Students of international relations (IR) are familiar with realist, liberal, marxist, and, more recently, constructivist paradigms. Each has a set of assumptions that reflects a singular perspective on how international relations work. A perspective that is less familiar-in large part, because it is only now emerging-is the evolutionary paradigm. There is no one evolutionary paradigm, just as other IR paradigms also possess multiple variations. But the core assumptions go well beyond the most minimal meaning, and perhaps most common employment, of evolution, that is an emphasis on change. The most critical assumptions involve variation and selection. The principal unit of analysis, whether it be states, regime types, economic innovations, ideologies, strategies, or policies, exists in different formats. For instance, at a given point in time, there may be variation in the types of states (city-states, empires, nation-states) or the types of ideologies (fascism, communism, liberal democracy, socialism) that exist. At a subsequent point in time, some of the state types and ideologies will have disappeared. Nation-states, by and large, have been selected over city-states. Liberal democracy has been selected over fascism and communism. The question is then why one approach is selected and others abandoned or ignored. The general answer is found in the interaction between changing environments and actors. As environments and actors change, so too do the probabilities that some approaches will survive and flourish while others wither and may even disappear.