As the title of Nietzsche's work suggests, his work is a genealogy. What he wants to do is to demonstrate that the origin of concepts which we hold dear have a genesis that is contrary to the meaning we attribute to them. In this case, he wants to argue that the concept of an objective moral order has an origin that can be explained naturalistically. Thus, for Nietzsche, there is a distinction, as well as a disjunction, between origin and purpose, as Sadler points out. The meaning we attribute to a specific institution may be radically different from its origin. For example, what is the origin of punishment?
We tell ourselves that its purpose is remedial. We want to discourage people from doing something wrong for their own good. But according to Nietzsche, the origin of punishment had nothing to do with correcting someone's behavior. On the contrary, it had only to do with revenge, and the expression of anger. The sociologist, as Sadler pointed out, might say that the purpose of punishment is to reinforce social norms. But as Sadler points out, the sociologist's explanation is simply a specific, sedimented expression of the will to power.
In many fundamental respects, Nietzsche appeals to evolutionary processes to make his point. He sees life as fundamentally oriented around will to power. Life is fundamentally about violence, desire, and the imposition of our own desire through violence. As Sadler points out, this dynamic is operative even in supposedly 'passive' forms of life like plants (which are really not passive at all). But he is not a strict Darwinist. Indeed, he sees a purely Darwinian explanation of psychology as reductionistic as well as reactive. Adaptation, for example, is pure reaction.
We encounter a situation and we respond to it by changing our behavior. In contrast to this, Nietzsche wants to explore spontaneous and active behaviors, and he sees adaptation as an inadequate explanation for all of our psychological processes. Many of our behaviors, he argues, is not the result of adaptation, but the result of the imposition of our wills onto others for reasons that have nothing to do with adapting to our environment, but are explicable only in terms of purely affirmative will to power.
Sadler, Gregory. [Gregory B. Sadler]. (2013, January 18). Existentialism: Friedrich Nietzsche, Genealogy of Morals (part 2). Retrieved from: http://www.youtube.com/watch?v=i5gnYCyKLeQ