Lecture notes for Ecology

  • MONDAY FEBUARY 12, 1995 LAB In lab we will investigatethe effect of this phenomenon on the genetic pool in a series of generations of corn (Zea mays)carrying the albino allele for disruption of chlorophyll synthesis: Genotype / Phenotype AA = green (viable) Homozygous dominant Aa = green (viable) Heterozygous dominant aa = white (lethal) Homozygous recessive

    Hardy & Weinberg

    Hardy and Weinberg asked, why don't dominat alleles push out recessive ones, especially lethal area, over time? They applied classical Mendelian genetics to answer this question. Using the Punnett square method can determine (predict) allele frequencies and derive a single formula for calculating them in subsequent generations.

    What factors can Change Allele Frequencies? Mutations - changes in the nucleotide sequence of the DNA; results in new alleles of given genes on chromosomes. - always change the genotype, but may not change the phenotype. - generation of new alleles through mutation, changes the future allele frequencies within a specific population. e.g. - Brown allele in rabbits and albino allele in rabbits are a mutation in the same gene.

    Migration - movement of individuals from one place to another; either into a population or out of : intoduction or reduction (removal) of exsisting alleles into or out of the population. OR: Introduction of new alleles other than the currently exsisting ones. e.g. - Suppose 10 of the albino rabbits migrate out of the population. 84 + 6 = 90

    Genetic drift - random changes in gene or allele frequency due to non-random mating; i.e. one phenotype is preferred over another for some unknown reason. A specific, finite, sample of the gene pool is selectively used for the allele contribution to the next generation. The non-random mating may or may not persist in future generations. Allele frequencies fluctuate over time.

    Non - Random Mating - if a phenotype is preferred ove another, for mating, allele frequencies will change. Also, lethal mutations, in effect, result in non - randdom mating because one phenotype is lost from the "mating pool" (possible candidate for mating) e.g. - corn albinisim

    Selection - survival of the fittest. If one phenotype is less likely to survive to reach reproductive maturity, allele frequencies will change; lethals - corn albinisim. natural selection - selection acts upon the phenotype rather than the genotype.

    Other specific types of selection:
    Founder principle selection:
    Isolated islands to where organisms migrate result in rare alleles, if present, becoming more prevalent, if the new envision mental conditions favor a more rare phenotype.
    Bottle Neck effect:
    Drastic reduction of a population due to natural disasters; may result in a population of only one type of allele and being represented.
    Directional:
    Elimination of extreme phenotypes; lethals fit in best here because they are the ultimate in extreme phenotypes.
    Stablizing Selection:
    Results in phenotypes; 2 extreme phenotypes, if present; reduces large numbers of extremes and results in a more even distribution of alleles within a population.
    Disruptive Selection:
    Eventually results in two drastically different phenotypes being equally represented; results in selection for rather than against extreme phenotypes, and average phenotypes are reduces and eventually eliminated. Tests of Natural Selection
    1. E. coli resistance to antibiotics
    2. Drosophila melanogaster, fruit fly resistance to DDT insecticide.
      Resistance occurs gradually due to increased use of DDT (selective pressure); many genes are involved, therefore many mutations are necessary for resistance to occur; fruit flies are dipliod, but most types of resistance are phenotypes are dominant, therefore, only one allele of a given gene would have to be affected; sexual reproduction decreases the rate of increase in DDT resestance, but provides facilitation of increased genetic variability in not only the population but with in the entire sp.

    Three types of predictions based on natural selection models

    1. Dominant homozygotes with highest fitness; heterozygotes with intermediate fitness:
      In most cases, dominant homozygotes and heterozygotes have the same fitness; however, some dominant phenotypes are dosage dependent and require the homozygous condition for full expression of the trait. Suppose that an allele which affects fertility is dosage dependent:
      • F is the dominant fertility allele
      • f is the recessive fertility allele
        Possible genotypes and phenotypes are:
        • FF - most fertile
        • Ff - intermediate fertility
        • ff - least fertility

      Natural selection will act upon each genotype to different degrees with regard to fertility. All have equal survivorships, but due to reduction in fertility, fitness i s ultimately reduced.
    2. Heterozygotes with highest fitness (heterosis):
      Sometimes heterozygotes have pheotypes that are more fertile and have higher survivorships than either dominant or recessive homozygotes. The heterozygote can function differently than either homozygote for reasons that are poorly understood. The gene products of each allele appear to work synergistically with regard to certain aspects of fertility.
    3. Heterozygotes with lowest fitness:
      Occurs when dominant and recessive alleles are incompatible with one another and the gene products work antagonistically.

    From the above three examples, it is obvious that knowing the level of heterozygosity with in a population is of great importantce with regard to determining the degree to which selective pressures will have an effect on a given population.

    Determination of heterozygosity levels with in a population by Electrophoresis (see page 123 of Ehrlich and Roughgarden, 1987).

    To determine wether 2 individuals have slightly different proteins that perform the same function (a silent heterozygous condition), electrophoresis is used.
    Electrophoresis - separation of compounds (usually nucleic acids or proteins) based on their net electrical charge.
    For determination of heterozygosity, protein electrophoresis is used as follows:
    1. Proteins are extracted from individuals and loaded into a well of a gelatin-like slab (gel; usually agarose, polyacrilamie, starch, or some other type of polymer).
    2. The protein samples are subjected to an electric field and migrate according to their net electrical charge and molecular mass.
    3. Typically, proteins that differ in one or more amino acids move through the gel at slightly different rates.
    4. The proteins are visualized by a variety of staining techniques that will reveal these slight differences (polymorphisms).
    If such proteins differ slightly in amino acid sequence, yet still are equally functional, they are called isozymes.
    If they are encoded by different alleles of the same genetic locus (gene) they are called allowzymes - alternative forms of the same enzyme.
    A typical example includes the starch degradation enzyme amylase which breaks down starch into glucose monomers.

    Suppose we want to know the heterozygosity of a specific population. How can we use electrophoresis and the analysis of allowzymes to determine heterozygosity with in that population?

    1. A1 - one functional amylase allele
      A2 - another functioal amylase allele
    2. Suppose that the following genotypes are present:
      A1A2 (heterozygote)
      A1A1 and A2A2 (homozygotes)
    3. Extract proteins from individuals with in the population and separate the proteins by electrophoresis on a starch-containing gel.
    4. After running the gel, let it incubate for a few hours so that the amylase enzymes will degrade the starch where ever the enzymes are located on the gel.
    5. Visulize the amylase location by soaking the gel in KI solution (starch wil bind with the I and produce a dark blue color on most of the gel). Clear areas (bands) will indicate the presence of amylase because the starch has been broken down in the vicinity of the enzyme.
    The expression of A1 or A2 cannot be seen with out using this technique, but individuals heterozygous for this allele will be present in the population and would go undetected otherwise.

    Allowzyme analysis can reveal additional heterozygosity with in a population that would also, otherwise go undected.

    The amount of heterozygosity as determined by this technique reveals how much genetic variability is present with in a given population.

    The greater the genetic variablity the greater the potential for natural selection pressures to ultimately facilitate adaptation with in the population and eventually the sp.

    Reference

    Ehrlich PR, Roughgarden J (1987) The Science of Ecology. MacMillan, New York