Epigenetic Integration, Complexity, and Evolvability of the Head rethinking the functional matrix hypothesis

As I get older, I find myself increasingly hesitant to use the word epigenetics because I worry about employing a term that is so liable to engender confusion and disagreement. Many biologists define epigenetics in a narrow sense solely as heritable changes in the phenotype that derive from molecular mechanisms other than sequence changes in the genotype (the classic example being methylation). However, Waddington, who coined the word in 1942, and other early users of the term had a broader concept in mind, one that captured the variable effects of interactions between genes, embry-onic development, and the environment. According to this original and more encompassing definition, epigenetics refers to the vast set of processes by which alternative, variable phenotypes––cellular, anatomical, physiological , even behavioral––derive from a given genotype (see Haig, 2004). Not surprisingly, there is a rich literature on epigenetics in complex organisms because many layers and types of epi-genetic interactions are essential to initiate the development and integration of diverse units so that they grow and function together appropri-Epigenetic interactions occur at each hierarchical level of development including within the genome; among cells, tissues, and organs; and between an organism and its environment.