Which scientist is credited with formulating the theory of evolution by natural selection?

Charles Darwin set out the theory in On the Origin of Species in 1859, after more than twenty years of gathering evidence. The argument itself is short. Organisms produce more offspring than can survive, individuals vary, some of that variation is inherited, and traits that improve survival and reproduction become more common over generations. Given enough time, that gradual filtering produces new species. What made the idea powerful was the mechanism: earlier thinkers had suggested species change, but Darwin explained how, without appealing to purpose or design. His voyage on the Beagle supplied crucial observations, including the way related species on separate islands differed in ways suited to local conditions. Darwin published when he did partly because Alfred Russel Wallace, working in the Malay Archipelago, sent him an essay outlining nearly the same idea, and the two were presented jointly. What Darwin lacked was any account of heredity, a gap genetics later filled.

The combination of Darwin's selection with Mendel's genetics in the twentieth century is known as the modern synthesis.