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Chapter Summary

  1. In biology, the question "Why do we see a particular trait or phenomenon?" can be answered at multiple levels. Proximate explanations specify immediate mechanisms, developmental explanations specify changes that occur during an individual’s lifetime, evolutionary explanations specify how selection and other evolutionary processes have shaped a trait, and phylogenetic mechanisms specify when and where in the history of life the trait arose.
  2. An evolutionary perspective on human disease does not ask why disease is evolutionarily advantageous, but rather it asks why evolution has left the body vulnerable to disease.
  3. Nesse and Williams distinguished six different evolutionary explanations for vulnerability to disease: (a) coevolutionary arms races, (b) not enough time for selection to catch up with environmental conditions, (c) trade-offs, (d) historical contingency and path dependence, (e) selection favoring reproductive success at the expense of health and well-being, (f) some symptoms possibly being defenses rather than pathologies. These explanations need not be mutually exclusive.
  4. Fever appears to reduce the duration and severity of microbial infection, but usually it can be treated without major negative consequences.
  5. The smoke detector principle suggests that defenses will tend to be overly sensitive because the cost of a false alarm is much less than the cost of failing to respond to a true threat.
  6. Immune systems help hosts cope with pathogens that typically evolve far more rapidly, but pathogens evolve ways of subverting immune responses.
  7. Selection due to immune responses can have a major impact on the phylogenetic structure of viral clades.
  8. Evolution is unable to plan ahead for future contingencies; as a result, organisms may be susceptible to problems that could have been avoided by structuring the anatomy in a different way. Human susceptibility to choking provides an example.
  9. Organisms senesce because natural selection is strong on traits that are manifest early in life but weak on traits that appear later in life. The mutation accumulation hypothesis suggests that drift leads to an accumulation of alleles with deleterious effects later in life. The antagonistic pleiotropy hypothesis suggests that alleles with beneficial effects early in life but deleterious effects later in life will be favored by selection and therefore accumulate in genomes. The disposable soma hypothesis focuses on a trade-off between investment in reproduction and repair.