The "upper echelon" literature has mainly produced static empirical studies on the impact of top management team composition on organizational outcomes, ignoring the dynamics of industrial demography. Organizational ecology explicitly studied the dynamics of organizational diversity at the population level, however largely ignoring how the entry and exit of executives shapes organizational diversity over time. In this paper, we try to integrate both streams of demography research and develop a multi-level behavioral theory of organizational diversity, linking selection processes at both levels of analysis. The behavioral mechanism connecting the two levels of analysis is the stylized empirical fact that small groups, including top management teams, routinely reproduce their demographic characteristics over time. We argue that, under certain conditions, the potent forces of team homogenization coevolve with those of population-level selection to sustain between-firm diversity.