One discontent with the role of the epigenetics in the evolutionary thought is that it´s, by definition, just soft inheritance. But what is rough inheritance?
What determine that one inheritance system could be soft or rough? is not this merely an investigator´s opinion, instead a biological property? This because a gene is far from inmortal, the genome structure is highly dynamic and the acclaimed intrinsic stability is just a methodological simplicity, a lab phenomen
http://www.sciencedirect.com/science/article/pii/S0092867407001262
I may be wrong, but my opinion is that they key to understanding this difference is natural selection. I would say that what you call "rough inheritance" is something selection can act upon. Epigenetic mechanisms could be subject to "rough inheritance" in this terminology. Consider this hypothetical example: in response to certain environmental conditions (such as: low food) a promoter of a gene becomes methylated in germline cells and this affects the future offspring and makes them more adapted to these environmental conditions. We can clearly see the advantage of this mechanism, selection will shape the genes involved in this mechanisms so that it works in the proper conditions, but not in the improper ones. However, the organism couldn't have possibly known that these epigenetic changes give an advantage in advance: only through the trial and error of previous generations, only through selection on the genetic level. The short term epigenetic changes themselves are usually not subject to selection ("soft inheritance"), because their persistence depends on the genes ("rough inheritance"). Without changes in the genes that regulate epigenetic modifications, they can probably persist for a few generations, but that is almost nothing in the timescale of evolution.
Thanks for the answer
Your answer have done two different reactions in the moment of answer back
1. The first or the definition of the "hardness" of inheritance. I think that is a great position, because it moves the attention from the details or properties of the molecules, to the function of each marker over the fitness. Or in the terms of the philosophy of biology, your position does the "soft inheritance" a potential historical cause of variation, more than a proximate mechanism
2. But .. the next part of the answer denies back the possibility that you've finished of give. Supported by a bad argument
The short term epigenetic changes themselves are usually not subject to selection ("soft inheritance"), because their persistence depends on the genes ("rough inheritance")
a) The persistence of the epigenetic marks is crucial for the normal development of the organism. e.g. the total lab conditions dependency of the plant without epigenetic marks or with mutation over the DMT.
b) The persistence of the marks is not a necessary condition for the Natural selection act's
Generally dog population show promiscus nature in reproduction, even after facing every odd dogs pass on characters and sensitivity to materials and odors to their filialsIt is an example of rough inheritance.
Is that a question to better understand the historical development of such semantics? Then you will find some insights in old papers on soma/germ pasm by august Weismann (e.g., A. Essays upon Heredity and Kindred Biological Problems (Clarendon Press, Oxford, 1891)
If the question is about the comparison of inheritance mechanisms and their evolutionary significance, some questions remain to be answered about "soft" inheritance (e.g., stability) but some insights can be find in this paper:
https://www.researchgate.net/publication/51226751_Beyond_DNA_integrating_inclusive_inheritance_into_an_extended_theory_of_evolution._Nat_Rev_Genet
Article Beyond DNA: Integrating inclusive inheritance into an extend...
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