3.1.4—Natural selection

Natural selection changes trait or allele frequencies when heritable variants differ in survival or reproduction under current conditions.

Selection acts each generation; adaptation is the longer-term result. It has no planned goal and a trait is advantageous only relative to the environment.

If darker moths leave more offspring on soot-darkened trees, their frequency can rise over generations.

Heritable variants must have different reproductive success; mere presence of variation is insufficient.

Selection is not organisms choosing useful traits; the environment filters existing inherited differences.