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Natural selection is tricky to catch in action. As Darwin put it, “A grain in the balance will determine which individual shall live and which shall die.” The grain in the balance—the slightly increased chance that organisms carrying one gene variant will fail in the struggle for existence—is the cost of selection. It is almost invisible, only becoming statistically evident when viewed across thousands of individuals, who may display only subtle differences in the affected character.
In the human population, the toll of natural selection is hidden within millions of deaths and births around the world every year. Everyone dies, many tragically young. And while obvious patterns sometimes emerge from early deaths—certain diseases, traffic accidents, drug overdoses—these are often challenging to connect to the action ...
