Epigenetics — the study of heritable changes in gene expression that do not involve alterations to the underlying DNA sequence — has substantially complicated the classical picture of heredity. In classical genetics, information flowed in one direction: from DNA to RNA to protein. Traits were determined by alleles, and variation between individuals reflected variation in nucleotide sequence. Epigenetic mechanisms, by contrast, operate through chemical modifications to DNA or its associated histone proteins. Methylation of cytosine residues in DNA, for example, can silence gene expression by preventing transcriptional machinery from accessing a gene, even though the gene's nucleotide sequence remains entirely intact.

Of particular interest to evolutionary biologists is whether epigenetic marks can be transmitted from parent to offspring and thereby influence the phenotype of subsequent generations — a phenomenon called transgenerational epigenetic inheritance. In most multicellular organisms, epigenetic marks are largely erased during the formation of gametes and the early stages of embryonic development, a process called epigenetic reprogramming. However, a growing body of evidence suggests that reprogramming is incomplete: some marks escape erasure and are transmitted across generations.

The evolutionary significance of transgenerational inheritance is contested. Some researchers argue that it constitutes a form of soft inheritance — a mechanism by which environmentally induced epigenetic changes can persist across generations — and that this challenges the strict gene-centric view of evolution. Others counter that, even if marks are occasionally transmitted, the rates of transmission and reversion are too high and too erratic to provide the stable, heritable variation that natural selection requires. On this view, epigenetic inheritance is evolutionary noise rather than signal.

Resolution of the debate will require large-scale studies tracking epigenetic marks across many generations and environmental contexts, as well as clearer theoretical frameworks for distinguishing evolutionarily relevant inheritance from developmental carryover effects.