Stephen J. Gould made some interesting comments regarding a phenomena he called
textbook orthodoxy, which I quote (in context) the following:
Gould wrote:Professional writing tends to be nuanced and judicious. Even the strongest partisan finesses his commitment and adds at least a footnote or tangential comment, so that any charge of oversimplification or dogmatism may be countered by stating: "but look on page 381 (in the small print); you see, I raised the caveat myself." (Gould 2002: 576)
To learn the unvarnished commitments of an age, one must turn to the textbooks that provide "straight stuff" for introductory students. Yes, textbooks truly oversimplify their subjects, but textbooks also present the central tenents of a field without subtlety or apology -- and we can grasp thereby what each generation of neophytes first imbibes as the essence of a field. Moreover, many textbooks boast authorship by the same professionals who fill their technical writings with exceptions, caveats, and complexities. (Gould 2002: 576)
I have long felt that surveys of textbooks offer our best guide to the central convictions of any era. What single line could be more revealing, more attuned to the core commitment of a profession that bathed in the blessings of Victorian progressivism, and aspired to scientific status in Darwin's century, than the epigram that Alfred Marshall placed on the title page to innumerable editions of his canonical textbook, Principles of Economics: "natura non facit saltum." (Gould 2002: 576)
The changing foci of 20th century textbooks provide direct insight into the history of evolutionary thought and the eventual triumph of Darwinism. In particular, if the Synthesis truly hardened, as I have argued, then texts following the 1959 centennial celebrations -- the apogee of strict selectionism [my emphasis] -- should describe evolution in unambiquously panadaptationist language, and should extol the sufficiency of natural selection to craft the entire range of evolutionary phenomena at all scales, ecological to geological. (Gould 2002: 576)
(....) I have consulted everything I could find, including nearly all major American books for introductory college biology (and several high school textbooks as well). A more complete search, extended back in time to cover the early days of the Synthesis, and the pre-synthetic period as well, would provide a fascinating topic for a dissertation in the history of science .... This field of vernacular expression has been neglected by scholars, though the subject would yield great insight (for such material obviously represents the only formal contact that most students ever receive with any given discipline). (Gould 2002: 576-577)
I appologize for my almost anecdotal approach, but I think that I have identified a robust pattern supporting the hypothesis of hardening. I will focus on the two topics that authors of texts found most congenial in their efforts to explain synthetic evolutionism to introductory audiences: the centrality of adaptation, and the sufficiency of synthetic microevolution to explain events at all scales. (I consider here only the evolution chapters of comprehensive biology texts for introductory courses, not entire textbooks on evolution....) (Gould 2002: 577)
-- Gould, Stephen J. (2002) The Structure of Evolutionary Theory. Belkamp: Harvard.
In a sidebox in the introductory textbook for biology
Principles of Human Evolution the following is stated:
Lewin wrote:Beyond The Facts
Is Lamarkism Dead?
Lamarkism collapsed in the face of the clear evidence that the biological information that an individual acquired during its life could not be passed back to the genes, ans so could not be transmitted to the next generation. There is little to challenge this in the main corpus of biology, and so the Darwinian dogma remains to this day.... However, .... that does not mean that the underlying principles were entirely wrong. In Lamark's case there are two general principles that are important.
The first is the question of whether evolution is driven by internal characteristics of the organism, or by the environment. Darwinism of course showed that the later was of major importance, and Neo-Darwinism emphasized that because the source of variation internal to the organism -- mutation -- was random, therefore this was not an important element. While modern biology has largely supported this view, it is also clear that int he emerging field of developmental molecular genetics, the processes occurring internally are more complex than the term "random" suggests, and that an understanding of these mechanisms, some of which may occur across generations, needs to be taken into account.
The second is the question of acquired inheritance .... [w]here Medelian genetics does not operate -- for example, in cultural inheritance, in certain aspects of immunology, in aspects to do with maternal condition -- it appears there may be a chink in the Darwinian armor through which some element of Lamark may enter evolutionary biology.
Roger Lewin and Robert A. Foley (2004) Principles of Human Evolution. Second Edition; Oxford: Blackwell Publishers. p. 45
For a overview of the field of epigenetics in a popular science journal, see
Lamarkianism Revisited. For a more in-depth study see Byran M. Turner's (2001)
Chromatin and Gene Regulation: Molecular Mechanisms in Epigenetics. Another source for a historical overview of the issues involved see
Epigenetic Inheritance and Evolution: The Lamarckian Dimension by Eva Jablonka and Marion Lamb (2005). The following
description is given:
Oxford University Press wrote:Does the inheritance of acquired characteristics play a significant role in evolution? In this book, Eva Jablonka and Marion J. Lamb attempt to answer that question with an original, provocative exploration of the nature and origin of hereditary variations. Starting with a historical account of Lamarck's ideas and the reasons they have fallen in disrepute, the authors go on to challenge the prevailing assumption that all heritable variation is random and the result of variation in DNA base sequences. They also detail recent breakthroughs in our understanding of the molecular mechanisms underlying inheritance--including several pathways not envisioned by classical population genetics--and argue that these advances need to be more fully incorporated into mainstream evolutionary theory. Throughout, the book offers a new look at the evidence for and against the hereditability of environmentally induced changes, and addresses timely questions about the importance of non-Mendelian inheritance. A glossary and extensive list of references round out the book. Urging a reconsideration of the present DNA-centric view prevalent in the field, Epigentic Inheritance and Evolution will make fascinating and important reading for students and researchers in evolution, genetics, ecology, molecular biology, developmental biology, and the history and philosophy of science.
An interesting review of Jablonka's book raises the question of the nature of chromatin marking, and whether it might not be a form of
regulatory genome.
See the folllowing for an interesting historical review of the evolutionary nature of the very definition of the term
epigenetics, including the definition given by the renowned molecular biologist Robin Holliday, who defines "epigenetics to include transmission of information from one generation to the next, other than the DNA sequence itself" (Holliday 1994, page 454).
Wu et al. wrote:In 1987, Robin Holliday, renowned by this time for his studies of the molecular mechanism by which chromosomes physically recombine, will write, "The properties of genes in higher organisms can be studied on two levels: first, the mechanism of their transmission from generation to generation, which is the central component of genetics and is well understood, and second, their mode of action during the development of the organism from the fertilized egg to adult, which is very poorly understood. The changes in gene activity during development are generally referred to as epigenetic, a term first introduced by Waddington [with reference given here to Waddington, Symp. Soc. Exp. Biol. 7,186 (1953); Principles of Embryology]. Thus, epigenetic switches turn particular genes on or off during the developmental process, producing either transient changes in gene activity or a permanent pattern of activities" (Holliday 1987, page 163).
Seven years later, Holliday will again consider epigenetics. This time, however, he will develop the idea of epigenetics beyond Waddington’s original definition. He will begin with, "The key feature is the nfolding of the genetic programme, which ultimately depends on the activation or inactivation of specific genes, or the interactions between genes and the products of genes" (Holliday 1994, page 453) and then suggest two variations of this definition with the intention of integrating some intriguing observations of gene function. The point we wish to emphasize is that these two variations will incorporate two new concepts to our understanding of epigenetics.
First, Holliday will point out that changes in gene expression occur not only during development but also during the adult stage of an organism. We believe that it is with this thought in mind that he will propose his first variation, a definition of epigenetics that "is not restricted to development, but to organisms that have several or many types of differentiated cells" (Holliday 1994, page 453). Accordingly, he will suggest epigenetics to be the "study of the changes in gene expression, which occur in organisms with differentiated cells, and the mitotic inheritance of given patterns of gene expression" (Holliday 1994, page 453). Holliday will emphasize that this definition "says nothing about mechanisms, so it can include all types of DNA-protein interactions, as well as changes at the DNA level, as seen in the production of genes coding for immunoglobulins. It could also include the alternative splicing of pre-mRNA transcripts to produce protein isoforms, which can be cell type specific" (Holliday 1994, page 453). (Immunoglobulins are proteins that mediate the ability of organisms to fight infection, and mRNA transcripts are RNA products of genes.)
This new definition will also clearly raise a second issue, which is the notion of inheritance. He will note that as changes in gene activity can be inherited through cell division, the "stable mitotic inheritance of given patterns of gene activity is a key feature of epigenetic controls" (Holliday 1994, page 453). How is this inheritance effected? Holliday will first remind us that DNA can undergo permanent changes in sequence during development and that such changes would be expected to be heritable through cell division. (It’s true. DNA will prove to be quite the dynamic molecule!) Holliday will then move on to heritable changes in gene expression that can be reversed at a later stage, sometimes after meiosis. As most reversible changes in gene regulation are not expected to entail alterations of DNA, it is here that Holliday suggests his second variation, which brings the role of non-DNA elements into the limelight. He proposes a "supplementary definition of epigenetics to include transmission of information from one generation to the next, other than the DNA sequence itself" (Holliday 1994, page 454), in other words, "Nuclear inheritance which is not based on differences in DNA sequence" (Holliday 1994, page 454).
So, here we are, at the brink of, but not quite arrived at, the definition of epigenetics which you have found so puzzling. There remains but one more step to reach this final destination, and that is the simplification, in the form of a fusion, of Holliday’s two definitions. Specifically, the most current interpretation of epigenetics combines the concept of changes in gene expression and the implication of mitotic inheritance (from the first variation) with the use of DNA as a reference point and the implication of generational, including meiotic, inheritance (from the second variation) to give rise to our current definition: the study of changes in gene function that are mitotically and/or meiotically heritable and that do not entail a change in DNA sequence.
-- C.-t. Wu and J.R. Morris. Genes, Genetics, and Epigenetics: A Correspondence. Science, vol. 293.
Jablonka et al. wrote:The idea that the inheritance of acquired characters plays an important role in evolution has been the subject of controversy for over a century. Enthusiasm for the idea, which is usually associated with the name of Lamarck, has sometimes led to charlatanism and fraud, while opposition to it has led to 'Lamarkist' being used as a term of abuse. Nowdays, biologists usually regard ideas about the inheritance of acquired characters as nothing more than an interesting part of the history of biology. Lamarkian evolution is rejected on the grounds that there is no evidence for it, no mechanism that can produce it, and no need for it in evolutionary theory. Some people go even further and argue that the inheritance of acquired characters is theoretically impossible--it is incompatible with what is known about genetics and development.
... [T]here are now well recognized mechanisms by which some acquired characters can be transmitted to the next generation, and that such characters have probably played a significant role in evolution. We want to make it clear right at the outset that although we argue that some types of Lamarkian evolution are possible, there is nothing in what we say that should be construed as being anti-Darwinian.[1] We are firm believers in the power and importance of natural selection. What we do maintain, however, is that some new inherited variations are not quite as random as is generally assumed, but arise as a direct, and sometimes directed, response to environmental challenge, and that the effects of such induced variations deserve more recognition in evolutionary theory.
[1] We feel it necessary to stress our belief in Darwinian evolution because recent history has shown than any argument suggesting that Darwinian evolutionary theory should be modified is liable to be used by Creationists as evidence that the theory of evolution is wrong. Like most Darwinians, we believe that Darwinian evolutionary theory is a flexible theory, quite capable of accommodating modifications and amendments.
[Similarly, some Panselectionists attempt to label any "argument suggesting that Darwinian evolutionary theory should be modified" must be a Creationist or Intelligent Design arguments in disguise, and they therefore distort and twist arguments (like Creationists do with the arguments of scientists, both of which is a dishonest tactic I might add) for their own rhetorical purpose. Indeed, they have a lot in common when it comes to such fallacious rhetorical tactics.]
-- Jablonka, Eva and Lamb, Marion J. (1995) Epigenetic Inheritance and Evolution: The Lamarkian Dimension. Oxford. p.1.
West-Ehberhard wrote:In mammals, and to an unknown extent possibly in other groups, male-female reproductive interdependence extends to embryonic gene expression through the phenomenon of genomic imprinting. Genomic imprinting is parent-specific gene expression, that is, gene expression that depends on which parent contributed the gene (Trivers and Burt, 1999), or the differential modification of genes such that maternal and paternal alleles are distinct (Wei adn Mahowald, 1994). For exmaple, the maternally derived allele of the Igf2r gene of the mouse is methylated at intron region 2, which permits the gene to be expressed, evidently because methylation blocks a silencer of transcription (Wei and Mahowald, 1994). The paternal allele is not methylated and therefore is not expressed. Parental imprints are evidently erased, and then remarked, during gametogenesis, to accord with the sex of the individual. Experimentally constituted mouse embryos with an entirely paternally imprinted genome (androgenomes) or with an entirely maternally imprinted genome (gynogenomes) are incapable of normal development and show severe abnormalities. (West-Ehberhard 2003: 635)
(....) Lyon (1993) called genomic imprinting a kind of "epigenetic inheritance." This brings us full circle from modern molecular studies of gene action back to Darwin, who, with his primitive but molecular gemmular theory of pangenesis, insisted that all inheritance is epigenetic, a product of both the transmission and the development of traits. (West-Ehberhard 2003: 637)
-- West-Ehberhard, Mary Jane. Developmental Plasticity and Evolution. Oxford: Oxford University Press; 2003; pp. 635-637.