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    BONUS QUIZZES GO LIVE TODAY!

    Starting today, you can receive points for your responses to
    questions embedded in the lectures.

    You can receive points ONLY if you click on the icon to the left
    and submit the requested response/agreement
    .

    If I do not have your response recorded before you answer
    any quiz question, you will not receive bonus points.

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(click on pic for source)

    Evolutionary Force #2:
    Non-infinite Population Size

    Random evolutionary change is caused by

    • mutation
    • non-infinite population size
    • migration
    • non-random mating
    • natural selection

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    Molecular Clock Relies on Neutral Mutations

    The Molecular Clock Hypothesis states that
    • DNA evolves at a relatively constant rate
      • over time
      • among different organisms

    • The genetic difference between any two species is proportional
      to the time since they last shared a common ancestor.

    But the Molecular Clock works only if the mutations in question
    do not affect the Darwinian fitness of the organism inheriting them.

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    The Neutral Theory of Molecular Evolution

    In 1968, Motoo Kimura proposed that a large proportion of new mutations
    did not affect their host's Darwinian fitness.

    These neutral mutations should not be subject to natural selection
    and would either

    • spread throughout a population be present in all members
      OR
    • be lost entirely

      ... in a random process called genetic drift.

    Neutral mutations, by definition, do not affect the Darwinian fitness of the individual bearing them.

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from: Casillas, Sonia and Barbadilla, Antonio. (2017) Molecular Population Genetics. Genetics. 205. 1003. 10.1534/genetics.116.196493.

    Changing Alleles Randomly

    When a wild type allele mutates, the new mutant allele
    is substituted for the original allele.

    Kimura demonstrated that, for neutral mutations,
    rate of substitution = rate of mutation

    Thus, if mutation rate is constant across species,
    then substitution rate should remain constant throughout the Tree of Life.

    This is the basic premise of the Molecular Clock Theory.

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(click on pic for more Neutral Theory information)

(In reality, mutation rates are not entirely constant across genomes nor across time.

However, modifications to the Neutral Theory are ongoing,
ensuring that it is still a highly useful model.)

    Not Dissing Darwin

    The neutral theory does not imply that
    • organisms are not adapted to their environments
    • all morphological variation is neutral
    • all genetic variation is neutral
    • natural selection does not contribute to evolution

    The neutral theory does imply that all populations
    exhibit genetic polymorphisms that may or may not
    affect the Darwinian fitness of individuals having the variants.

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    Gene Flow

    Gene flow is the transfer of genetic variation
    from one population to another.

    Gene flow tends to homogenize the population in which it is occurring.
    Lack of gene flow can lead to genetic differentiation of isolated demes.

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    Biogeography and Genetic Change

    Recall that biogeography is the distribution and diversity of living things and ecosystems over time and space.

    The effects of geographical separation on the evolution of populations can be observed at any many levels:

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    Distributions Mirror Continental Drift

    The original supercontinent, Pangaea, split into

    Gondwanaland to the south,
    which further split to give rise to
    • Africa (Columbian "Old World")
    • South America (Columbian "New World")
    • Antarctica
    • Australia
    • India
    • Arabian peninsula
  • Laurasia to the north,
    which further split to give rise to
    • Eurasia (Columbian "Old World")
    • North America (Columbian "New World")

  • Living organisms rode the land masses as they drifted apart.
    Separated populations became reproductively isolated.
    In isolation, the populations genetically diverged.

    The evolution of related lineages can be traced by comparing
    the descendants' current geographic distributions with their genetic makeup.

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    Island Biogeography: Repeated Cladogenesis

    The same patterns can be observed on a smaller scale
    on islands derived from a single land mass that split.

    In this example...

      1. Ancestral taxon X lives on the orignal island.
      2. The island splits, dividing the X population.
      3. Ancestral X populations evolve into descendant taxa A and B.
      4. The island housing B splits, dividing the B population.
      5. Ancestral B populations evolve into descendant taxa C and D.
      6. A mountain range on D's island splits the D population.
      7. Ancestral D populations evolve into descendant taxa E and F.
      8. Some individuals from population E migrate to a new island.
      9. Ancestral E populations evolve into descendant taxa G and H.

      (Note: Once an ancestral taxon splits into descendant taxa
      the ancestor is considered to be extinct. Remember that it existed
      in the past, and its descendants are different individuals.)

      In all of these instances, the size of the separated population can have a profound effect on how rapidly they are likely to undergo evolutionary change.

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(artwork by Oliver Cleve)

    It's All About Random Chance

    Does every possible allele of every gene make it into the next generation?

    • n = an individual's number of genes
    • 2n = its number of genetically unique gametes
    • The average mammal has about 20,000 genes.
    • The potential: 220000 genetically unique gametes

    Clearly, not every gamete ends up as a zygote.

    This is the basis of Genetic Drift.

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    Genetic Drift: Genetic Change due to Sampling Error

    Genetic Drift is the genetic change in a population
    that occurs due only to the random chance of some alleles
    not making it into the next generation.

    • The smaller the population, the smaller the gene pool.

    • The smaller the gene pool, the lower the genetic diversity.

    • The smaller the gene pool, the more likely that any given allele
      will be lost due only to random chance.

    Two flavors: Founder Effect and Bottleneck Effect

    Genetic drift is a major evolutionary force.
    Although it works more quickly in small populations,
    genetic drift will contribute to evolution in any population
    that is not infinitely large.

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Founder Effect:

    Founder Effect

    The Founder Effect describes genetic drift
    due to random loss of alleles in a small population that

    • has colonized a new area separated from its source population
    • can no longer interbreed with the source population

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Bottleneck Effect:

    Bottleneck Effect

    Bottleneck Effect describes genetic drift
    due to random loss of alleles in a small population that

    • remains in its original source population location
    • is small because of a random, catastrophic event
      that killed off the rest of the source population

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