Prolonged exposure to constant environmental conditions prompts nonrandom genetic variation

Long-term environmental exposure under selection-free conditions has no consequences for fitness under the neo-Darwinian paradigm but it may provoke adaptive developmental buffering if environmental pressures foster directional organismal changes. To test this hypothesis, we revisited a Mutation Accumulation (MA) experiment where isogenic lines of the ciliate Paramecium were propagated for >40 sexual cycles ([~]4 years) in a nearly selection-free and nutrient-rich environment. We find that these MA lines somatic genome is enriched with intervening segments of DNA (IESs), which are normally eliminated during germline-soma differentiation. Across independent replicate MA lines, an excess of these somatic IESs fall into a class of epigenetically controlled sequences, map to the same genomic locations, and preferentially disrupt loci that regulate nutrient metabolism. Although further work is needed to assess the phenotypic consequences of somatic IESs, these findings support a model where environmentally induced developmental variants may restore an adaptive fit between phenotype and environment. In this model, positive selection is surprisingly dispensable for adaptation.