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    Microevolution:
    Genetic Change Within a Population

    The raw material of evolution is genetic variation,
    which can manifest as polymorphism at different levels.

    Behold the humble bacterium, Staphylococcus aureus.

    In wild type, it can be killed by a wide variety of antibiotics.
    But one small genetic modification can change it into MRSA
    (Methycillin-resistant Staphylococcus aureus)

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Evolution of Antibiotic Resistance in MRSA

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    Macrolide Antibiotics Bind Bacterial Ribosomes

    Macrolide antibiotics (e.g., erythromycin) bind to
    the bacterial 50S ribosomal subunit,
    inhibiting translation of vital proteins.

    They are bacteriostatic, slowing the growth of bacterial colonies.

    The antibiotic binds to the L4 protein of the ribosome,
    blocking the ribosome tunnel.

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    The L4 Ribosomal Protein Has Two Alleles

    The a allele of the L4 protein encodes L4 to which macrolides can bind.

    A mutant form of the allele (g), changes the protein's configuration.
    Macrolides can no longer bind, rendering the bacteria resistant.

    It's a simple missense mutation,
    changing one codon from aaa --> gaa.

    The result: Antibiotic Resistance.

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    The Luria-Delbruck Fluctuation Experiment:
    Mutations are NOT Induced

    In 1943, Max Delbruck and Salvador Luria published their experiment demonstrating that mutations arose in the absence of natural selection.
    • A small number of E. coli were inoculated into several culture tubes.
    • Each tube was incubated until bacteria formed a larger population.
    • Bacteria from each tube were plated on agar containing T1 bacteriophage.

    Prediction:

    • If mutations are induced by the presence of virus,
      then all plates should have ~ same number of resistant colonies.

    • If mutations are random, not induced,
      then the plates should differ in the number of resistant colonies.

    Result:

      The plates varied drastically in the number of resistant colonies,
      demonstrating that T1 resistance mutations had occurred at random.

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    The Lederberg Experiment: Mutations are NOT Induced

    In 1952, Esther and Joshua Lederberg demonstrated
    a similar pattern in bacteria exposed to antibiotics.
    • E. coli colonies were grown on an agar plate.
    • Colonies were "stamped" onto streptomycin-infused plates.
    • Some colonies died, and some survived.

    Question:

      Did the surviving colonies evolve streptomycin resistance
      because they were exposed to streptomycin?

    Test:

    • The original colonies were then grown in streptomycin medium.

    Result and Inference:

    • The SAME colonies survived, though never previously
      exposed to streptomycin.
    • The mutants had already been present in the original population.

    This is the basis of natural selection.

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Population Genetics: Measuring Evolutionary Change

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    Microevolution

    Microevolution is change in allele frequencies within a population.
    These changes do not involve reproductive isolation within the population.

    All the alleles of every gene
    in every individual of a population comprise that population's GENE POOL.

    A population geneticist studies

    • the genetic composition of a population's gene pool
    • the forces that determine and change the gene pool

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    What Drives Genetic Change?

    In natural populations, over any given span of time
    • some genes may be evolving
    • other genes may not be evolving

    The genetic makeup of a population can change by means of five mechanisms:

    • mutation
    • small (i.e., not infinitely large) population size
    • non-random (assortative) mating
    • migration into or out of the population
    • natural selection

    In the absence of all of these, a population will NOT EVOLVE.
    Such a hypothetical, equilibrial population is rare to non-existent in nature.

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    Hardy-Weinberg: A Null Model for Microevolution

    In 1908,

    • Godfrey H. Hardy (British mathematician)
    • Wilhelm Weinberg (German physician)

      independently reported that the expansion of the binomial equation
      could be used to predict relative allele frequencies
      in a population that is not evolving.

    In any given population, if gene "A" has two alleles, one dominant and one recessive

    • the dominant allele is represented as A
    • the dominant allele's frequency (= proportion = %) is represented as p

    • the recessive allele is represented as a
    • the recessive allele's frequency (= proportion = %) is represented as q

    If a gene has only two alleles, then p + q = 1.0 (100% of the alleles)

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    Genotypes, Genes, and Alleles, oh my.

    Let's continue to consider a single gene ("A") with two alleles (A and a).

    The number of alleles determines the number of possible genotypes.
    Our "gene A" has two alleles, so there are three possible genotypes:

    • AA - homozygous dominant
    • Aa - heterozygous
    • aa - homozygous recessive
    The Hardy-Weinberg equation is an expansion of the binomial equation (p + q)2:

    p2 + 2pq + q2 = 1.0

    In which...

    • p2 is the relative frequency of homozygous dominant (AA) individuals
    • 2pq is the relative frequency of heterozygous (Aa) individuals
    • q2 is the relative frequency of homozygous recessive (aa) individuals

    Given initial frequencies of each allele, the Hardy-Weinberg Model predicts relative frequencies of all three possible genotypes in the population if that population is not evolving.

    Remember, for any gene (with two alleles) in your population:

    • The value of p is simply the proportion of alleles that are dominant.
    • The value of q is simply the proportion of alleles that are recessive.


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    An Evolving Population? Using a Gene as an Indicator

    You are studying a population of Australian Jewel Beetles (Family Buprestidae).

    Beetles can have one of two different colored elytra (wing covers).

    A literature search reveals that elytra color is controlled by
    a single gene locus ("E") segregating two alleles:

    • The dominant E allele encodes green elytra
    • The recessive e allele encodes blue elytra

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    One Gene, Two Alleles, Three Genotypes

    With two alleles, there are three possible genotypes:
    • EE - green elytra
    • Ee - green elytra
    • ee - blue elytra

    There are other populations of beetles in other forests in Australia,
    but only your study population has the blue beetles.

    You wonder whether this unusual trait is adaptive, maladaptive, or neutral.

    By quantifying changes in genotype frequencies over generations
    might enable you to answer this question.

    To begin, you must determine p and q for the E locus.

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Question #1:

Question Set #2:

    Good Beetle Hunting

    You are a persistent scientist.
    Also, you have a touch of Obsessive Compulsive Disorder.
    You counted 10,000 beetles in one day.
    Of these, 6400 were green and 3600 were blue.

    Question Set #3

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    Hardy-Weinberg Equilibrium

    The Hardy-Weinberg Equilibrium Model states that allele and genotype frequencies in a population will remain constant from generation to generation in the absence of the five factors listed before:

    • mutation
    • small population size
    • non-random (assortative) mating
    • migration into or out of the population
    • natural selection

    In other words, a population will evolve (i.e., the allele frequencies of some of its genes will change from one generation to the next) if

    • genes change to generate new alleles (mutation)
    • the population is not infinitely large (genetic drift)
    • mating is non-random (assortative mating)
    • individuals immigrate into or emigrate out of the population (migration)
    • certain alleles are more adaptive than others (natural selection)

    With respect to a particular locus under study...

    • A population that is not evolving is in Hardy Weinberg equilibrium.

    • If relative genotype frequencies are significantly different
      from those predicted by the HW model, then the population is evolving.

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    Are Genetic Changes Meaningful? Statistical Analysis

    One can determine whether change in genotype frequency over generations
    is significant with statistical analysis.

    In this case, we are quantifying non-parametric data
    (numbers of individuals of particular genotypes).

    An appropriate statistical test is the Chi (Χ2) Square Test.

    You will this formula to calculate a Chi (Χ2) Square statistic for your data.

    But first, you must return after one beetle breeding season to count again.

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    Degrees of Freedom

    In statistics, the degrees of freedom (df) is the
    number of values in the final calculation of a statistic that are free to vary.

    (Ever play Yahtzee? Let's illustrate.)

    For the Chi-square test, df = n - 1,
    where n = the number of independent categories in the system.

    Each genotype comprises an independent category.
    Our beetle elytra gene has three possible genotypes (EE, Ee, and ee).
    Therefore, our df = 3 - 1 = 2.

    Next, we use the Chi Square Table of Critical Values to determine
    the P value associated with our Chi2 statistic.

    The Chi square value associated with an α of 0.05 is 5.991.

    Do we reject or fail to reject our null hypothesis?

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Heterozygosity: A Measure of a Population's Genetic Diversity

Hardy-Weinberg calculations can help us predict expected levels of heterozygosity of various gene loci in a population.

The higher the heterozygosity of a population, the greater its genetic variability.

The higher the homozygosity of a population, the lower its genetic variability.

Heterozygosity and Deleterious Alleles

    Heterozygosity and Deleterious Alleles

    Why might it be advantageous to be heterozygous, rather than homozygous for a particular trait?

    The major histocompatibility complex The MHC, or Major Histocompatibility Complex is a relatively large gene family found in most vertebrates. The MHC genes encode MHC polypeptides, which are constructed into important players in the immune system.

    The significantly longer survival of MHC heterozygotes over MHC homozygotes is another bit of evidence that heterozygosity at certain gene loci can be adaptive. The more heterozygous the individual, the longer that patient staves off full-blown AIDS.

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(click on pic)

    Inbreeding Depression

    Hybrid individuals usually exhibit a high degree of heterozygosity, which results in hybrid vigor.

    The more closely related parents are, the less heterozygous their offspring are likely to be.

    While inbreeding does not always produce misfit offspring, it often does result in reduced vigor of the products of inbreeding.

    Reduction in biological (as opposed to evolutionary) fitness in an inbred organism is known as inbreeding depression (<-- required link).

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You might reasonably ask:



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Explanation for Facial Preference:


The face on the right is a computer-generated composite (averaging) of the two faces to the left.
The more faces averaged into a composite, the more symmetrical it becomes,
mimicking a high degree of heterozygosity.

We are "hard wired" to prefer symmetry, which is generally a strong indicator of heterozygosity.