Population ecology or natural selection has been considered in social systems
since the 1940's, but was best defined by Hannan and Freeman (1977) and
Aldrich (1979). It primarily applies to populations of organizations, and
tries to explain why some organizational forms survive and others fade away.
They propose that organizational forms with the best fit to environmental
characteristics will be selected and proliferate. In natural selection,
a variety of forms are created, some are selected for better fit, and the
form is retained through reproduction or duplication. They contend that
organizations usually don't change or adapt but are replaced by organizations
with forms that better fit the current environment. It has become very popular
due to its defined and accepted biological paradigm and it's quantitative
One important concept from population ecology is density
dependence, which suggests that the concentration of a certain organizational
form follows a curvilinear approach -- rising legitimacy increases founding
rates early on and later competition reduces founding rates.