Ch 21

• Penicillin, a product of Penicillium mold, was the first antibiotic drug based on a naturally occurring substance
• Just 4 years after the public started using penicillin, 14% of the Staphylococcus strains in patients in a London hospital were resistant – by 1950, more than half were resistant
• According to the Centers for Disease Control and Prevention, 30,000 to 40,000 Americans die each year from infections caused by antibiotic-resistant bacteria
• Microevolution


  A heritable change in the genetics of a population
• Population


  All individuals of a single species that live together in the same place and time
• Types of phenotypic variation

  • Qualitative
  • Quantitative
• Qualitative variation


  Characters that exist in two or more discrete states (polymorphisms), described by the percentage or frequency of each trait
• Quantitative variation


  Most characters that differ in small, incremental ways that can be measured and displayed on graphs
• Examples of qualitative and quantitative traits


  • Blood pressure (quantitative)
  • Eye Color (qualitative)
  • Blood Type (qualitative)
  • Religion (qualitative)
  • Weight (quantitative)
  • Attached Ear Lobes (qualitative)
  • Pregnancy (qualitative)
  • Neck size (quantitative)
• Sources of phenotypic variation

  • Genetic differences
  • Environmental factors
  • Interaction between genetics and environment
• Only genetically based variation is subject to evolutionary change
• Breeding experiments can demonstrate the genetic basis of phenotypic variation (e.g., activity levels in mice)
• Sources of genetic variation

  • Production of new alleles
  • Rearrangement of existing alleles into new combinations

  
• Most new alleles arise from small-scale mutations in DNA
• Rearrangements of existing alleles result from larger scale changes in chromosome structure or number and from several forms of genetic recombination (crossing over, independent assortment, and random fertilizations)

  
• DNA studies suggest that every locus exhibits some variability in its nucleotide sequence

  
• Variability is seen among individuals from a single population, populations of one species, and related species
• Some variations in protein-coding regions of DNA do not affect phenotypes because they do not change the amino acid sequences of the proteins for which the genes code
• Gene pool


  Includes all gene copies at all gene loci in all individuals in the population

  
• Genotype frequencies


  The percentages of individuals in a population possessing each genotype
• Allele frequencies


  The relative abundances of the different alleles
• For a gene locus with two alleles, there are three genotype frequencies, but only two allele frequencies (p and q)
• The sum of the three genotype frequencies must equal 1 and so must the sum of the two allele frequencies
• If each genotype has a unique phenotype, as in incomplete dominance, allele frequencies can be counted directly

  
• Snapdragon flower color genotypes

  • RR (red)
  • rr (white)
  • Rr (pink)
• Null models


  Conceptual models that predict what would happen if a particular factor had no effect, used as theoretical reference points against which observations can be evaluated
• Hardy–Weinberg principle


  A mathematical model that describes how genotype frequencies are established in sexually reproducing organisms
• The Hardy–Weinberg principle specifies conditions under which a population of diploid organisms achieves genetic equilibrium – the point at which neither allele frequencies nor genotype frequencies change in succeeding generations
• Conditions for Hardy-Weinberg genetic equilibrium

  • No mutations are occurring
  • The population is closed to migration
  • The population is infinite in size
  • All genotypes survive and reproduce equally well
  • Individuals mate randomly with respect to genotypes
• Agents of microevolutionary change

  • Mutation
  • Gene flow
  • Genetic drift
  • Natural selection
  • Nonrandom mating
• Mutation


  A spontaneous and heritable change in DNA
• Effects of mutations


  • Deleterious
  • Lethal
  • Neutral
  • Advantageous
• Deleterious mutations


  • Huntington's disease, cystic fibrosis, Tay-Sach's disease, sickle-cell anemia
• Nylonase, a bacterium discovered in 1975, is able to digest and live off waste chemicals from nylon manufacture – chemicals that had not existed before nylon production began
• Gene flow


  The movement of organisms or their gametes (e.g., pollen) from one population to another (migration)

  
• Genetic drift


  Chance events (such as founder effects and population bottlenecks) that cause allele frequencies in a population to change unpredictably
• Population bottleneck


  A dramatic reduction in population size due to factors such as disease, starvation, or drought
• Population bottleneck example


  • Elephant seals in the Pacific Northwest exhibit no variation in 24 proteins studied by gel electrophoresis
• Founder effect


  The change in the gene pool when a few individuals start a new population, carrying only a small sample of the parent population's genetic variation

  
• Founder effect example


  • Ellis–van Creveld syndrome in the Old Order Amish community, Lancaster County, Pennsylvania