Abstract: An evolutionary model of the growth, decline, & demographic dynamics of voluntary organizations is developed & tested. The model, rooted in Peter M. Blau's notion that social groups exist in a multidimensional space of sociodemographic dimensions in which social differentiation occurs (see Inequality and Heterogeneity, Free Press, 1977), demonstrates a strong analogy between the adaptive landscape of Sewall Wright ("Evolution in Mendelian Populations," Genetics, 1931, 16, 97-159) & the exploitation surfaces generated by a model of member selection & retention for voluntary associations. The processes of membership recruitment & loss are linked to the social networks connecting individuals. The model generates dynamic hypotheses about the time path of organizations in sociodemographic dimensions, & suggests that membership selection processes at the individual level produce adaptation in communities of organizations. An empirical example is provided, & implications of the model for a variety of research literatures are considered.