Andrew Marr discovers something surprising about his own evolutionary history as this epic series continues with an exploration of Darwin's impact on politics and society. Under the banner of Survival of the Fittest, Darwin's theory of natural selection has been used to justify imperial expansion and the oppression of indigenous peoples; to inform the science of eugenics - the selective breeding of humans which was implemented in the United States in the early 20th century; and to provide a veneer of scientific respectability to Nazi plans to create an Aryan master race. It was also used quite explicitly to explain the twisted logic of the final solution. But Andrew Marr also finds a kind of redemption for Darwin's theory of evolution. After the Second World War, it was a founding idea behind the democratic, anti-racist values of the United Nations. More recently, it has also been used to help eliminate a fatal genetic disease from the Orthodox Jewish community in Brooklyn. Marr goes on to consider the difficult social and political choices presented by predictive DNA testing - the final frontier of Darwin's Dangerous Idea.