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Whether phenotypic plasticity generally facilitates or hampers adaptive evolution is a matter of much debate. By comparing gene expression changes in guppy populations, Ghalambor et al. 1 suggest that ‘adaptive’plasticity hampers adaptive evolution, whereas ‘non-adaptive’plasticity facilitates adaptive evolution 2. Here, we argue that the classification of individual gene expression changes as representing either adaptive or non-adaptive plasticity is problematic. Instead of indicating adaptive evolution, the expression changes of suites of genes may be caused by potentially random changes in underlying regulatory processes. There is a Reply to this Comment by Ghalambor, CK et al. Nature 555, http://dx. doi. org/10.1038/nature25497 (2018). Ghalambor et al. 1 did not directly measure organismal plasticity or evolutionary adaptation at the organismal level 2, but inferred evolutionary changes in plasticity from …