. Mutation a heritable change in the
    nucleotide sequence of the genetic nucleic acid,
    resulting in an alteration in the products coded
    for by the gene
  •  

6
  • 2. Gene flow introduction or loss of new
    alleles into the population through immigration
    or emigration.
  •  
  •  

Wilson Bossert, 1971
7
  • 3. Genetic drift stochastic shifts in allele
    frequencies in small populations
  •  
  •  

Wilson Bossert, 1971
8
  • 4. Selection change in allele frequencies over
    generations due to differential survival and
    reproductive success of genotypes
  •  

Darwinian evolution is evolution by natural
selection
9
Natural selection leads to adaptive radiation and
speciation
10
What is the mechanism of natural selection?
  • 1. Genotypes within populations vary and this
    variability is heritable.
  •  
  • 2. Biotic and abiotic components of an organisms
    environment act as selection pressures.
  •  
  • 3. Genotypes that are best adapted to these
    selection pressures leave the most offspring.
  •  

11
Closely examine these three premises
  • What introduces variability among genotypes?

12
Closely examine these three premises
  • What introduces variability among genotypes?
  • Mutations
  • introduce new genetic variation

13
Closely examine these three premises
  • What introduces variability among genotypes?
  • Mutations
  • Plasmids
  • Transformation
  • Transduction
  • Conjugation
  • can all introduce genetic variability to
    bacterial populations

Horizontal gene transfer
14
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15
Closely examine these three premises
  • What introduces variability among genotypes?
  • Mutations
  • Anastomosis
  • can introduce genetic variability to fungal
    populations

16
  •  Populations with diverse gene pools have a lot
    of variation in alleles.
  • How is this variability passed on (heritable)?

17
genotypes pass on this variability through
reproduction
18
genotypes pass on this variability through
reproduction
  • In sexually reproducing organisms (eg. many
    species of algae, zooplankton, fungi, and
    protozoa), recombination occurs with reproduction
    (the genetic deck of cards gets shuffled every
    generation). That means that novel alleles that
    arise through mutations are immediately placed in
    a diversity of genetic environments.

19
genotypes pass on this variability through
reproduction
  • In contrast, recombination is not tied to
    reproduction in asexual organisms (e.g. bacteria,
    archaea, many species of algae, fungi ....).
    Recombination happens in asexual organisms, but
    it is not necessarily tied with reproduction.

20
genotypes pass on this variability through
reproduction
  • Recombination has major ramifications on how
    natural selection acts on variance in the
    populations.
  • Although sexual recombination is rare in bacteria
    (Cohen, 1996), horizontal gene transfer appears
    to be more common than previously thought
    (Pennisi 2004)

21
2. What are selection pressures in an organisms
environment?
22
2. What are selection pressures in an organisms
environment?
  • Examples of biotic factors
  • predators
  • competitors
  • mutualists
  • Examples of abiotic factors
  • resource availability
  • physical conditions
  • chemical conditions

23
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24
Selection can be
  • natural or anthropogenic

25
3. Genotypes that are best adapted to these
selection pressures leave the most offspring
  • Premise 3 leads to the concept of adaptation and
    fitness

26
3. Genotypes that are best adapted to these
selection pressures leave the most offspring
  • Premise 3 leads to the concept of adaptation and
    fitness
  • Adaptation a genetically determined
    characteristic that improves an organisms
    ability to survive and reproduce in a particular
    environment.

27
Premise 3 leads to the concept of adaptation and
fitness
  • Adaptation a genetically determined
    characteristic that improves an organisms
    ability to survive and reproduce in a particular
    environment.
  • Adapt the evolutionary process by which
    organisms become better suited to their
    environments

28
  • Fitness

29
  • Fitness
  • the relative contribution by an individuals
    descendants to future generations.

30
Some important properties of fitness
  • Fitness is specific to a particular
    environment.(Consider both the biotic and abiotic
    environment).
  • As the environment changes, so do the fitness
    values of the genotypes
  • Notice the connection between ecology and
    evolution.

31
Some important properties of fitness
  • Fitness is a property of a genotype, not of an
    individual or a population.
  • Individuals with the same genotype share the
    same fitness within the same environment.
  • Fitness is measured over one generation or more.

32
  • New genotypes and alleles enter the population
    through mutation, immigration (horizontal gene
    transfer) etc. A new genotype that is fitter
    than the current one will gradually replace it.
    If the current genotype cannot be replaced by an
    invading one, it is said to represent the
    evolutionarily stable strategy or ESS (Maynard
    Smith and Price, 1973).

33
  • The concepts of fitness and adaptation are
    relevant ONLY in a particular ecological context.
    There is no such thing as fitness in an absolute
    sense.

34
Which of the 4 evolutionary mechanisms generates
adaptation?1. mutation2. gene flow3. genetic
drift4. selection
35
Which of the 4 evolutionary mechanisms generates
adaptation?1. mutation2. gene flow3. genetic
drift4. selection
  • Only natural selection, the other mechanisms
    generate change, but the change has no linkage to
    improved survival in the environment

36
  • There can be multiple paths to higher fitness in
    response to many but not all types of natural
    selection

(e.g. Contrast the results of Lenskis
experiments of glucose starvation in E. coli with
Bulls experiments with high-temperature stress
in a bacteriophage
37
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38
The role of genetic exchange (recombination of
alleles) in evolution.
  • The paradigm of geographic speciation was
    developed from studies of sexually reproducing
    populations
  • This paradigm assumes
  • Allelic combinations are reshuffled every
    generation.
  • Successful mating only occurs between individuals
    that are closely related.

39
geographic speciation allopatric speciation
40
This paradigm falls apart with bacteria and other
asexual organisms because
  • 1. Allelic combinations are NOT reshuffled every
    generation. Only a small amount of genetic
    material is exchanged (via conjugation,
    transformation, transduction, plasmid transfer).
  • Cohan suggests this exchange happens at a low
    frequency (10-8 to 10-7 exchanges per gene
    segment per genome per generation). But Pennisi
    suggests this exchange rate is MUCH higher,
    particularly in stressful environments.

41
This paradigm falls apart with bacteria and other
asexual organisms because
  • 2. Successful genetic exchange occurs between
    individuals that are NOT closely related
    ("promiscuous genetic exchange").

42
  • The process of periodic selection in bacteria
    purges diversity in populations gene pools.
    (Figure 3 in Cohan, 1996)

43
  • Even with relatively low levels of recombination,
    there is enough genetic exchange, so that diverse
    allelic combinations can arise.
  • (Figure 4 in Cohan, 1996)  

44
Cohan (1996) concluded that
  • Recombination does NOT preserve genetic
    diversity in bacteria.
  • Genetic exchange does NOT threaten the
    integrity of population adaptations.
  • Genetic exchange can transfer adaptations
    across bacterial taxa.

45
Implications of this
  • Adaptive mutations in bacteria have the potential
    to purge diversity from the populations.
  • In contrast, in sexually reproducing organisms,
    adaptive mutation is transferred into many
    genetic backgrounds and does not follow the
    entire genome of the individual carrying the
    original mutation.

46
Implications of this
  • 2. At recombination rates gt 10-5 exchanges per
    gene segment per genome per generation,
    ecologically distinct populations may be
    indistinguishable (variance within populations is
    as great as variance between populations )
    because of sufficient neutral sequence variance.

47
Implications of this
  • 3. Adaptive gene sequences can go ANYWHERE
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