When I first heard about epigenetics, I hated it. I still do, in fact, but I am slowly beginning to accept the fact that epigenetics is a fact. And it’s a terrible fact, like all facts that force one to radically change one’s long-held ideas.
In the 19th century everyone knew about inheritance, but nobody knew about genes. Two similar theories of evolution were proposed, one in 1809 by Lamarck, and 50 years later a more complete one by Darwin. Both men proposed that species can change their characteristics over time and that such changes can even produce new species, and both saw the role of the environment as critical to this process. Lamarck’s theory was that when creatures experienced environmental changes to their characteristics, these changes could be inherited, resulting in evolution of the population.
Darwin’s theory saw evolution proceeding in two major steps. First there were heritable changes, and then environmental conditions selected which organisms would survive and reproduce based on the phenotypes produced by these inherited changes.
The 20th century saw the flowering of Mendelian inheritance into a major science. The concept of the gene – the hypothetical inheritance particle – was substantiated by the discovery of the structure and function of DNA. Darwin’s theory was confirmed, Lamarckian ideas were utterly rejected – both on theoretical and observational grounds – and neo-Darwinism was born.
And that’s how things have stood for the past 60 years or so. The inheritance of acquired characteristics, as Lamarkian evolution has been summarized, was dismissed as nonsense, with no evidence and lots of counterevidence.
Until recently. Don’t you love science? It’s never boring. I first heard about epigenetics when I was researching the environmental causes of cancer. The idea was that in some circumstances, certain parts of the DNA molecule are modified with methyl groups that change the expression (usually by repression) of a gene. This modification of gene function can act like a mutation, since if a gene is not expressed, it’s just as if it had undergone a mutation. By itself, these findings were not terribly dramatic, because such methylation and its effects could not possibly be inherited, so I (and everyone else) thought of these epigenetic changes (called marks) as having no impact on basic Darwinian theory.
Oops. Over the past decade, it has become clear that sometimes these epigenetic changes, due to various exposures or environmental influences, can in fact be inherited, even after the exposure is over. At first it seemed that this only happened for a few generations. But no. That would be too easy. It now appears pretty likely that some epigenetic effects are long-lasting, perhaps permanent, just like a mutation in the DNA sequence. The crucial difference is that mutations are very rare, accumulate slowly, and are random, while epigenetic effects can be very rapid, much more frequent, and targeted to specific genes.
Welcome back, M. Lamarck. Having been educated for over four decades that Lamarckian evolution is a nonsensical, utterly wrong idea, I was not happy. But that’s science. Physicists have gone through this sort of thing routinely in the past century or so, and now it’s the turn of biologists.
The implications of long-term inheritance of epigenetic alteration of genomes are staggering. If epigenetics is a real mechanism of evolution, as real as the neo-Darwinian mechanism of mutation and natural selection, then everything in biology changes. The good news is that a lot of tough question might be answered, such as how phenotypic innovations can lead to major and rapid appearance of new biological features, without needing the very slow, laborious process of mutation accumulation (allelic variation) followed by selection. The bad news is that almost everything we thought we knew about evolution must now be re-thought.
As in all such paradigm shifts, traditional Darwinian genetic evolution is not disproven; it has been modified to include a host of new mechanisms (epigenetics being a major example, but not the only one). Lamarck is back, with his own molecular mechanism of epigenetics to be joined with Darwin’s genetic mechanism. The theoretical and predictive implications of this merger are both incredibly complex and very exciting. I propose that the new theory of evolution combining inherited germline mutations and inherited epigenetic marks be called Darmarckian evolution. And to all my colleagues who share my horror at such a heretical vision of how biology actually works, I can only say (as I have had to say to myself), “It’s science – deal with it”.