| Themes > Science > Life Sciences > Physical Anthropology > Heredity and Variation > On the Origin of Variation |
Recent experiments at Harvard University and elsewhere revealed mutations in bacteria induced by nutrient deprivation. These mutations, of the lac operon and other genes, arose under conditions ordinarily associated with selection pressure. One interpretation of these experiments is that genetic variation and natural selection are two sides of a unitary and simultaneous process, the two sides of a general response to the struggle for existence and the Malthusian doctrine. Far from being random, the production of genetic variation (mutation) must be an innate capability of life, in the same way that the genetic conservatism, metabolism and the reproduction of organisms are innate and anything but random. The need to examine non-random mechanisms for evolutionary variation is also suggested by the dubious value of sexual reproduction if mutations are random and exceedingly rare. If generally true, this principle of selection-induced variation could be the basis for engineering evolution. Variation and Selection Must Have The Same Cause The ghost of Jean Baptiste Lamark is back, casting a shadow over a series of recent experiments that could be the most important experiments ever for the theory of evolution. Hidden in a subsequent debate over whether nutritional deprivation in bacteria caused genetic changes that are either directly adaptive (Lamarkian) or something else, is the starkness of the fact that the environmental induction of inheritable variation calls into question one of the foundations of the Modern Synthesis. Beginning with the pioneering 1984 experiments of James Shapiro at the University of Chicago on selection-induced mutations and with the 1988 experiments of John Cairns et. al. at the Harvard School of Public Health on the environmental induction of genetic variation in Escherichia Coli, meticulously verified, entirely unexpected genetic changes were observed under conditions of insufficient or inappropriate nutrition. A lively debate ensued and continues over the nature and possible general importance of these mutations. Based on work with transposable elements in maize Barbara McClintock proposed a genomic stress hypothesis, that enviromental stresses may trigger genomic reorganizations and mutations capable of overcoming the stressful conditions (1). In flax plants subjected to certain soil and nutrient conditions stably inherited size differences in progeny were found, associated with inherited differences in ribosomal DNA (2). Convergent evolution, especially the work of Milford Wolpoff on the Multiregional theory of human origins, is entirely consistent with large-scale, non-random variation. Random mutations cannot explain the simultaneous evolution of man in different places. The convergent evolution of chichlid fishes in three areas of East Africa within the last 14,000 years is perhaps the best known example of this, with identical morphological, behavioral and ecological specializations arising independently accordingly to molecular biologists(3). Simultaneous cultural developments, such as agricutural and animal husbandry in many places all at once 10,000 years ago (4), suggests something similiar about the mode and mechanism of human variation-evolution, or consciousness (5). Proceeding from the expectation that mutations can be induced by the real environment of the cells and that the time-scale of such evolutionary events is on the order of days or weeks, D. H. Robinson and deNovo Biologicals LLC have undertaken the commercialization of natural evolutionary modifications of bacteria by oxygen-starvation-induced gene transfer from host cells to their procaryotic symbionts(www.denovo-bio.com.). Despite evidence for a spectrum of simultaneous competitive mutations, continued use of the term adaptive mutation unnecessarily returns to the specter of Lamarkism. These selection-induced mutations represent a broad genomic search for many mutations at once, some or very few of which may be adaptive. These experiments and many that continue to follow imply, in their most general interpretation, that competition causes or induces, through a variety of specialized mechanisms, both genetic variation and natural selection simultaneously. Variation and selection, then, if these experiments are taken at face value, are two sides of a unitary process, the two sides of the totality of the process of adaptation-evolution. The expression of the antagonism between reproduction as the mode of life and reproduction in crisis as the agency of death, is in fact of the nature of a cause for altering the genetic constitution. The duality of variation and selection caused by competition, which Darwin and Wallace only saw one side of, is not the expression of new law of nature. The survival of living matter must have been possible only on this basis. MORTALITY It is necessary to suppose that in the beginning the main enemy was not hostile protoplasm which caused a high fatality, but the growth of competition due to the failure to die, and the incidence of great changes which would cause much greater fatality. Fatality is the product of a lack of adaptation: it is a negative process. a process hostile to the species, and a by-product of a condition to which the organism is continually adapting. Mortality, on the other hand, itself constitutes an adaptation, the accommodation of the species to the condition of change which is an imminent quality of nature. Each represents a different type of evolutionary mechanism. Fatality is the expression of the struggle for existence. Mortality, which is more fundamental, is the mechanism which underlies adaptation itself. One is the failure of adaptation, the other is itself a very high degree of adaptation, fundamental to all others. The definite limitation of the term of life is itself an evolutionary mechanism of first rate importance. It provides plasticity to the species by giving opportunity to frequent genetic recombination and the elimination of the old as an element of competition. There could not be adaptation if there were not a sufficiently frequent succession of generations containing news potentialities. The succession of generations is the foundation of plasticity of species. Natural death was not one of the earliest features of life. Bacteria, fungi, protozoans and many algae can be immortal. The dormant spores produced by many of these developmentally less complex phyla is a further contravention of natural death. The asexual phase of the alternation of generations life cycle can be immortal. Reliance on sexual reproduction and the reduction of the period of viability of dormant spores were stages in the development of mortality, were faculties that had to be in place first. The development of the true extinction of the parent generation is a great advance in creating evolutionary possibilities. The most likely cause of the development of natural death as an evolutionary mechanism was the prevention of intraspecific competition. Variation is Neither Lamarkian or Random. The conception most often offered as the sole alternative to the genetic account of the origin of variation is that there is a molding of the species by nature so to speak in its image, a molding which causes the species to change adaptively and to transmit this change to the next generation, which continues the adaptive tendency, until a new form appears as the result of a lengthy accumulation of such directed changes. This idea involves the conception of the inheritance of acquired characteristics or Lamarkism, which has often been refuted. But in fact the process is rather more complicated than this. The species cannot "adapt" strictly speaking: the fund of its possible transformations is precisely the series of genetic forms which it assumes in the course of its development its recapitulation of the history of the development of its predecessors; and no degree of mal-adaptation can do more than draw upon these inherited resources by an emphasis of certain of them. "One of the great lessons of evolutionary biology is that organisms differ largely because of combinatorial variations of a comparatively few features rather than as a result of an unlimited capacity to endlessly add new features to those that came before. Thus the diversity of life is more a consequence of permutation than of simple addition" (8). New expressions of homeobox genes controlling the body plans, eyes and hearts of both insects and all vertebrates are examples of the possible permutations of long-established qualities that have actually taken place (6). "Rapid, drastic changes governed by regulatory genes have most often occured in association with speciation" (7). Examples based on this mechanism are known for both sides of heterochrony, paedomorphosis and hypermorphosis, the shortening and lengthening of ontogeny by the action of regulatory genes. For plants the mechanism of the alteration of the timing of flowering versus vegetative growth through mutations of regulatory genes and their protien products have been elaborated in detail (8). For animals numerous extant examples of the genetic regulation of paedomorphosis and hypermorphosis show that this is no mere isolated, species-specific mechanism. (7,8). Whether and to what degree these mutations can be induced by competition has never been studied and remains to be investigated. It is necessary to make the distinction between poor conditions as such, and competition as the source of variation. Not the direct molding of the heredity by the "environment": rather the winnowing of all of the resources of the past development by the instrument of competition, which is the universal, absolute consequence of a change in conditions leading to a lack of adaptation. THE EVOLUTION OF SELECTION The process of natural selection has been modified by its own action, so that the process of natural selection in all of its ramifications produces the means to limit the operation of natural selection by limiting the competition which causes it, and by producing modification and restriction of fertility, and other agencies of selection than natural, to accomplish the adaptations. Sexual selection safeguards the heredity of the new generation by distilling the germ plasm of the group into its best essence. This is a species-wide process, which would not exist unless variation could be a large-scale event. Sexual reproduction means that there is no clonal fidelity through the generations. The sexual mixing of heredity would seem to be on an equal plane with overcoming the contradiction between metabolism and generation (9, 10). In creating a new generation and allowing the evolution of mortality, sex and natural death are two sides of a coin. The only immortality is among the protists. Definite limitation of the term of viability of their dormant spores is a step in the direction of the achievement of mortality. Subsequently, a succession of sexual generations becomes the exclusive and necessary mode of life: the so-called alternation of generations come to end. Sex and mortality imply, require, a definite mechanism for generating variation in appropriate intensity when it is required (11). THE ROLE OF THE NURTURE A chicken is the way DNA makes mores DNA, according the famous dictum of Samuel Butler. The following aphoristic blank-verse is a good reply to this: In a general way, one can see that the fact that there is mortality, that the old are required to depart, means to make way for the young, the new. It is a systematic way to admit the new, to allow the new to flourish. So that, one must conclude, the possibility, the inevitability of newness has to exist in the succession of generations. So then the heredity is not given by God, but changes. The seed is now one thing, now another. The germ plasm cannot be immortal because the Weissman barrier cannot exist, there is no separation of the genetic variation of the germ cells from that of the somatic tissue. Whatever mechansism alters the development of the soma must simultaneously modify both the soma and the germ cells. And in a way the next generation repeats this as a genetically inherited characteristic. Not for years but for generations plant scientists have known absolutely that no barrier exists between the germ cells and somatic tissues. They have quietly been inducing cells from any part of almost any plant to form embryos and whole plants. Such somatic cell embryogeneis has been the source of much still-to-be-explained variation, which they call somaclonal variation. Such techiques have also been applied to many invertebrates, proceeding with no deference at all to the Weissman barrier. Everything above depends on the simultaneous modification of the germ cells and the soma as the mechanism of evolution (12). Immortal germ cells, random mutations, selfish genes and social Darwinism are relics of a secular and anachronistic world view. Mutations induced under selection conditions in bacteria of the lack operon and genes associated with antibiotic resistance have been called examples of natural genetic engineering (13-19). Similiar but unexplained examples of mutated plants regenerated from tissue cultures are a second well-documented example of induced genetic variation and a possible basis for a new theory about evolution (20, 21). |
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