Social closure is what turns the vague spectrum of social status into a specific ladder of social classes. Somewhat arbitrary distinctions, such as educational credentials or occupations, are turned into real social distinctions that affect who you make a life with. The idea of social closure, which was advanced by Max Weber, has been most fully developed by Frank Parkin. I have written about marriage and social closure before.
Richard Conniff wrote an interesting book on The Natural History of the Rich. As the title suggests, he uses insights from animal behavior, especially primatology, to look at the behavior of rich people. One of the characteristics of the very rich is that they put up strong boundaries between themselves and other people. They live in fear of thieves, parasites, and gold-diggers. For their own physical, financial, and psychological safety they tend to make a life only with other rich people. They also tend to marry only within the rich group. They adopt the metaphorically biological language of "the tribe" and create a vast interrelated "cousinage." Conniff says all these forms of separation by the rich amount to "pseudo-speciation."
Pseudo-speciation is the ultimate tendency of social closure.