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Open-Source Sol~.-warc (OSS) development is regarded as a successful model of cnenuraging "natural product evolution". To understand how this "natural product evolution" happens, we have conducted a case study of four typical OSS projects. Unlike most previous studies on software evolution that focus on the evolution of the system per se, our study takes a broader perspective: It examines not only the evolution of OSS ystems, but also the cvolution of the associated OSS communities, as well as the relationship between the two types of evolution.Through the case study, we have found that while collaborative development within a community is the essential characteristic of OSS, different collaboration models exist, and that the difference in collaboration model results in different evolution patterns of OSS systems and communities. To treat such differences systematically, we propose to classify OSS into three types:Exploration-Oriented, Utilio,.Oriented , and Service-Oriented.Such a classification can provide guidance on the creation and maintenance of sustainable OS5 development and communities.