Genetic and cultural inheritance interact, each altering the selective landscape of the other, so neither can be understood alone.
Cultural practices can change gene frequencies faster than ecological change does, because a practice spreads in a generation and then persists. Lactase persistence, alcohol metabolism and starch digestion all show the pattern. The framework treats culture as a second inheritance system with its own transmission rules rather than as an output of the first.
In practice
- A profession that selects hard on one trait finds the trait more common among its members' children, by two routes at once.
- A tool that becomes universal makes the skill it replaced rare within one working lifetime.