By Richard Dawkins and Yan Wong
We saw in the Mouse’s Tale that the main differences between mice and men reside in the pattern of choosing genetic routines from the available toolbox. It is of even greater importance in the case of differentiated cells in a single multicellular body. What distinguishes your nerve cells from your liver cells, your muscle cells from your skin cells, is not the DNA instruction set, but which instructions have been switched on or off. Unlike the case of mice and men, this is not a genetic difference, for your cells have essentially identical DNA. The difference is epigenetic — outside of the genome. This has been known since before the discovery of DNA — the word was coined by Conrad Waddington in 1942 — but it has become disappointingly popular to trumpet this as something unexpected, even as a threat to our conventional understanding of genetics. Newspapers state with ill-informed astonishment that new research shows the environment can modify which genes are turned on and off. Well of course it can! Embryology depends on it. And it’s not just when switching between different types of cell. Compared to a couch potato, we would expect a body builder to have a different pattern of genes being switched on in his muscles.
Responding to environmental cues — be those hormones in embryonic development or external influences in an adult — is exactly what genes do. This has been known since Jacob and Monod’s investigations of bacteria in the early 1960s. And wasn’t it obvious even before?
There is an extension to epigenetics which is more controversial. This is the idea that the pattern of gene use can be passed on to future generations: epigenetic inheritance. We are fêted with stories of characteristics being passed on from parent to offspring, in a modern resurgence of the Larmarckian idea of a blacksmith passing on his strong muscles to his children. There seems to be something in the human psyche to which this disastrous concept appeals — disastrous because it would also mean the blacksmith’s child inheriting his father’s gammy leg, scarred face and political attitudes (of whatever persuasion). Most of us would consider it fortunate that we do not inherit all the acquired characteristics of our parents. There is a lot to be said for starting from scratch.
Nevertheless, if you define epigenetic inheritance to include cellular inheritance within the body, it is undoubtedly true. Liver cells beget liver cells, muscle cells muscle cells, although the DNA is the same in all. Would it be too surprising if the same epigenetic inheritance carried over to a new body, that of the offspring? There is some suggestive evidence that mothers pass on the effects of starvation to their offspring and even their grandchildren. It’s perhaps not too surprising that chemicals in the egg should carry an epigenetic whiff of the mother. But some stories have the whiff of implausibility. Recent experiments claim that male mice, conditioned to fear certain smells, pass on the fear through their sperm. This result may be solid, but extraordinary claims require extraordinary evidence.
We should make the point that in order for such effects to be of evolutionary significance in the same sense as true mutations, they would have to be passed on not just to the grandchild generation but indefinitely through future generations. In fact, to the extent that they occur at all, they seem to die away over generational time. This is why we expect all adaptive evolution to be caused by Darwinian natural selection on the DNA sequence. In fact, all epigenetic inheritance we know of is under the control of the genome, the most important case being the cellular inheritance required to build a multicellular body. In contrast, pseudo-Lamarckian examples of epigenetic inheritance from parent to child are insignificant, and threaten to distract us from a much more important thought. We said all adaptive evolution is based on DNA, but that is not true for humans. We pass on acquired characteristics in the form of ideas, a case of non-genetic inheritance if ever there was one. This alternative form of information transmission is the root of our culture, and in many senses distinguishes humanity from the rest of life on Earth.