Copy number variation facilitates rapid toggling between ecological strategies.

A central question in evolutionary biology is how different types of mutations shape ecological strategy. Whereas both single nucleotide variants (SNVs) and copy number variants (CNVs) can alter gene dosage, it remains unclear whether CNVs confer unique adaptive advantages. Here, we show that CNVs do not mediate adaptive tracking in fluctuating environments, but instead act as reversible toggles between ecological strategies. Using a dual-fluorescent CNV reporter system in , we tracked CNV dynamics at two transporter loci during long-term evolution under static and fluctuating nitrogen limitation. CNVs arose with high repeatability in static conditions but showed dampened or divergent dynamics in fluctuating environments, suggesting they do not track environmental change. Instead, we found that the ratio of copy number between the two loci, but not copy number at either locus alone, predicted ecological strategy: imbalanced CNV ratios defined specialists, while balanced ratios defined generalists. Evolution in static environments favored specialists whereas fluctuating environments favored generalists. Applying this framework to over 3,000 sequenced yeast genomes, we found generalist CNV signatures in both wild and domesticated strains, but specialist signatures exclusively in domesticated strains. These findings introduce a generalizable framework for predicting ecological strategy from genome structure, positioning CNV ratios as a molecular signature of niche breadth across evolutionary and ecological contexts.