Environmental heterogeneity can favor phenotypic plasticity, but whether evolutionary responses in one environment predict trait expression after environmental change remains unclear for threshold traits. Polyphenisms-plasticity characterized by discrete alternative morphs-rely on developmental thresholds, and as a consequence, are often expected to permit partial evolutionary decoupling among alternative phenotypes. Using replicated experimental evolution and artificial selection of a resource polyphenism in the nematode Pristionchus exspectatus, which develops either a predatory morph or a microbivorous morph, I tested whether evolutionary changes in morph frequency are coupled across environments. Across independent lineages, I asked whether evolutionary increase in predatory morph frequency under selection predicted phenotypic change when populations were reared back in the ancestral environment. I found a strong positive cross-environment coupling: for both sexes, lineages showing larger evolutionary responses during the first 25 generations, and across the full 50-generation experiment, also showed larger increases in predatory morph expression after environmental reversal. Artificial selection on mouth-form produced the same pattern. These results show that even a discrete threshold trait can retain strong and repeatable correlated responses across environments, consistent with morph-frequency evolution being channeled by developmental architecture shared across environments.
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