• Natural selection occurs when there are differences in fitness among members of a population.
  • The example of sickle-cell anemia is described in the Figure below and Table below.
  • It shows how natural selection can keep a harmful allele in a gene pool.
  • Natural selection for polygenic traits is more complex, unless you just look at phenotypes.
  • It may have a stabilizing, directional, or disruptive effect on the phenotypic distribution.
  • It also shows that fitness may depend on the environment.
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