Evolution by Natural Selection These different traits or variations are part of natural selection. Darwin proposed that natural selection is a cause of evolution. In this context, evolution is a change in the inherited characteristics of a population from one generation to the next. An adaptation is an inherited trait that is present in a population because the trait helps individuals survive and reproduce in a given environment. Note that Darwin’s theory refers to populations and species—not individuals—as the units that evolve.

Forms of Natural Selection In nature, natural selection is most commonly stabilizing. The average members of the population, with intermediate body sizes, have higher fitness than the extremes. Natural selection now acts against change in form, and keeps the population constant through time. Natural selection can also be directional and disruptive.

Stabilizing Selection In stabilizing selection, individuals with extreme characteristics die off or fail to reproduce resulting in populations of individuals with intermediate characteristics. Stabilizing selection is most common in unchanging environments.

Disruptive Selection Extreme traits are favored

Directional Selection Kettlewell’s Moths model this form of selection Gene frequencies shift in one direction

Natural Selection Example: Peppered Moths