The theory of evolution developed by (among others) Charles Darwin in 'The Origin Of Species' (first published 1859).
The theory states (put very simply) that the organisms living today are the result of a design process, evolution, where life forms compete for resources in order to survive and procreate and, in the end, ensure the survival of the genes, a concept first conceived by Gregor Mendel. The competition ensures that only the best (fittest) designs survive, and the process can as such be referred to as natural selection.
There has been much controversy about this idea that challenges the traditional view that mankind is created by a God, but Darwinism has become one of the basic building blocks of modern science, and even Charles Darwin himself was hesitant about publishing his work.
Darwinism has also been misused and misunderstood and used as justification for elitism, racism, the state as an organism and thus providing an incentive for aggression and incorporation of neighbouring states (e.g. Germany during the early 20th century).
Traditional Darwinism has often focused on the individual or group as the unit of selection. Modern Darwinism, neodarwinism has mainly focused on the gene as the unit of selection, mainly through the work of Richard Dawkins (The Selfish Gene, 2nd ed. 1989), and John Maynard Smith and many others.
Daniel Dennett's book, Darwin's Dangerous Idea discusses the consequence of the theory on our beliefs and world view and provides a theory of design space.