Gingers are Lorax-kin

For real though redness (red hair, freckles) comes from pheomelanin: a divergent form of traditional brown/black melanin (eumelanin) which both branch off from a shared precursor.

The selective pressure (if any) isn't settled but is thought to be potentially some combo of: relaxed pressure against skin camcer protection, Vitamin D synthesis, sexual selection, and (low evidence for) pain modulation, immune signaling, and inflammation.