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(photo by Phil Gates)

    Evolutionary Force #4:
    Non-random Mating

    A fourth factor that can change relative allele and genotype frequencies is
    • mutation
    • non-infinite population size
    • migration
    • non-random mating
    • natural selection

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    Random Mating

    Random mating occurs in a population when

    • Any female has an equal chance of mating with any male, regardless of phenotype.

    • Any male has an equal chance of mating with any female, regardless of phenotype.

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    Non-random Mating

    Non-random mating in a population can be either

    • Assortative (positive) - similar individuals mate together
      more frequently than they mate with dissimilar types

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    • Disassortative (negative) - dissimilar individuals mate together
      more frequently than they mate with similar types

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MORE PRECISELY...

    Assortative Mating

    is occurring if phenotypically similar organisms mate together more often than would be predicted by their relative frequencies in the population.

    <--

    Disassortative Mating

    is occurring if phenotypically dissimilar organisms mate together more often than would be predicted by their relative frequencies in the population.

    <--

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    Non-random Mating and
    Genotype Frequencies

    Non-random mating is usually described with respect to
    a particular trait (monogenic or polygenic) or set of traits.

    Non-random mating restricts gene flow between preferentially mating groups.

    Assortative mating, in particular

    • decreases effective population size of each sorting group
    • can lead to further isolation and evolution of assortatively mating groups

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    Autozygosity

    Autozygosity is a special case of homozygosity in which
    both alleles are identical by common descent.

    In an autozygous genotype, the alleles are exact copies
    of an ancestral allele because both parents were genetically related.

    A recessive allele is often a non-functional version of a wild-type allele.

    When wild type function is lost, there can be significant phenotypic consequences.

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(click for source)

    Risk of Recessive Homozygosity

    In a very small population, the probability of matings
    between relatives will increase.

    The resulting genome-wide homozygosity can affect individual vigor,
    lowering survival and fertility due to inbreeding depression.

    The cure: OUTBREEDING.
    (This was attempted with the Florida Panther population
    ...with mixed success.)

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    Positive Assortative Mating Happens. A lot.

    Positive assortative mating is more common than negative assortative mating.
    Many species have been documented to assort based on
    • body size
    • pigmentation/color patterns
    • other phenotypic characters

    Humans may mate assortatively based on
    • genetic/phenotypic factors
      • height
      • general morphology
      • armspan (!)
      • facial features (males, not females)

    • sociological factors
      • ethnicity
      • socio-economic status/education
      • religious beliefs

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    Disassortative Mating Also Happens

    Although negative assortative mating is less common,
    there are documented examples.

    White-throated Sparrows (Zonotrichia albicollis) can express
    one of two distinct feather color phenotypes.
    In both sexes, head stripes can be either white or tan.

    In this species, ~98% of matings/pair bonds
    are between individuals of opposite phenotype
    .

    And then there's the famous MHC connection
    in our own species, among others.

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    Assortative Mating and Evolution

    How can assortative mating drive evolution?

    Imagine the ancient Artiodactyl ancestor of today's whales and hippos.
    A 2005 study identified this extinct animal as an anthracothere.

    Although not fully aquatic, it lived near and foraged in water.

    Once upon a time...

      ...there was a population of anthracotheres living around an estuary.

      Some of the herd had a genetic predisposition to prefer salty water.
      Some of the herd had a genetic predisposition to prefer fresher water.

      You take the story from there.

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    Sexual Selection vs. Assortative Mating

    Sexual Selection is defined as natural selection occurring because of preference by one sex for certain characteristics in individuals of the other sex.

    There is a subtle but important difference between this and assortative/disassortative mating.

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An Illustration: Random vs. Non-random Matings

Here is a population of Norwegian Tutpips, polymorphic for (at least) two traits:

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    The Difference: Differential Reproduction

    Both assortative mating and sexual selection are non-random, but...

    • In assortative mating, all individuals have equal chances to mate.
    • In sexual selection, some individuals get more chances to mate than others.

      Thus sexual selection provides the opportunity for
      differential reproduction among individuals.

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https://phys.org/news/2017-01-evidence-humans-partners-assortative.html