CHAPTER 14 Summary

• Populations are groups of individuals of the same species living together in the same geographic area.
• Bacteria populations exist nearly everywhere, including on and in our bodies; most are harmless or even beneficial, but some can cause disease.
• Within any population, there is genetic variation among individuals.
• Bacterial populations, which reproduce asexually, acquire genetic variation by mutation and gene transfer; populations of sexually reproducing organisms acquire genetic variation by mutation and by meiosis and fusion of gametes.
• Genetic variation in a population gives rise to corresponding phenotypic variation in the population.
• Individuals with different phenotypes will have differing ability to survive and reproduce in a population; that is, they will differ in fitness.
• The differential survival and reproduction of individuals in a population over time in response to environmental pressure is called natural selection.
• Natural selection is one cause of evolution, which is defined as a change in the allele frequency of a population over time.
• Individuals with higher fitness in a given environment reproduce and pass on their alleles more frequently than do individuals with lower fitness, resulting in evolution by natural selection.
• Over time, natural selection leads to adaptation: advantageous traits become more common in the population, which as a result becomes more suited to its environment.
• Natural selection can shift the allele frequencies in a population in one or other of several patterns: directional selection, diversifying selection, or stabilizing selection.
• Antibiotic-resistant populations of bacteria emerge by directional selection in the presence of antibiotics.

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  MORE TO EXPLORE

  • Infectious Diseases Society of America: Patient Stories http://www.idsociety.org/Patient_Stories/
  • Klevens, R. M., et al. (2007) Invasive methicillin-resistant Staphylococcus aureus infections in the united States. Journal of the American Medical Association 298(15):1763–1771.
  • Aiello, A. E., et al. (2007) Consumer antibacterial soaps: effective or just risky? Clinical Infectious Diseases 45:S137–S147
  • Aiello, A. E., et al. (2005) Antibacterial cleaning products and drug resistance. Emerging Infectious Diseases 11(10)1565–1570.
  • Kallen, A. J. (2010) Health care-associated invasive MRSA infections, 2005–2008. Journal of the American Medical Association 304(6):641–647.