In a joint publication with Alfred Russel Wallace, he introduced his scientific theory that this branching pattern of evolution resulted from a process that he called natural selection, in which the struggle for existence has a similar effect to the artificial selection involved in selective breeding.
I en fælles publikation med Alfred Russel Wallace introducerede han sin videnskabelige teori, at denne evolution var resultat af en proces, som han kaldte naturlig selektion, hvor kampen for overlevelse har en lignende effekt som den, der ses ved kunstig udvælgelse inden for selektiv avl.