Evolution, the science of how populations of living organisms change over time in response to their environment, is the central unifying theme in biology today. Evolution was first explored in its semi-modern form in Charles Darwin 's 1859 book, Origin of Species by means of Natural Selection. In this book, Darwin laid out a strong argument for evolution. He postulated that all species have a common ancestor from which they are descended. As populations of species moved into new habitats and new parts of the world, they faced different environ ....