Population Genetics and Microevolution––#

POPULATION GENETICS AND MICROEVOLUTION

  1. Introduction

    1. Darwin based his theory of natural selection on the supposition of heritable variation
    2. Heritable variation requires the existence of hereditary units
    3. Gregor Mendel was the first person to infer the nature of these units, which today we call “genes” but Darwin was unaware of Mendel’s work
    4. During the early twentieth century “neo-Darwinism” (synethic theory of evolution) was developed as a synthesis of

      1. Darwinian natural selection
      2. Modern genetics
      3. Paleontology

    5. People primarily responsible for this synthesis

      1. J. B. S. Haldane
      2. Th. Dobzhansky
      3. George Gaylord Simpson
      4. Ernst Mayr
      5. Ronald Fisher

    6. Here we will introduce the area of genetics that deals with neo- Darwinism––population genetics
    7. Population genetics developed in response to a desire to understand

      1. Why harmful, recessive genes are not eliminated from the human population
      2. How natural populations change from one generation to another

  2. Basic concepts of population genetics

    1. Population

      1. Local group of interbreeding organisms
      2. A species often contains several or many populations

    2. Gene Pool––All the genes in all the individuals in a population
    3. Gene frequency

      1. This is the basic unit of interest to population geneticists
      2. It is calculated in the following way: Gene frequency=number of alleles of a given type / total number of alleles for gene in population

    4. The Hardy-Weinberg equilibrium principle

      1. G. H. Hardy (English mathematician) and Wilhem Weinberg (German physician) independently developed a mathematical model to describe the behavior of gene frequencies in populations
      2. This principle states: In an equilibrium population, gene frequencies remain constant from one generation to another

    5. Assumptions for an equilibrium population:

      1. Population is large
      2. Mating within the population is random
      3. All genotypes are equally viable––There is no natural selection
      4. There is no mutation
      5. There is no migration
      6. There is no genetic drift

    6. The mathematical model: (p + q)2 = 1.0

      1. Where p = freq(A)
      2. q = freq(a)
      3. Note that

        1. The number of terms in the expression is equal to the number of different alleles in the gene pool
        2. The power to which the expression is raised is equal to the number alleles of a gene per individual (monoploid=1; diploid=2; triploid=3)
        3. There are three types of frequencies of interest

          1. Allele frequency
          2. Genotypic frequency
          3. Phenotypic frequency

    7. We will now examine processes that upset the Hardy-Weinberg equilibrium––forces of microevolutionary change

  3. Processes upsetting the Hardy-Weinberg equilibrium––Forces of microevolutionary change

    1. Mutation

      1. Mutations are sudden, random alterations in the genotypes of individuals
      2. Only germ cell mutations are important in altering genetic equilibrium
      3. Mutation rates are generally quite low for any particular gene––estimated to effect 1/100,000 gametes for any particular gene locus in Drosophila or humans
      4. However, considering all the genes present in an organsim, the chance of at least one mutation on any gene in a gamete is quite high

        1. Assume that humans have 50,000 genes
        2. Then, with the chance of 1/100,000 (0.00001) for a mutation on any particular gene, we can predict the average number of mutations/gamete = 0.5
        3. Thus , about half the egg or sperm cells produced by an individual would likely carry a new mutation

      5. We can now estimate how often new mutations might affect a single offspring:

        1. Let p = prob(new mutation on gamete) = 0.5
        2. Let q = prob(no new mutation on gamete) = 0.5
        3. Then, chance of carrying 2 new mutations: prob(2 mutations) = p2 = (0.5)2 = 0.25
        4. Chance of not carrying any new mutation: prob(no mutation) = q2 = (0.5)2 = 0.25
        5. Chance of carrying 1 new mutation: prob(1 mutation) = 2pq = 2(.5)(.5) = 0.5
        6. Thus the chance of offspring with at least 1 new mutation is: prob(1 or 2 mutations) = p2 + 2pq = 0.25 + 0.50 = 0.75

      6. Interestingly, roughly a third of all conceptions end up as spontaneous abortions
      7. Most spontaneous abortions are the result of genetic mutations
      8. Because mutation rates are so low for any given gene

        1. By itself mutation, a random event, is not considered an important directional force in evolution
        2. Mutation, is however, a significant source of new genes in populations
        3. Natural selection, a non-random process, then works on the resultant genetic variation

    2. Gene flow

      1. Two forms

        1. Immigration
        2. Emigration

      2. Examples

        1. Gradient of IB frequency across Eurasia

          1. High frequency of IB in Asia
          2. As one moves west, frequency of IB diminishes

            1. In Basque community (in Pyrenees Mountains), the frequency of IB < 3%
            2. Basque people have little gene exchange with other Europeans

          3. Probably at one time, no Europeans carried IB
          4. Apparently, migration westward by Mongolians between the 6th and 16th centuries A.D. was responsible for bringing the IB allele to Europe

        2. Frequency of the FY-NULL*1 allele (a RFLP)

          1. Africans: freq = 0
          2. Europeans: freq = 1.0
          3. African-Americans: freq = 0.1 to 0.2
          4. European-Americans = slightly less than 1.0

        3. Parra et al. estimate that

          1. African-American populations derive between 11.6 and 22.5 percent of their ancestry from Europeans
          2. European-American populations derive between 0.5 and 1.2 percent of their ancestry from Africans

    3. Sampling Error

      1. Genetic drift

        1. Random fluctuations in gene frequency
        2. Very effective in small populations
        3. Example

          1. Suppose allele b has a frequency of 0.02 in a population
          2. If there are 50 animals in the population, only 1 will carry allele b
          3. If that one individual dies before breeding, allele b is lost
          4. If population contains 1,000,000 individuals, 20,000 individuals will carry allele b
          5. It is unlikely that all 20,000 individuals will die before breeding
          6. Thus, genetic drift is much more effective in small than in large populations

        4. As time goes on, isolated populations diverge from one another through random drift
        5. Divergence can lead to speciation

      2. Bottleneck effect

        1. Decimation of large population followed by rebuilding of the population from a few survivors
        2. Results in restricted genetic diversity
        3. Examples:

          1. Elephant seals
          2. Cheetahs
          3. Florida panthers

            1. Of 30 subspecies of cougers, Florida panther is the most rare
            2. During late 1990s, only 30-50 animals remained
            3. Current population show signs of loss of genetic diversity

              1. Cryptorchidism (failure of testicles to descend)
              2. Low sperm counts
              3. Congenital heart defects
              4. Immune deficiencies
              5. Kink in the tail
              6. Whorl of fur on the back

            4. Efforts to save Florida panthers includes, among other measures, the introduction of Texas panther into the Florida population

      3. Founder effect

        1. Founder population carries only a fraction of genetic diversity of parent population to new locality
        2. Results in shifts in gene frequencies in founder compared with parent population
        3. Examples:

          1. Dunkers of E. Pennsylvania

            1. Descendents of Old German Baptist Brethren
            2. 300 individuals came to U.S. in 1700’s (90 parents)
            3. Kept reproductively isolated
            4. Compared with Old World relatives, Dunkers exhibit:

              1. IA more common
              2. i, IB very rare
              3. Hitchikers thumb and attached ear lobes are rare

          2. Galapagos finches

    4. Natural selection––Next lecture from one generation to another

II. Basic concepts of population genetics

A. Population

1. Local group of interbreeding organisms
2. A species often contains several or many populations

B. Gene Pool: All the genes in all the individuals in a population
C. Gene frequency

1. This is the basic unit of interest to population geneticists
2. It is calculated in the following way:

Gene Frequency = No. of alleles of a certain type
Total No. of alleles for gene in population

III. The Hardy-Weinberg equilibrium principle

A. G. H. Hardy (English mathematician) and Wilhem Weinberg (German physician) independently developed a mathematical model to describe the behavior of gene frequencies in populations
B. This principle states that : In an equilibrium population, gene frequencies remain constant from one generation to another.
C. Assumptions for an equilibrium population:

1. Population is large
2. Mating within the population is random
3. All genotypes are equally viable -- There is no natural selection
4. There is no mutation
5. There is no migration
6. There is no genetic drift

D. We will now examine genetic equilibrium using several genetic models: