In this article we apply one of the key concepts in cognitive linguistics, the radial category, to inflectional morphology. We advance the Paradigm Structure Hypothesis, arguing that inflectional paradigms are radial categories with internal structure primarily motivated by semantic relationships of markedness and prototypicality. It is possible to construct an expected structure for a verbal paradigm, facilitating an empirical test for our hypothesis. Data tracking an on-going morphological change in Russian documents the distribution of conservative vs. innovative forms across the (...) cells of the verbal paradigm. A logistic regression model that takes into account the sources of variation (the frequencies of individual verbs and paradigm slots, and individual verb preferences) shows that the language change is implemented differently across the paradigm forms, confirming the expected structure. In addition to markedness and prototypicality, we investigate the impact of frequency and show that there is a good, albeit not perfect match between the expected hierarchy and frequency. We conclude that the diachronic change analyzed in this article gives evidence for the structure of paradigms modeled on the radial category. (shrink)