Googling ‘natural selection + Stalin’, sure enough, I end up at Evolution News for a useful essay on the connection. But this conservative site would never acknowledge the connection of Darwinism/social Darwinism to capitalism, thus this piece/site while useful is hardly trustworthy.
In any case, our critiques of Marx are from the left and there the connection of Marx and Darwin needs careful examination. I am suspicious that Marx suspected from the first that Darwinism’s natural selection was ideological and then changed his mind, or else dissembled, under the influence of Engels, Marx one of the first to be wary of the rising orthodoxy set into motion by Darwin and his generation and which has produced so many hypocrites who must get on with their academic careers.
Biographically speaking, reading Darwin’s Origin was seminal in Stalin’s own march toward a godless communism.