Genetic Variation: Prerequisite for Darwinian Change––#

GENETIC VARIATION: PREREQUISITE FOR DARWINIAN CHANGE

  1. Introduction

    1. During the 20th century biologists have learned a great deal about how organisms change over time
    2. Microevolutionary mechanisms are well established and understood
    3. Macroevolutionary mechanisms remain more elusive
    4. Here we will begin to examine biological models of change
    5. In this lecture we will focus specifically on natural variation, a prerequisite for Darwinian change

  2. Darwin and natural selection

    1. Darwin was put off by the non-mechanistic models for change proposed by his contemporaries and predecessors
    2. He searched for a totally naturalistic model of change
    3. He called the process whereby change occurred “natural selection”
    4. He developed his model of natural selection on the basis of:

      1. Observations of organisms during his around-the-world voyage, particularly of those living on the Galapagos islands
      2. A deep appreciation for the principle of design as a result of his reading of Rev. William Paley’s Natural Theology
      3. Acceptance of geological uniformitarianism as a result of reading Charles Lyell’s Principles of Geology
      4. An understanding of some basic population principles presented by the Rev. Thomas Malthus in An Essay on Population
      5. Personal experience in the breeding of a variety of organisms, particularly domestic pigeons

    5. The model was developed over a period of many years, and was finally published in his book On the Origin of Species by Means of Natural Selection (1859).
    6. This book argues two major points:

      1. That organisms change through time
      2. That this change occurs through natural selection

    7. The four postulates of natural selection theory:

      1. Individual members within a species exhibit variability
      2. Some of this variability is heritable
      3. More offspring are produced than can survive
      4. Organisms with features best suited to the environment pass on more genes than organisms less well suited––differential reproductive success or natural selection

    8. Thus, through time as environments change, so do organisms

  3. Genetic variation in natural populations––the prerequisite for natural selection

    1. At one time variation was considered to be a deviation from the “normal”

      1. Perfect heavenly pattern––an ideal
      2. Real organisms were imperfect reflections of this ideal.
      3. This view grew out of Platonic dualism

    2. Today we know that variation IS the norm––it is present in all populations all of the time
    3. While not all of this variation is based in the genes, much of it is heritable
    4. In fact, Darwin would have been shocked to learn how much genetic variation there really is in natural populations
    5. Genetic variation can be evaluated at several levels:

      1. Variation perceived at the level of gross morphology

        1. Rat snakes (Elaphe obsoleta)

          1. Live in eastern United States
          2. Consist of six subspecies that interbreed where ranges meet
          3. The six subspecies exhibit strikingly distinctive color patterns

        2. Drongos (Dicrurus paraiseus)

          1. Crested birds that live in southern Asia
          2. Crests vary markedly from region to region

        3. Drosophila bristle number

          1. Average number of bristles on the bottom of the 4th and 5th abdominal segments is 36.
          2. Artificial selection created two lines

            1. Low bristle line

              1. Average of 30 bristles
              2. Line eventually died out due to sterility
              3. Apparently something related to low reproductive potential was selected along with low bristle number

            2. High bristle line

              1. After 21 generations an average of 56 bristles was achieved
              2. The high bristle line eventually began to show signs of sterility, so the flies were allowed to breed without selection
              3. The average bristle number fell to 40
              4. Selection was once again initiated
              5. The average of 56 was once again achieved, but this time with no signs of sterility
              6. Apparently the genotype became reorganized into more favorable combinations of alleles

      2. Variation perceived at the level of proteins

        1. Differences in proteins imply differences in DNA
        2. Proteins can be extracted from organisms and separated by chromatography or electrophoresis (since they tend to be charged molecules)
        3. Comparisons can then be made among the patterns of separated proteins
        4. Protein variation in wild populations of Drosophila pseudoobscura (Hubby and Lewontin, 1966)

          1. Show table
          2. Proportion of polymorphic gene loci (of the 18 tested) within the populations ranged from 0.28 to 0.33.
          3. Proportion of genes heterozygous within individuals ranged from 0.081 to 14.8.

        5. Human hemoglobin variation––Vernon Ingram’s (1957) protein fingerprinting study
        6. Protein variation has been studied in a variety of organisms

          1. Show table
          2. Invertebrates generally exhibit higher levels of protein variation than vertebrates, both at the population and at the individual level

      3. Variation in the DNA––the most fundamental form of genetic variation

        1. A variety of techniques allow for comparison of DNA within and between species:

          1. RAPD analysis
          2. RFLP analysis
          3. Sequence analysis

        2. All these techniques show high levels of genetic variation at the level of DNA

    6. Preservation and promotion of genetic variability in nature

      1. Evolutionary biologists have not only shown that natural populations exhibit tremendous genetic variation, but also that mechanisms exist that function to preserve and promote this variation.
      2. Mechanisms for preserving and promoting genetic variability

        1. Mutations

          1. Mutations can be categorized as

            1. Gene
            2. Chromosomal

          2. Both types can result in morphological variability that can be acted upon by natural selection
          3. Mutations are occurring all the time in all organisms
          4. Often mutations are harmful or simply harmless
          5. But occasionally they can result in something helpful (e.g., a protein that can withstand a higher temperature)
          6. Mutations are the ultimate source of genetic variability among organisms because mutations produce brand new genetic material

        2. Sexual reproduction

          1. Produces variability through recombination events

            1. Independent assortment during meiosis
            2. Crossing over during meiosis
            3. Promotion of outbreeding

              1. In plants

                1. Anatomical barriers to self-fertilization
                2. Dioeciousness
                3. Self sterility

              2. In animals

                1. Anatomical features to prevent self-fertilization
                2. Movement away from birthplace to mate
                3. Incest taboos among humans

          2. Production of variability may be the most important function of sexual reproduction, an otherwise costly enterprise

        3. Diploidy

          1. Rare recessive alleles are hid from selection in the heterozygous state
          2. Here they can remain until a favorable environment develops

        4. Balanced polymorphism––snails eaten by thrushes

          1. Exist in two color morphs

            1. Banded––Found in bogs and woodlands
            2. Unbanded––Found in habitats with uniform backgrounds

          2. Predation pressure in heterogeneous environment thus maintains genetic variability

        5. Heterozygote superiority

          1. Heterozygotes are often better fit than homozygotes
          2. Obviously, this would promote variability.
          3. Example: Allele for sickle cell anemia

            1. HbNHbN genotype

              1. Subject to malaria
              2. Less fertile

            2. HbSHbS genotype

              1. Extreme anemia
              2. Die before reproduction is possible

            3. HbNHbS genotype

              1. Resistent to malaria
              2. Only mild anemia
              3. fertile, thus highest fitness
        6. Clines

          1. A cline is heritable variation within a species that follows some ecological gradient
          2. Example: House sparrow body size

            1. Smaller in south
            2. Larger in north
            3. This variation in body size is genetically based.

  4. Summary

    1. Far from being an aberration, genetic variability is the rule, not the exception
    2. Natural populations exhibit extensive variability, much more than Darwin could have imagined
    3. In fact, nature actively preserves and promotes genetic variability
    4. It is this genetic variability that serves as the raw material for natural selection