The current theory of the process of evolution, formulated between about 1920 and 1950, that combines evidence from classical genetics with the Darwinian theory of evolution by natural selection (see darwinism). It makes use of modern knowledge of genes and chromosomes to explain the source of the genetic variation upon which selection works. This aspect was unexplained by traditional Darwinism. Recent discoveries of epigenetic inheritance have led some to call for modification, or even replacement, of neo-Darwinian theory, invoking neo-Lamarckism. But this is still regarded by many as heresy.