Software evolution is commonly characterized as a slow process of incremental change. Researchers have observed that software systems also exhibit characteristics of punctuation (sudden and discontinuous change) during their evolution. In this paper, we analyze punctuated evolution from the perspective of structural change. We developed a color-coded visualization technique called the Evolution Spectrograph (ESG). ESG can be applied to highlight conspicuous changes across a historical sequence of software releases. We describe evolution spectrographs and present the empirical results from our studies of three open source software systems: OpenSSH, PostgreSQL, and Linux. We show that punctuated change occurred in the evolution of these three systems, and we validate our empirical results by examining related software documents such as change logs and release notes.