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3.13: A short note on pedagogical weirdness

Many students are introduced into the field of population genetics and evolutionary mechanisms – that is, how phenotypes, genotypes, and allele frequencies change in the face of selective and environmental pressures – through what is known as the Hardy-Weinberg (H-W) equilibrium equation. Many H-W equation problems have been solved, but the question is why? From a historical perspective, the work of G.H. Hardy and Wilhelm Weinberg (published independently in 1908) resolved the question of whether, in a non-evolving population, dominant alleles would replace recessive alleles over time. So what does that mean? Remember (and we will return to this later), in a diploid organism two copies of each gene are present. Each gene may be represented by different alleles. Where the two alleles are different, the allele associated with the expressed (visible) phenotypic trait is said to be dominant to the other allele, which is termed recessive.84 Geneticists previously believed that dominant alleles and traits were somehow “stronger” than recessive alleles or traits, but this is simply not the case and it is certainly not clear that this belief makes sense at the molecular level, as we will see. The relationship between allele and trait is complex. For example, an allele may be dominant for one trait and recessive for another (think about malarial resistance and sickle cell anemia, both due to the same allele in one or two copies.) What Hardy & Weinberg demonstrated was that in a non-evolving system, the original percentage of dominant and recessive alleles at various genetic loci (genes) stays constant. What is important to remember however is that this conclusion is based on five totally unrealistic assumptions, namely that: 1) the population is essentially infinite, so we did not have to consider processes like genetic drift (discussed below); 2) the population is isolated, no individuals leave and none enter; 3) mutations do not occur; 4) mating between individuals is completely random (discussed further in Chapter 4); and 5) there are no differential reproductive effects, that is, no natural selection.85 Typically H-W problems are used to drive students crazy and (more seriously) to identify situations where one of the assumptions upon which they are based is untrue (which are essentially all actual situations).

Questions to answer & ponder:

  • Why does variation never completely disappear even in the face of stabilizing selection?
  • What would lead stabilizing selection to be replaced by directed or disruptive selection?
  • Explain the caveats associated with assuming that you know why a trait was selected.
  • How could phenotypic variation influence random mating?
  • By looking at a population, how might you estimate the strength of selection with respect to a particular trait?

References

  1. In the context of the ABO gene for blood type, A and B alleles are dominant to O, which is recessive. Neither A nor B are dominant or recessive with respect to one another.
  2. Hardy-Weinberg Equilibrium: http://www.tiem.utk.edu/~gross/bioed...-weinberg.html

Contributors

  • Michael W. Klymkowsky (University of Colorado Boulder) and Melanie M. Cooper (Michigan State University) with significant contributions by Emina Begovic & some editorial assistance of Rebecca Klymkowsky.