ABSTRACT
The emerging theme of the present work is that an understanding of heredity and individual development will allow not only a clear picture of how an adult animal is formed but that such an understanding is indispensable for an appreciation of the processes of evolution as well. This is a tall order, of course, and the writer would be glad if the present sketch provides what many practicing scientists would agree to be an appropriate abstract or conceptual orientation to the problem. The cardinal assumption of the present work is that the persistence of the nature-nurture dichotomy reflects an inadequate understanding of the relations among heredity, development, and evolution, or, more specifically, the relationship of genetics to embryology. This is merely another way of saying that to the extent that it is correct to say that alterations of ontogeny are responsible for evolution (phylogeny), an understanding of the relationship of genetics and embryology will go a long way toward clarifying the process of evolution. At
the heart of this difficult synthesis is the fact noted previously that early in this century genetics and embryology were practiced as independent sciences even though both of them took for their essential datum the phenotype that was realized as an outcome of a particular pathway of development. Geneticists inferred the structure and function of genes or genetic activity based on particular phenotypic outcomes of their experiments. The experimental embryologists likewise inferred cellular and tissue interactions based upon the presence or absence of distortions produced in the normal or species-typical phenotype caused by their experimental manipulations. In all of this the gene or genetic material took on a separate existence that made it stand somewhat outside of, or distinct from, the developmental process as such. This was a very subtle intellectual event, and even today it is hard to portray it in a way that makes it readily comprehensible as an intelligible state of affairs. I know this firsthand, not only from my own personal difficulties of grappling with the synthesis of heredity and development, but from sustained interactions with colleagues whose appreciation of issues in developmental analysis I had profited from myself. One of these colleagues once told me that genetics and embryology had been conceptually integrated for a long time as could be seen, for example, in Thomas Hunt Morgan’s book, Embryology and Genetics, published in 1934. I thought it was some obtuseness on my part that made it impossible for me to appreciate Morgan’s supposed conceptual integration, so I was relieved when I read in another expert’s book that the only “integration” of the two topics was achieved in the title of Morgan’s book (Dunn, 1965, p. 190)! The difficulty of integrating the topics of genetics and embryology was, of course, appreciated by Morgan himself, and, as Dunn points out, Morgan was insightful as to the conceptual obstacle and a possible escape from it:
As I have already pointed out, there is an interesting problem concerning the possible interaction between the chromatin of the cells and the protoplasm during development. The visible differentiation of the embryonic cells takes place in the protoplasm. The most common genetic assumption is that the genes remain the same throughout this time. It is, however, conceivable that the genes also are building up more and more, or are changing in some way, as development proceeds in response to that part of
the protoplasm in which they come to lie, and that these changes have a reciprocal influence on the protoplasm. It may be objected that this view is incompatible with the evidence that by changing the location of cells, as in grafting experiments and in regeneration, the cells may come to differentiate in another direction. But the objection is not so serious as it may appear if the basic constitution of the gene remains always the same, the postulated additions or changes in the genes being of the same order as those that take place in the protoplasm. If the latter can change its differentiation in a new environment without losing its fundamental properties, why may not the genes also? This question is clearly beyond the range of present evidence, but as a possibility it need not be rejected. The answer, for or against such an assumption, will have to wait until evidence can be obtained from experimental investigation. (Morgan, 1934, p. 234)
What Morgan is proposing here, in the final paragraph of his 1934 book, is that the integration of genetics with development could be achieved only when the gene is actively incorporated into the developmental process and that its activity is seen as reciprocally altered thereby (i.e., not only feedforward effects [gene protein] but feedback effects as well [gene protein]). To those who viewed (and view) the integrity of the gene as absolutely constant, operating essentially outside the reciprocally interactive developmental system, the notion that “the genes are changing in some way” was, and is, indeed a radical suggestion. But if there is to be a truly developmental genetics, genes will have to be viewed in some sense as Morgan suggested (and as the geneticist Sewall Wright suggested) and thus become part of the entire system of mutual interactions that is the hallmark of embryological analysis and that characterizes the epigenetic development of the individual. Viewing the genes in this way continues to be a conceptual obstacle even for certain of the eminent biologists of today, so one would certainly not expect a ready appreciation of such a momentous insight among scientists outside the field of biology (e.g., in psychology, anthropology, or sociology). For example, on the concluding page of his opus magnum on The Growth of Biological Thought, the eminent evolutionary biologists Ernst Mayr has this to say:
The pathway from the DNA of the genome to the proteins of the cytoplasm (transcription and translation) is strictly a one-way track. The proteins of the body cannot induce any changes in the DNA. (Mayr, 1982, p. 828)
How different is Mayr’s view, stated elsewhere in the same book, “that the DNA of the genotype does not itself enter into the developmental pathway but simply serves as a set of instructions” (Mayr, 1982, p. 824) from Sewall Wright’s physiological view of DNA (see Figure 10-5, Chapter 10) and the view advocated here. That there seems to be some kind of physiological pathway back to the genes is suggested by the observation that genetic mutations can be induced by environmental causes such as extreme temperature or exposure to ionizing radiation. Also, the currently held notion that during individual development genes are “turned on and off” in their activity suggests again some mechanism of feedback from the products that genes produce back to the genes themselves. For example, DNA-binding proteins are known to regulate gene expression (protein DNA). In sum, there has to be some means for the activity of genes to be regulated by events and processes occurring at other levels of the developmental pathway, otherwise it would not be possible to cause gene mutations by environmental agents, nor to “turn genes on and off” during normal individual development (i.e., as occurs in the construction of a normal individual of the species as opposed to a mutant). As an example of genes being turned on during development, in a leading textbook, Comparative Embryology, by Gorbman, Dickhoff, Vigna, Clark, and Ralph (1983), the schematic diagram shown in Figure 12-1 is presented to show how a steroid hormone diffuses into the nucleus of a cell to activate DNA transcription that results in protein secretion.
