Roundworms may provide a clue.
From “New Role for RNAi Discovered: Epigenetic Memory May Pass RNA Silencing from One Generation to the Next” (ScienceDaily, June 26, 2012), we learn:
Now, UMass Medical School researchers have identified a mechanism related to RNAi that scans for intruders not by recognizing dsRNA or some other aberrant feature of the foreign sequence, but rather by comparing the foreign sequences to a memory of previously expressed native RNA. Once identified, an “epigenetic memory” of the foreign DNA fragments is created and can be passed on from one generation to the next, permanently silencing the gene.
“If a worm modulates gene expression by carrying a memory of the genes it expressed in previous generations, perhaps other organisms (including humans) can as well. If so, mechanisms of this type could have an important impact on evolution,” said Craig C. Mello, PhD, Howard Hughes Medical Institute Investigator, Blais University Chair in Molecular Medicine and distinguished professor of molecular medicine and cell biology. “The RNAe mechanism could accelerate evolutionary change by increasing heritable phenotypic variation (without the need for DNA mutations). There is growing evidence that many organisms can track and respond epigenetically to gene expression patterns. Our findings provide insight into a whole new level of sophistication in the recognition and memory of gene expression programs.”
Once the DNA is identified as foreign and silenced, an epigenetic memory is created that silences the foreign gene from one generation to the next. While the inheritance of this memory requires further exploration, the authors showed that successive generations of C. elegans are unable to express the foreign DNA even if the corresponding piRNA is absent.
To the extent that epigenetics plays a role in evolution, it undermines the ubiquitous just-so stories for how Darwin’s natural selection on the random mutation of stable existing genes (via differing survival rates of offspring) did it all.
In epigenetics, the genome is being modified by the environment during the life form’s existence, and the mutation isn’t random. It is a (programmed?) response to a threat, and the information is passed on.
Of course there are differing rates of survival of offspring. But what difference that difference makes surely depends in part on whether the environment stays the same or changes. If the previous generation’s intruder is still a threat, then the epigenetic change helps. If the intruder wanes in importance, it doesn’t particularly. If the next generation features a different intruder, we go back to square one.
Shortly, we will be featuring Woodward and Gills’ book, The Mysterious Epigenome, to unpack what all this mean for Darwin vs. design.