Synonym of
演变 (yǎnbiàn) and the two are often interchangeable.
衍 carries the nuance of 'spreading out / branching from a root', so
衍变 leans toward 'evolve as a derivation from';
演变 leans toward 'evolve as a process'. In practice modern editors prefer
演变 —
衍变 reads slightly more literary.
