Commentary: A conceptual revolution limited by disciplinary division.

we could not discuss in 1995 was information transmission through small non-coding RNAs. This was not recognized until the late 1990s, when the discovery of gene regulation through RNA interference and the realization that it provided a powerful research tool gave epigenetics research an enormous boost. It soon became clear that the small RNAs that mediate gene silencing can not only be transmitted to daughter cells, but also move to more distant cells, including germ cells. Thus, another route through which epigenetic changes can affect future generations was revealed, one that might have significant effects in all species, including those with early segregation of the germ line. Weismann's barrier was breached in a surprising way, one that was reminiscent of Darwin's unaccepted idea that hereditary information is transmitted from body cells to the reproductive organs through small molecules called gemmules. 10 Today, 'epigenetic inheritance' has become an umbrella term covering the many interacting ways through which variations that do not depend on DNA differences are transmitted in lineages of cells and organisms. In disciplines as diverse as animal behaviour, plant ecology and epidemiology, it is recognized that through epigenetic inheritance acquired variations can have an impact on later generations. Sadly for us, in our own field of evolutionary biology, with a few exceptions, 11 either the significance of epigenetic inheritance is down-played or the subject is treated with hostility; many evolutionary biologists have not bothered to get acquainted with epigenetic research. We have continued to argue that epigenetic and other types of non-DNA variations must be included in evolutionary thinking. 12 Our hope is that now that studies of heredity and development have moved away from the gene-centric approach that characterized the last half of the 20th century, the developmental system approach that is replacing it will lead to more interest in evolutionary epigenetic research and to a more widespread acceptance of the importance in evolution of the inheritance of acquired epigenetic variations. In the past decade or two we have witnessed a remarkable thaw in the long chilly relations between the biological and social sciences. For more than a century it seemed that there was only one basic way the two modes of enquiry