Chapter 13.4: Selection Can Be Stabilizing, Directional, or Disruptive

Flashcards from Chapter 13.4 of Principles of Life, 3rd Edition.

Fitness

The reproductive success of a particular phenotype, relative to other phenotypes

Qualitative traits

Traits that are distinguished by discrete qualities, such as smooth or wrinkled pea texture

Quantitative traits

Traits that are likely to show continuous qualitative variation, such as body size

Stabilizing selection (purifying selection)

Natural selection on quantitative traits favoring average individuals

• Reduces variation while maintaining the mean

  • Often selects against deleterious mutations to the usual gene sequence

• Most common action of natural selection

Directional selection

Natural selection on quantitative traits favoring one direction away from the mean of the population

• For a specific variant, this is referred to as positive selection due to the increase in frequency

• Seen with Texas Longhorns evolving longer horn length due to predation in the wild

Disruptive selection

Natural selection on quantitative traits favoring both directions away from the mean of the population

• This increases variation in the population