Defining Epigenetics in Deterministic Terms

become the causes, and some causes become the effects. In turn, the “revolution” in The Epigenetics Revolution is not so much a revolution as it is a new form of government regulation—DNA sequence variation is a blueprint and the cause for phenotypic variation, but it has now acquired an additional layer of interpreters and modifiers whose origins and evolutionary destinies are not clear. There are some similarities between these two books: Both illustrate the extent to which epigenetic processes seem to rule every aspect of our lives. We learn that these processes create and regulate uniquely medically relevant tradeoffs between stem cell and cancer cell lineages; they regulate parthenogenesis through imprinting and methylation and thus shape mating and life-history systems; they underlie sex determination, reconfigure our skeleton for current mechanical needs, and shape our memories and create behavioral phenotypes; they assimilate past environmental events and create new ecological cycles. But how did these epigenetic effects become so powerful in evolutionary processes? What is their historical origin and evolutionary future? When do epigenetic processes get involved in evolution by natural selection? And are they, themselves, a product of natural selection? Answering these questions would require a synthesis of epigenetic patterns into evolutionary processes or, at the very least, an examination of the relationship between epigenetic effects and associated DNA sequences. Neither of these books ventures parti cularly deep into these areas, however. Instead, both titles invoke the conceptual framework of Conrad Waddington (1953, 1957), who used the concept of canalization to understand developmental inte gration of genetic and environmental influences in the appearance of distinct tissue types in place of a continuous variation. The and function of those organisms’ phenotypes. Instead, such data often unveil a complex epigenetic road across multiple levels of organization—from transcription regulation to cell interactions and tissue feedbacks—that is needed to take the genotype on a journey to the pheno type that is most appropriate in a particular developmental or ecological context. The crucial question is whether traveling on this road from genotype to phenotype is synonymous with moving from cause to effect. To this question, two recent books—one academic, Epigenetics: Linking Genotype and Phenotype in Development and Evolution, and one popular, The Epigenetics Revolution: How Modern Biology Is Rewriting Our Understanding of Genetics, Disease, and Inheritance—give different answers.