Realism in Naturalism
Naturalism: keeping it real (and depressing) since 1859.
Realism was the literary movement that preceded Naturalism, and out of which Naturalism grew like a Darwinian weed. Realist writers like Gustave Flaubert and Leo Tolstoy were interested in recording people and events as they really happened, almost journalistically.
The line between Naturalism and Realism is often blurred, and many Naturalist writers can also be considered Realist. The difference, however, is that, unlike Realist writers, Naturalist writers often portrayed people as having no control over their fate. They believed that social environment—as opposed to free will—had a huge influence on the actions and behavior of people.
Basically, Realism + A Depressing Lack Of Control Over Your Life = Naturalism.
Chew On This
The line between Realism and Naturalism can be a bit blurry. Have a look at this analysis of the relationship between Realism and Naturalism in Émile Zola's novel Thérèse Raquin.
Edith Wharton's novel Ethan Frome can be considered a Realist text, but its emphasis on pessimism (whee!) and the environment also make it a Naturalist text. Check out this analysis of the novel's genre here.