<p>Gene duplication is the primary mechanism by which new genes emerge. Models and empirical studies have shown that paralogous genes are maintained because of dosage benefits, the partitioning of ancestral functions or the acquisition of new functions. However, the underlying molecular mechanisms and the relative importance of the factors driving evolution towards one fate or another have remained difficult to quantify. Recent advances in experimental and computational methods, such as gene editing, deep mutational scanning and ancestral sequence reconstruction, have enabled molecular analyses of duplicated gene evolution across timescales. Combined, these approaches are revealing how adaptive and non-adaptive evolutionary forces shape the modern fates of gene duplicates.</p>