Evolution theory, developed by Charles Darwin, exchanged the concept of fixed species with the idea of evolution, made possible by the vehicle of natural selection. Evolution theory is often associated with Marxism and could explain the social theory due to the similarities between the two.
LiveScience.com explains that evolution theory happens over many generations as changes occur in the traits of biological populations. The process of evolution explains the rich diversity of the natural world and is made possible by natural selection. Natural selection takes place when certain traits within a species become more or less prevalent as organisms reproduce with like species over the course of long periods of time. Creation.com describes how Karl Marx, one of the founders of Marxism, recognized how the struggle Darwin discovered in the natural world, as certain traits fight to subsist, paralleled his theory. According to Marx, a struggle exists among the classes of people too. Marx believed that evolution happens in the social world in the form of revolution. As the classes develop and change due to things like greater opportunity or revolt, an economic shift takes place, resulting in a social evolution. Evolution in the biological and social senses does not happen in giant bounds, it is a gradual process that takes times.