01Selection and the environment
Which phenotype is favored depends on the environment. Change the environment and the favored trait changes: peppered moths darkened during industrial pollution and lightened after clean-air laws.
Modes: directional (one extreme favored), stabilizing (average favored — human birth weight), disruptive (both extremes favored — seed-cracking finches).
02Sexual selection
Traits that increase mating success can spread even if they hurt survival (peacock tails). Intrasexual = competition within a sex; intersexual = mate choice.
03Notebook box
Graph shapes: directional = curve shifts; stabilizing = curve narrows; disruptive = two peaks.
Worked example: After a drought, only large-beaked finches survive. Mode? → Directional.