Epigenetics and adaptive evolution: which wins?

Yesterday we discussed some aspects of epigenetics, that is, the modification of DNA that does not alter its nucleotide sequence but does induce or repress expression of genes in the modified area of the chromosome.  There were a lot of comments and replies on that post that you might want to browse.  

However, one reader tweeted a very interesting question that we’d like to address here where others could see his and our thinking. He wrote: Could epigenetic inheritance make genetic assimilation more potent since environmentally induced phenotype is multigenerational?

We thought this was worth addressing here, as it raises important issues about how evolution works.  

For readers who may not know the term, ‘genetic assimilation’ refers to a situation in which some environmental factor induces one of a particular set of possible states (sometimes called polyphenism).  This has long been studied in many different species, though much of the work was done in the early 20th century’s pre-gene era–that is, when actual known genes were few and far between.  Earlier studies had to rely on traits for which there was reasonably specific evidence, even if indirect, that their variation was due to genetic variation.

Clearly we now know that many traits, including behavioral traits of various kinds, are affected by epigenetic changes as well as DNA sequence variation.  In itself, traits due to epigenetic changes have not seemed to be inherited:  Although since gene expression is directly the result of environmental changes a cell detects, epigenetic changes are a major mechanism of local adaptation.  However, the idea has been that state of an organism’s trait would not be inherited.  Each organism starts life afresh in terms of its gene usage.

Yet there are by now many studies in various species, direct and indirect, observational and experimental, that show not just that changes in cell behavior involve epigenetic mechanisms, but that epigenetic changes may sometimes be inherited–perhaps even for multiple generations.  This has been seen as a threat to Darwinian theory, to the extent that such assertions have been sneered at as almost ante-diluvian Lamarckian nonsense.  Still, there is the evidence, and it’s from legitimate investigators, not crack-pots, and from legitimate journals (well, some of them are like Nature and Science, that go for sensationalistic stories, sometimes not looking all that closely at the evidence).  So those who react against anything that smacks of environmentalism should stop and take a breath. 

There is nothing at all surprising about organisms reacting to their environment and since we are made of cells that express genes, about that reaction involving gene-usage changes.  Epigenetic changes are not mystic, mechanisms are clearly known, and they at most would change the criteria to which the term ‘inheritance’ is applied.  This in the same way that, because of increased understanding of DNA, the term ‘gene’ has rapidly lost its standard 20th century meaning–with no threat to basic genetic or evolutionary theory.

Genetic assimilation
This is a term coined by CH Waddington in the mid-20th century, but that applies to phenomena studied in the late 19th century under names like the Baldwin effect (for more on this history, you could see my 2004 Evolutionary Anthropology article, “Doin’ what comes naturally”).  Classic experiments were done to show that genetic assimilation can happen.  [Waddington was a quirky character who made lots of enemies and was dismissed by many, often for political reasons but that's irrelevant here]

Traits, including behavior, were thought occasionally to be such that they might lead to higher reproductive success, and if in the same environment this success was consistently achieved by having the trait, it could be beneficial for mutations that arose, which generated the trait, to increase in frequency.  That is, if such mutations arose, selection could favor them so that eventually traits that had once been environmentally induced became genetically hard-wired.

How often this actually happens in nature is debatable and controversial because it has seemed by some to verge on non-Darwinian evolution.  But if or when it happens,, genetic assimilation would, in a sense, guarantee that the individual had the advantageous trait. This is what led our correspondent to ask whether epigenetic changes that could be passed down over some generations might give normal genetic evolutionary mechanisms a chance to occur, by presenting the favored trait to the environment more stably and consistently than if it depended on chance epigenetic mechanisms each generation.

The obvious answer is that this is certainly plausible. But it would have to persist for far longer than has been observed, to our knowledge, to match the slowness and random-mutational aspect of normal evolution.  As important, to us, is that if one thinks carefully about evolution, it is not so clear what would actually happen.

But good or bad?  Is epigenetic or genetic causation more ‘fit’?
If the trait were hard-wired because of mutations, environmental induction wouldn’t be necessary.  If the environment is present, the organism doesn’t have to rely on any chance aspects of epigenetics to make it fit, relative to natural selection, compared to a less likely competitor who had to rely on that chance.  Of course, the mutations shouldn’t reduce the chance of a response, say by erasing the DNA signal for epigenetic marking. But if they guaranteed the trait, the organism starts out life in an adapted state.

Yet one can ask when it is a good for a species to be hard-wired.  One could argue that it is not such a good thing, because the organism may be far less able to adapt to different or changing circumstances.  Depending on the species, its population size and habits and reproductive biology, it might be far better for each individual or local set of kin to adapt in an epigenetic way, when or if the environmental circumstance arises.  An epigenetic response would be reversible when environments change.  Let the environment do the talking, so long as the organism can respond to it.

However, an obvious analogue to natural selection applies to epigenetic traits: if they are really passed down from one generation to the next, that is itself like a form of hard-wiring that’s only somewhat ‘softer’ than incorporating the trait into a deterministic DNA sequence.  It might be better not to transmit the trait even epigenetically.

If the environmental factor is utterly inevitable, hard-setting by DNA sequence might be as good as or even surer than epigenetic transmission.  But otherwise, maybe the risk of having to experience the environment and then adjust to it epigenetically, is worth the ability not to make that epigenetic change until it’s necessary.  In other words, both genetic and epigenetic pre-determination may both be risky.  In a less certain environment, it may be better for each individual to learn by experience how to respond to what it faces; in that case, epigenetic marking is a good way to go, but not necessarily epigenetic transmission across generations.

Here we have some testable, sensible scientific issues with no obvious answer.  Of course, one first has to accept the reality of epigenetic effects and their transmission under at least some conditions, before one can even ask the question.  But whether conceptual assimilation by scientists is as real as genetic assimilation is not so clear.