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5. 21: Natural Selection

What's fitness?

Does this type of fitness have anything to do with natural selection? Usually not. There are countless ways in which an organism can be more "fit," or better adapted to its habitat. And we probably do not know about most of these adaptations.

Natural Selection

Natural selection occurs when there are differences in fitness among members of a population. As a result, some individuals pass more genes to the next generation. This causesallele frequencies to change.

Sickle Cell and Natural Selection

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. You can also watch a video about natural selection and sickle-cell anemia at this link:

Sickle-cell anemia blood cells, and normal red blood cell

Sickle Cell and Natural Selection. Notice the normal-shaped red blood cell on the left, and the sickle-shaped cell on the right.

Genotype Phenotype Fitness
AA 100% normal hemoglobin Somewhat reduced fitness because of no resistance to malaria
AS Enough normal hemoglobin to prevent sickle-cell anemia Highest fitness because of resistance to malaria
SS 100% abnormal hemoglobin, causing sickle-cell anemia Greatly reduced fitness because of sickle-cell anemia

Here’s how natural selection can keep a harmful allele in a gene pool:

  • The allele (S) for sickle-cell anemia is a harmful autosomal recessive. It is caused by a mutation in the normal allele (A) for hemoglobin (a protein on red blood cells).
  • Malaria is a deadly tropical disease. It is common in many African populations.
  • Heterozygotes (AS) with the sickle-cell allele are resistant to malaria. Therefore, they are more likely to survive and reproduce. This keeps the S allele in the gene pool.

The sickle-cell example shows that fitness depends on phenotypes. It also shows that fitness may depend on the environment. What do you think might happen if malaria was eliminated in an African population with a relatively high frequency of the S allele? How might the fitness of the different genotypes change? How might this affect the frequency of the S allele?

Natural Selection and Polygenic Traits

Sickle-cell trait is controlled by a single gene. Natural selection for polygenic traits is more complex, unless you just look at phenotypes. Three ways that natural selection can affect phenotypes are shown in Figure below. You can also watch an animation comparing the three ways at the link below.

  1. Stabilizing selection occurs when phenotypes at both extremes of the phenotypic distribution are selected against. This narrows the range of variation. An example is human birth weight. Babies that are very large or very small at birth are less likely to survive. This keeps birth weight within a relatively narrow range.
  2. Directional selection occurs when one of two extreme phenotypes is selected for. This shifts the distribution toward that extreme. This is the type of natural selection that the Grants observed in the beak size of Galápagos finches.
  3. Disruptive selection occurs when phenotypes in the middle of the range are selected against. This results in two overlapping phenotypes, one at each end of the distribution. An example is sexual dimorphism. This refers to differences between the phenotypes of males and females of the same species. In humans, for example, males and females have different heights and body shapes.

Stabilizing, disruptive, and directional selection

Natural selection may affect the distribution of a polygenic trait. These graphs show three ways this can happen.

For a review of natural selection and genetic drift, and how they relate to evolution, see Mutation, natural selection, genetic drift and gene flow are discussed at (8:45).


  • Natural selection occurs when there are differences in fitness among members of a population.
  • Natural selection for a polygenic trait changes the distribution of phenotypes. It may have a stabilizing, directional, or disruptive effect on the phenotype distribution.

Explore More

Explore More I

Use this resource to answer the questions that follow.

  1. Describe natural selection in each of the following:
    1. the peppered moth,
    2. the Galapagos finches,
    3. peacocks,
    4. the deer mouse,
    5. and humans.

Explore More II


  1. What is natural selection and what are its effects on allele frequencies?
  2. Describe three types of natural selection for a polygenic trait.
  3. How does the recessive sickle-cell allele stay in the gene pool?