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    Evolutionary Force #5. Natural Selection

    The final factor that can drive evolution is the most familiar
    • mutation
    • non-infinite population size
    • migration
    • non-random mating
    • natural selection

    Of all the forces that drive evolution,
    only natural selection results in organisms
    better suited to live and reproduce in their environment.

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Recall Darwin's four tenets of evolution by means of natural selection:


Huge numbers of individuals are not necessary for natural selection to occur. But this observation contributed to Darwin's idea of a "struggle for existence".

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    Heritable Variability

    Much of the observed variation in traits (polymorphism)
    among individuals in a population is heritable.

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    Competition

    Resources are limited.
    Members of a population must compete for
    • food
    • space
    • mates
    • other resources
    ...as well as avoid disease and predation.

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    Differential Reproduction

    Individuals with heritable, adaptive traits will leave
    a disproportionate number of genes to the next generation
    .

    Over time, those heritable, adaptive traits
    will become more frequent in the population.

    Darwinian fitness, a measure of reproductive success,
    has two components:

    • viability - probability of surviving to reproduce
    • fertility - number of offspring an individual produces

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(click on pic for source)

    Darwinian Fitness Depends on Context

    In a given environmental context, an individual trait (or mutation) can be:
    • adaptive - increases Darwinian fitness
    • maladaptive - decreases Darwinian fitness
    • neutral - does not affect Darwinian fitness

    A trait adaptive in one environment may be maladaptive in another,
    or if the original environment changes.

    The adaptiveness of a trait is determined by environmental context.

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    Natural Selection acts on Phenotype

    The three conditions necessary and sufficient for natural selection are
    • variable heritability
    • competition
    • differential reproduction

    In Darwin's "struggle for existence", heritable traits
    are the difference between conspecific, competing survival machines.

    Conspecifics do not compete against their predators or pathogens.
    They compete against each other.

    Natural selection acts upon (heritable) phenotypes
    to change the genetic composition of populations.

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    Fitness is Relative

    A few definitions.
    • absolute fitness = progeny survival rate (N) of a particular genotype
    • progeny survival rate (N) = Nafter selection / Nbefore selection

    • relative fitness (ω) = N of competing genotype / N of fittest genotype
    • genotype with highest N is assigned relative fitness (ω) of 1.0

      Assuming equal fecundity for all genotypes:

        Genotype:

        Nbefore
        Nafter
        N
        (survival rate)
        ω
        A1A1

        100 80 0.8 1.0
        (NA1A1/NA1A1)
        A1A2

        100 60 0.6 0.75
        (NA1A2/NA1A1)
        A2A2

        100 40 0.4 0.5
        (NA2A2/NA1A1)

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    Selection Coefficient

    The selection coefficient (s) is a measure of selection against a genotype compared to selection against other genotypes in the same population.

    s = 1 - ω
    For our three genotypes above:

    Genotype:

    ω
    (relative fitness)
    1 - ω
    (relative selection)
    A1A1

    1.0
    (NA1A1/NA1A1)
    0
    A1A2

    0.75
    (NA1A2/NA1A1)
    0.25
    (25% more selection against A1A2 than against A1A1)
    A2A2

    0.5
    (NA2A2/NA1A1)
    0.5
    (50% more selection against A2A2 than against A1A1)

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    Natural Selection can be
    Stabilizing, Directional, or Diversifying

    A polymorphic population is usually made up of individuals expressing traits along a continuum.
    This can be expressed as a Gaussian (bell-shaped) curve.

    Natural selection can act on individuals in any region of this curve.

    • stabilizing selection
      • the average phenotype is most adaptive.
      • mode is stable
      • curve may become more narrow

    • directional selection
      • phenotype at one extreme of the range or the other is the most adaptive.
      • mode shifts right or left, depending on selective pressure

    • disruptive (= diversifying) selection
      • the phenotypes at either extreme are more adaptive than the average phenotype.
      • curve becomes bimodal
      • original average phenotype becomes rare or extinct

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    Pocket Mice, Camouflage,
    and Natural Selection

    View the video, then try the bonus questions.


    (a three-pointer!)

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    Directional Selection in Human Populations: Lactose Intolerance

    All mammals produce the enzyme lactase during infancy.
    Lactase, encoded LCT, breaks down lactose (milk sugar).

    In wild type mammals, the gene is permanently silenced after weaning.

    Around 9000 BC, Middle Eastern humans invented dairy agriculture.
    Milk from cows, goats, camels, etc. was a rich source of nutrition.

    But what about all that flatulence? Mutation to the rescue!

    Wild type MCM6 (mini chromosome maintenance complex) gene

    • is involved in silencing LCT expression in adult humans
    • has retained several different mutations in Africa and the Middle East
    • has retained a single mutation in European populations

    Mutant MCM6 product fails to shut down the lactase gene.
    In populations practicing dairy agriculture, these mutations are adaptive!

    Research reveals a relatively small fitness advantage to mutant MCM6.
    In a Scandinavian population it was measured at only ~0.09 - 0.19.
    But over 9000 years, the alleles accumulated in dairy farming populations.
    In some, it has nearly reached fixation.

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Examples of wild type and mutant phenotypes:

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    Why Multiple Alleles?

    If certain alleles are more adaptive than others, why do those others remain?

    The Classical (Darwinian) Model

    In any given population, at any particular locus,
    one allele functions better than the others.

    Natural selection will drive the population
    to a higher frequency of the more adaptive allele.

      But it's not always that simple.

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Variant frequencies change over time during natural selection.
Different colors represent the different variants of a gene. Most adaptive mutations do not reach 100% frequency but are maintained at intermediate, balanced frequencies until they are displaced by new beneficial mutations.
(click on pic for reference)

    The Balancing Selection Model

    Multiple alleles at a particular locus can be maintained in the gene pool
    if an allele that is maladaptive under certain conditions is adaptive
    under different conditions.

    In any population, balancing selection can

    • prevent one adaptive allele from displacing other alleles
    • maintain stable frequencies of two or more phenotypes
    • this is known as balanced polymorphism

    Balancing selection can occur via

    • heterozygote advantage
    • sex-influenced expression
    • frequency dependent selection

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    Balancing Selection: Sex-influenced Expression

    Heterozygote advantage describes the phenomenon of heterozygotes
    having higher fitness than either homozygote at a given gene locus.

    A slight variation on this theme has been reported regarding...

    Male Human Homosexuality
    Observation:

    • female maternal relatives of male homosexuals have higher fecundity than female maternal relatives of male heterosexuals.
    • This disparity was not found in female paternal relatives.

    Inference:

    • One to several (currently unidentified) heritable alleles that predispose a male to homosexuality could be adaptive when they occur in females.

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    Balancing Selection: Frequency-Dependent Selection

    In Frequency-dependent selection occurs when selection against a particular phenotype changes with that phenotype's relative frequency in the population.

    Positive frequency-dependent selection:
    a phenotype's fitness increases as it becomes more abundant.

    Negative frequency-dependent selection:
    a phenotype's fitness decreases as it becomes more abundant.

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Batesian Mimicry:

Mullerian Mimicry:

    Frequency-dependent Selection: Mimicry

    Predators develop a search image (<-- required link) for a specific prey item.
    Prey species can evolutionarily respond via phenotypic convergence.

    • Batesian mimicry
        An edible species (mimic) is protected by its resemblance to a
        toxic/noxious species (model) that predators have learned to avoid.

      <--The Viceroy is protected when the Monarch is relatively abundant.

    • Mullerian mimicry
        two or more sympatric toxic/noxious species living in the same area () evolve similar appearance.

      <-- The more toxic species converging on a common form,
      the more quickly predators learn to avoid them.

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Frequency-dependent Selection: The Monarch and the Viceroy

Monarch butterflies (Danaeus plexipus) is toxic and distasteful to birds.
Viceroy butterflies (Limenitis archippus) are harmless and edible to birds.

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    Frequency-dependent Selection: Nymphalid Butterflies

    Members of the genus Acraea (Family Nymphalidae) are all toxic to birds.
    In areas where different species co-exist, they converge on a similar color pattern.

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We are the Products of Ancient Selection

The discovery of neutral evolution provided a powerful tool for determining whether a particular gene has been under natural selection pressure. This involves comparison of the ratio of non-silent to silent mutations in a particular gene.

Negative (Purifying) Selection
The most common type of natural selection is the removal of maladaptive alleles.
A gene encoding for a product so vital that even small changes to its sequence are lethal should accumulate only silent (S) mutations, and not tolerate any non-silent (NS) mutations.
A gene that has undergone negative (purifying) selection has, figuratively, been "purified" of maladaptive variations.

In a gene that has undergone purifying selection, the ratio of non-silent (NS) to silent (S) mutations should be ZERO. (NS = 0)

~~~~~~~~~~~~~~~~~~~~~

Neutral Evolution
If a gene is not influenced by natural selection, neither non-silent nor silent mutations should affect phenotype. Thus, non-silent and silent mutations should accumulate at an equal rate.

In a gene changing only due to neutral evolution (genetic drift), the ratio of nonsilent to silent mutations (NS/S) should equal ONE. (NS = S)

~~~~~~~~~~~~~~~~~~~~~

Positive Selection
If the function of a gene product is somewhat less rigid, then non-silent mutations may be tolerated.
This can produce new alleles, some of which may be more adaptive than the original. These will be naturally selected.

In a gene changing due to positive selection, the ratio of nonsilent to silent mutations should be GREATER THAN ONE (NS > S).

~~~~~~~~~~~~~~~~~~~~~

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(click on pic for primary literature source)

    The FOXP2 Gene

    The FOXP2 gene encodes a transcription factor named forkhead box P2.

    FOXP2 protein may regulate the expression of hundreds of different genes,
    but only some of its targets have yet been positively identified.

    FOXP2 protein is active in multiple tissues during ontogeny and adulthood.
    Research has revealed that it regulates genes involved in

    • brain development
    • neuron and synapse development
    • synaptic plasticity (learning and memory)

    FOXP2 protein is essential for normal development of communication.
    • Normal baby mice squeak to get mother's attention and nursing.
    • FOXP2 knockout mice cannot squeak normally.
    • Something of a liability.

    • (Male) passerine birds with abnormal FOXP2 cannot produce normal song.

    • Humans with mutant FOXP2 have severe language & speech disabilities.
    • They cannot understand grammar or language construction.

    FOXP2 is highly conserved among vertebrates.
    Among mammals, it is nearly identical in sequence and function.

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    Positive Selection in the FOXP2

    A study examining monkey and ape genomes (10,000 genes) found
    • 9.8% of proteins were conserved (NS = 0)
    • 2.2% of proteins showed evidence of positive selection (NS > S)

    One of the latter was the FOXP2 gene.

    In humans, two non-silent mutations have occurred in the past 6 million years.
    These modified protein function compared to other great apes.
    The precise nature of the change is not fully understood.

    Might human capacity for complex language be traced to just two mutations?
    Did the FOXP2 gene contribute to making our species unique?

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