The goal of this paper is to document the evolution of a portfolio of related open source communities over time. As a case study, we explore the subprojects of the Apache project, one of the largest and most visible open source projects. We extract the community structure from the mailing list data, and study how the subcommunities evolve, and are interrelated over time. Our analysis leads us to propose the following hypotheses about the growth of open source communities: (1) communities add new developers by a process of preferential attachment; (2) links between existing communities are also subject to preferential attachment; (3) developers will migrate between communities together with other collaborators; and (4) information flow follows project dependencies. In particular, we are concerned with the underlying factors that motivate the migration between communities, such as information flow, co-worker ties, and project dependencies.