“This preservation of favourable variations and the rejection of injurious variations, I call Natural Selection.”Charles Darwin · On the Origin of Species · 1859
Species evolve by the gradual accumulation of small advantages in individuals that reproduce more.
Picture wild rabbits: the slightly faster ones escape foxes more often and have more babies. With each generation, the remaining rabbits are, on average, a little faster. Darwin called this slow process natural selection — nobody directs it, yet it has shaped life on Earth for millions of years.
Natural selection is not just about survival. It is about differential reproduction. A fast rabbit escapes the fox, but if it does not reproduce, its speed dies with it. What truly matters is how many offspring it leaves behind. Think of a bird that sings more beautifully. It attracts more mates and has more chicks. Its song spreads through the population. This is the core: not only the strongest survive, but those who leave the most descendants. And this subtly shifts the direction in which a species evolves.
Why it matters The same idea explains today why bacteria become resistant to antibiotics.