Darwin's Theory of Evolution by Natural Selection

Charles Darwin spent 20 years collecting evidence before publishing On the Origin of Species in 1859, presenting the mechanism of natural selection: heritable traits that improve survival become more common across generations, driving the divergence of species over deep time. Darwin's theory explained the stunning diversity of life on Earth — roughly 8.7 million known species — without invoking supernatural design. It unified biology the way gravity unified physics, providing a single explanatory framework for anatomy, behavior, ecology, and medicine. The modern evolutionary synthesis, incorporating Mendelian genetics, confirmed and extended Darwin's core insight, which remains the organizing principle of all life sciences 165 years later.