variation and natural selection chap 18 n.
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Variation and Natural Selection (Chap. 18). Two basic points of variation. Variation can occur in a wide array of traits (physical features, color, size....) We may not be aware of the variation (enzymes, disease susceptibility, blood type…). Variation comes from:. Sexual reproduction

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two basic points of variation
Two basic pointsof variation
  • Variation can occur in a wide array of traits (physical features, color, size....)
  • We may not be aware of the variation (enzymes, disease susceptibility, blood type…)
variation comes from
Variation comes from:

Sexual reproduction

·genetic recombination in meiosis continually produces variation by shuffling existing genes and alleles

·fertilization allows the alleles from two diff. individuals to be brought together in one new individual


·provides a steady input of new genes and alleles

maintaining genetic variation
Maintaining Genetic Variation
  • Recessive traits tend to be maintained in a population because of heterozygous individuals
    • natural selection may not operate on the recessive alleles.
  • Often results in balanced polymorphism, a process that inhibits the loss of recessive (even detrimental) alleles
    • the population’s ratios of alleles do not change much from one generation to the next
    • beneficial for a population to maintain two different alleles creating different phenotypes within the population.

Types/examples of balanced polymorphisms

  • Heterozygote advantage
  • heterozygous genotype has greater success than either homozygote
  • ex) sickle cell anemia:
    • heterozygous individuals do not develop anemia and are resistant to malaria. In some areas, 40% of a population are carriers.
    • Their red blood cells, containing some abnormal hemoglobin, tend to sickle when they are infected by the malaria parasite. Those infected cells flow through the spleen, which culls them out because of their sickle shape -- and the parasite is eliminated along with them.
      • SS – normal, get malaria
      • Ss – resistant to malaria and carrier of sickle cell
      • ss – get sickle cell disease

Sickle Cell Video clip


a. The distribution of the falciparum malaria (Plasmodium falciparum) in the Old World before extensive eradication of mosquitoes begun in the '20s. b. The distribution of the sickle-cell allele within the same area. Intensity of color refers to increasing allele frequencies. Note the correlation between the two distributions.

2. Frequency-dependent selection
  • occurs when the reproductive success of a genotype depends on its frequency in the population
  • Negative
    • a trait has advantage when rare but disadvantage when common which results in a balanced polymorphism
      • Fish example in text
  • Positive
    • a trait has higher fitness when it is common than when it is rare
      • bright colored and poisonous animals
3. Transient polymorphism
  • one allele is gradually replacing the other
    • ex) sickle cell anemia
      • If a population with Ss moves to a location where malaria is no longer present, the s allele will begin to disappear because the selective pressure in its favor has disappeared but the pressure against it (s) continues.
    • ex) industrial melanism
      • Black moths slowly replace white moths in dirty coal burning areas of England
Three observed examples of Natural Selection in response to environmental change that lead to the evolution of organisms
  • 1970’s Evolution of the beaks of Galapagos Island Finches (Geospiza fortis)
  • Periods of draught caused plants to produce large, tough seeds.
  • Sampling the G. fortisthat died as well as those that survived showed that:
    • the larger birds were favored over the smaller ones
    • birds with larger beaks were favored over those with smaller ones.

But did it produce evolution?

The answer turned out to be yes. As the population of G. fortis recovered after the rains returned, the average body size and beak depth of their offspring was greater than before (an increase of 4–5% for beak depth). The bell-shaped curve had been shifted to the right — directional selection.

2 multiple antibiotic resistance in bacteria
2. Multiple antibiotic resistance in bacteria
  • Case study of one bacterial species, Staphylococcus aureus:
  • 1943 - Penicillin becomes commercially available and initially is seen as a "magic bullet" against staph infections1947 - First staph resistance to penicillin reported1980s-1990s - 90% of clinical cases are now resistant1960s - Switch by hospitals to a new drug, methicillin, for serious staph aureus1961 - Methicillin resistance reported in Cairo1980s - Methicillin resistance rising1992 - 15% resistant to methicillin.1996 - 35% resistant to methicillin1990s - Switch to another drug, vancomycin, as last resort1996 - First resistance to vancomycin reported
3 peppered moth fig 18 10 in text
3. Peppered Moth (Fig 18.10 in text)
  • Study took place in England (1800’s)
  • Often referred to as Industrial Melanism
  • Two morphs – peppered/light color (recessive) and dark (dominant)
  • Black morph became more common in dirty, industrialized area.
  • As England improved air pollution, the black morph declined and peppered/white returned.
  • A measure of an organism’s contribution to the gene pool of the next generation—reproductive success
  • Natural selection determines, not favors, the fittest individuals.
  • Can be quantified between 0-1.