We investigate how synonyms evolve over time, focusing on two apparently contradictory tendencies: the Law of Differentiation, whereby synonyms diverge in meaning, and the Law of Parallel Change, whereby they co-evolve. We develop a framework that breaks down the laws into a fine-grained taxonomy, which we use to construct a diachronic dataset based on the Oxford English Dictionary. We rely on definition embeddings and the use of LLM-as-a-Judge to link synonyms. We validate our methodology by performing a human evaluation on a subset of the constructed dataset. Finally, we populate the taxonomy and conduct extensive analyses across different time spans to investigate the predominance of each law in English over time. We find that while the Law of Differentiation is larger in absolute numbers, it is driven by the general tendency for meanings to disappear from the language rather than by a property unique to synonymy.