The population ecology of social movements takes as its starting point theories of organizational evolution. First elaborated by Hannan and Freeman, what came to be known as organizational ecology provides a research paradigm for scholars interested in understanding how diversity in organizational forms develops and changes over time. Drawing on evolutionary models, organizational ecologists focus on competition and environmental selection as the central mechanisms driving the creation and institutionalization of new organizational forms. In addition to competition over scarce resources, processes of legitimation are also important in explaining how new organizational models become established. A central theoretical principle is that the development of organizational populations is “density-dependent”: early on, increases in the number, or density, of successful organizations improve the legitimacy of the forms and provide a template for action that creates resources and incentives for organizational formation; as the population expands, however, competitive pressures drive down the founding rate and increase the failure rate of existing groups.