As celebrations in honor of Charles Darwin’s 200th birthday are in full swing this month, a revolution is rumbling through evolutionary biology. In the 150 years since Darwin published his landmark book “On the Origin of Species, ” scientists have elucidated much of the molecular underpinnings of how evolution — natural selection, punctuated equilibrium and the like — works. One of the pillars of evolutionary theory has long been the notion that the DNA sequence is the only molecular information that is passed on from one generation to the next. But over the last decade or so, an avalanche of studies has shown that epigenetic settings — specific markings on the genome that can turn a gene on or off — can be heritable as well. Moreover, some of these epigenetic changes are triggered by environmental influences, such as stress or diet, suggesting that there is much more to evolution than just random variation followed by selection of the fittest.
Darwin knew that organisms pass certain traits on to their offspring, but he didn’t know how. Several decades later, genetics clarified the process: Random mutations and genetic shuffling during reproduction can change traits and then over time, those traits that make an individual more competitive become more common in future generations — or so scientists thought.
The first inkling that this model was only part of the story came in the 1990s when scientists looked at a batch of young water fleas that grew larger heads to avoid being eaten when they sensed that a predator was around. Such an early developmental change in response to environmental conditions was not unusual, but it turned out that subsequent generations had large heads as well, even when no predator was there. “That was one of the key things that alerted us that here is an effect that is caused by the environment, which is then inherited for a few generations, ” says Scott Gilbert, a biologist at Swarthmore College in Pennsylvania. It turned out that other studies throughout the 20th century showed such epigenetic inheritance as well. And in 1995, Eva Jablonka, an evolutionary biologist at Tel Aviv University in Israel, and Marion Lamb, a geneticist at the University of London in the United Kingdom, documented about 40 cases.
The fact that organisms develop differently according to environmental cues and that epigenetic changes are responsible is neither new nor surprising. Changes in epigenetic settings modify genes: For example, certain environmental cues sometimes trigger the tagging of DNA with methyl groups, tiny chemical attachments that can silence a gene. “If you have a tree that is growing on a hilltop where there is a lot of wind, it’s going to be stunted and a little bit thicker than the genetically identical tree that is living in a valley and doesn’t get a lot of wind, ” says Eric Richards, a biologist at the Boyce Thompson Institute for Plant Research at Cornell University in Ithaca, N.Y.
But unlike a genetic mutation, the changes in epigenetic settings do not alter the actual DNA sequence. Therefore, scientists assumed epigenetic changes weren’t passed down from generation to generation. So, the offspring of the stunted tree would grow to a normal size if it grew in an area with little wind. “Everything is reset, ” Richards says, “and the next generation does it all again depending on the environment it lives in.”
