Natural Selection is a theory developed by Charles Darwin and Alfred Wallace. Darwin, however, usually receives the most credit. Darwin developed the idea by taking notes on finches he saw on his voyage on the HMS Beagle. He noted that finches on the Galapagos Islands were similar, but different and that their beaks helped them survive with how they acted.
Charles Darwin also wrote On the Origin of Species, a book stating his theory with evidence of it. He suggested that species evolved over time with small genetic variations that helped them to survive. Over millions of years, today's species evolve into what we know today.