- Email: [email protected]
Evolving hierarchical systems
80 Because he is so sensible and so scientifically complete, I think Walker misses the "soul" of the planet. The Earth is the planet of life, not just some life, but "crawling with it", and yet Walker dismisses the two related fundamental and supremely fascinating enigmas of the Earth almost without thought. These fascinating topics are, in my opinion, the origin of life and geophysiology. With respect to the first, he says at the end of Chapter 5, "Our understanding of the origin of life is based on theory, laboratory simulation and analogy". Sorry, Jim, we have nothing as yet to call an understanding of the origin of life. What we have is a fabrication of guesses. There has certainly been a murder committed since the "body" (in the form of the teeming life on Earth) is clear enough to see, but by whom, employing what method, why and how remains fascinatingly unclear. By geophysiology of the Earth I mean the fundamental interactions between the biosphere and the rest of the planet. It is these interactions which are responsible for the chemical disequilibrium that characterizes our Earth and probably any other living environment; and yet Walker dismisses (on page 74) the Gaia hypothesis with a statement "The hypothesis of regulation is plainly falsified by the biologically driven transition from an anaerobic to an aerobic world." I am afraid you've got Gaia wrong, Jim. She isn't a committee of organisms that holds annual general meetings to decide on the next stage of planetary evolution. There is no requirement for pre-thought or pre-planning for the Gaia mechanism to work. I can understand why a scientist such as Walker might not be attracted by the incredible simplicity of Jim Lovelock's Daisyworld, but I recommend it nonetheless. In summary, we have in Earth History: The Several Ages of the Earth a first rate, enjoyable and scientific account of the evolution of the Earth, but to add spice to the meal I warmly recommend two other equally readable books. These are Cairns-Smith's Seven Clues to the Origin of Life (Cambridge University Press, 1985) and Jim Lovelock's Gala. A New Look at Life on Earth (Oxford University Press, 1979). Between them, Cairns-Smith and Lovelock complement Walker's factual account. Cairns-Smith writes a detective story about the "crime" of the origin of life and Lovelock describes the way in which an anaerobic world could become aerobic without the necessity for a pre-planned suicide pact on the part of the biota.
A. Henderson-Sellers Department of Geography University of Liverpool PO Box 147 Liverpool L69 3BX United Kingdom
Evolving Hierarchical Systems, Stanley N. Salthe, Columbia University Press, New York, 1985, 343pp. (US$32.50) The author notes, about midway through the book, that a complete understanding of sickle cell anemia would require information from at least six levels of organization. Mutation, solubility, and crystal structure of hemoglobin are pertinent molecular level properties. Hemoglobin concentration and membrane interactions are better described as cellular level phenomena. At the organism level it is necessary to understand the crucial oxygen delivery requirements of various tissues. Analysis at the level of the deme is necessary to a proper understanding of the allowable level of lethal mutations. The interactions among mosquitoes, malarial parasites, and humans constitute an ecosystem level phenomenon, while the geographical factors that control the coinci-
81
dence of hemoglobin polymorphism and malaria require a biogeographic regional level of description. If hierarchical analysis is necessary for the proper understanding of sickle cell anemia, why should it not be necessary for a proper understanding of "biology in general and organic evolutionary theory in particular"? Salthe's thesis, indeed, is that the reductionist attitude is inadequate for understanding evolutionary phenomena. "It is nothing short of myopic," he writes, "to believe that we can understand how the [Linneaen] hierarchy of taxa was produced by trying to construe it as having been only by way of organism level (or even demic level) processes." Higher level constitutive change, such as primary succession and coevolution, and more broadly, ontogeny of the surface of the earth "give rise to regulation governing microevolution while the results of the latter will aid in guiding the former." Organism evolution, in this broader view, is an interlocking of processes and multiple levels of organization, and as such will inevitably show a puzzling resistance to explication to the extent that explanations are restricted to one or a few levels. The hierarchical philosophy has in recent years been forwarded by a number of authors, including this reviewer, and one of the virtues of Evolving Hierarchical Systems is that it provides a rather comprehensive overview of the major works leading in this direction. The first six chapters develop the concept of hierarchy, and the author explores in depth the difficult problems associated with identifying hierarchical structure and entities. Some authors feel that hierarchy is merely a descriptive convenience, others that it represents bona fide entities in nature. Salthe, I believe, is of the latter persuasion. For him, there is a real hierarchy of nature, meaning an ontological interpretation of real world data. Criteria for identifying hierarchical entities include borders, scale, integration, and spatial and temporal continuity. An important distinction is made between economic (ecological) hierarchies based on generative processes, such as energy transformations, and genealogical hierarchies. Evolutionary biology, in Salthe's opinion, has been preoccupied with the genealogical perspective at the expense of the economic one. His own perspective, summarized in the final two chapters, give weight to both types of hierarchical analysis, and indeed to the interaction of economic and genealogical hierarchies. As illustrated by the sickle cell example, the hierarchical mode of analysis would appear to be called for whether the hierarchical entity is interpreted nominally or realistically, or even if some entities are interpreted nominally and others realistically. A particle physics approach to evolutionary theory would lose all the essential patterns. It would probably be impossible even to identify organisms if ecosystems were viewed solely in terms of the interactions of particles. Is the organism, as a pattern, a primitive ontological entity or a convenience of description? As organisms we tend to confer reality on ourselves. But perhaps we are more hesitant to confer reality on populations or higher level social organizations. It seems to this reviewer that in practice the validity of a hierarchical expansion of evolutionary theory does not critically depend on a watertight philosophical analysis of the problem. In practice biologists recognize patterns at various levels of organization and it is absurd not to utilize all the information collected around these patterns in our analysis of biological phenomena. I would even argue, from the computer science point of view, that precise, algorithmic definitions of patterns are impossible whenever ambiguity is present, and that the human facility in this respect is at best approximate and in practice not efficiently implementable in programmable machines (hence in practice irreducibly biological). But Salthe is perfectly correct that a careful analysis of the defining features of biological patterns is indispensible for arriving at proper interpretations. Salthe's monograph serves as a kind of manifesto of the hierarchical point of view, largely because it directly addresses, even challenges, the adequacy of the synthetic theory's focus on dynamics at the gene and organism levels. Though a more distant observer might see this corn-
82
mendable direction of thought as an extension of the synthetic theory, he might also note an element of sociological paradox. A large fraction of the biological explanations that are put forward in the literature are purely physiological in style, as if the authors were entirely unaware of evolutionary theory. In many cases the phenomena remain puzzling, and as a consequence further mechanistic or functional hypotheses are added. As Salthe suggests, this single track approach inevitably creates puzzles faster than it solves them. Possibly such refractoriness to evolutionary theory in many parts of the world reflects deep seated philosophical and religious convictions. Hierarchical modes of analysis promise to greatly enhance the interpretive and even predictive power of evolutionary theory, and hopefully they will encourage increased sensitivity to modes of analysis that attend to, rather than ignore, the fundamental conceptual scheme of biology.
Michael Conrad Departments of Computer Science and Biological Sciences Wayne State University Detroit, MI 48202, U.S.A.
The Transforming Principle, Maclyn McCarty, Norton and Company, New York, ISBN 0-39330450-7
The Transforming Principle by Dr. Maclyn McCarty of Rockefeller University is the first of the Commonwealth Fund Book Program series intended to present writings by scientists about their own work for a general, literate readership. It deals with the discovery by Avery, MacCleod and McCarty in the early 1940s that DNA provides the informational basis of heredity. While this book is unlikely to have the impact credited Paul de Kruifs Microbe Hunters or Rene V. Radot's The Life of Pasteur in directing another's career, The Transforming Principle should be on the reading list of those interested in the process of science and the roots of molecular biology. A unique feature of molecular biology is that by and large its practitioners are still young. The field is populated by post-war babies, a majority of whom may have been born after Watson and Crick developed their model for DNA structure. As one of those who tends to take molecular biology for granted, the struggles involved in having DNA accepted as the genetic material seem the substance of fiction rather than fact. The zeroing in on the truth and its ultimate acceptance by the scientific community provides a study in the workings of science that should fascinate many. This book also provides a good view of the acceleration of modern science as diverse fields such as physical chemistry, microbiology, enzymology and analytical chemistry merge to create the basis for the blossoming of what has become molecular biology. While the rigours required to establish the chemical nature of the transforming principle provide the main course, there are many side dishes. Described for example, is perhaps the first demonstration of the power of microbial genetics, the use of strain R36 by McCleod in 1934. This strain showed no background reversion, yet was an highly efficient recipient often yielding transformation in 100% of the cells. A particularly fascinating bit of technology was the creative conversion of an air driven cream separator spinning at 30,000 rev./min into a continuous flow centrifuge and the subsequent need for biological containment to prevent the release of pathogens in an invisible aerosol. Equally interesting is McCarty's response to the discovery that subcellular fractions could be sedimented in an early model of an analytical centrifuge. This merging of science and technology is taken for granted by this generation of molecular biologist, but it was not always the case.
A. Henderson-Sellers Department of Geography University of Liverpool PO Box 147 Liverpool L69 3BX United Kingdom
Evolving Hierarchical Systems, Stanley N. Salthe, Columbia University Press, New York, 1985, 343pp. (US$32.50) The author notes, about midway through the book, that a complete understanding of sickle cell anemia would require information from at least six levels of organization. Mutation, solubility, and crystal structure of hemoglobin are pertinent molecular level properties. Hemoglobin concentration and membrane interactions are better described as cellular level phenomena. At the organism level it is necessary to understand the crucial oxygen delivery requirements of various tissues. Analysis at the level of the deme is necessary to a proper understanding of the allowable level of lethal mutations. The interactions among mosquitoes, malarial parasites, and humans constitute an ecosystem level phenomenon, while the geographical factors that control the coinci-
81
dence of hemoglobin polymorphism and malaria require a biogeographic regional level of description. If hierarchical analysis is necessary for the proper understanding of sickle cell anemia, why should it not be necessary for a proper understanding of "biology in general and organic evolutionary theory in particular"? Salthe's thesis, indeed, is that the reductionist attitude is inadequate for understanding evolutionary phenomena. "It is nothing short of myopic," he writes, "to believe that we can understand how the [Linneaen] hierarchy of taxa was produced by trying to construe it as having been only by way of organism level (or even demic level) processes." Higher level constitutive change, such as primary succession and coevolution, and more broadly, ontogeny of the surface of the earth "give rise to regulation governing microevolution while the results of the latter will aid in guiding the former." Organism evolution, in this broader view, is an interlocking of processes and multiple levels of organization, and as such will inevitably show a puzzling resistance to explication to the extent that explanations are restricted to one or a few levels. The hierarchical philosophy has in recent years been forwarded by a number of authors, including this reviewer, and one of the virtues of Evolving Hierarchical Systems is that it provides a rather comprehensive overview of the major works leading in this direction. The first six chapters develop the concept of hierarchy, and the author explores in depth the difficult problems associated with identifying hierarchical structure and entities. Some authors feel that hierarchy is merely a descriptive convenience, others that it represents bona fide entities in nature. Salthe, I believe, is of the latter persuasion. For him, there is a real hierarchy of nature, meaning an ontological interpretation of real world data. Criteria for identifying hierarchical entities include borders, scale, integration, and spatial and temporal continuity. An important distinction is made between economic (ecological) hierarchies based on generative processes, such as energy transformations, and genealogical hierarchies. Evolutionary biology, in Salthe's opinion, has been preoccupied with the genealogical perspective at the expense of the economic one. His own perspective, summarized in the final two chapters, give weight to both types of hierarchical analysis, and indeed to the interaction of economic and genealogical hierarchies. As illustrated by the sickle cell example, the hierarchical mode of analysis would appear to be called for whether the hierarchical entity is interpreted nominally or realistically, or even if some entities are interpreted nominally and others realistically. A particle physics approach to evolutionary theory would lose all the essential patterns. It would probably be impossible even to identify organisms if ecosystems were viewed solely in terms of the interactions of particles. Is the organism, as a pattern, a primitive ontological entity or a convenience of description? As organisms we tend to confer reality on ourselves. But perhaps we are more hesitant to confer reality on populations or higher level social organizations. It seems to this reviewer that in practice the validity of a hierarchical expansion of evolutionary theory does not critically depend on a watertight philosophical analysis of the problem. In practice biologists recognize patterns at various levels of organization and it is absurd not to utilize all the information collected around these patterns in our analysis of biological phenomena. I would even argue, from the computer science point of view, that precise, algorithmic definitions of patterns are impossible whenever ambiguity is present, and that the human facility in this respect is at best approximate and in practice not efficiently implementable in programmable machines (hence in practice irreducibly biological). But Salthe is perfectly correct that a careful analysis of the defining features of biological patterns is indispensible for arriving at proper interpretations. Salthe's monograph serves as a kind of manifesto of the hierarchical point of view, largely because it directly addresses, even challenges, the adequacy of the synthetic theory's focus on dynamics at the gene and organism levels. Though a more distant observer might see this corn-
82
mendable direction of thought as an extension of the synthetic theory, he might also note an element of sociological paradox. A large fraction of the biological explanations that are put forward in the literature are purely physiological in style, as if the authors were entirely unaware of evolutionary theory. In many cases the phenomena remain puzzling, and as a consequence further mechanistic or functional hypotheses are added. As Salthe suggests, this single track approach inevitably creates puzzles faster than it solves them. Possibly such refractoriness to evolutionary theory in many parts of the world reflects deep seated philosophical and religious convictions. Hierarchical modes of analysis promise to greatly enhance the interpretive and even predictive power of evolutionary theory, and hopefully they will encourage increased sensitivity to modes of analysis that attend to, rather than ignore, the fundamental conceptual scheme of biology.
Michael Conrad Departments of Computer Science and Biological Sciences Wayne State University Detroit, MI 48202, U.S.A.
The Transforming Principle, Maclyn McCarty, Norton and Company, New York, ISBN 0-39330450-7
The Transforming Principle by Dr. Maclyn McCarty of Rockefeller University is the first of the Commonwealth Fund Book Program series intended to present writings by scientists about their own work for a general, literate readership. It deals with the discovery by Avery, MacCleod and McCarty in the early 1940s that DNA provides the informational basis of heredity. While this book is unlikely to have the impact credited Paul de Kruifs Microbe Hunters or Rene V. Radot's The Life of Pasteur in directing another's career, The Transforming Principle should be on the reading list of those interested in the process of science and the roots of molecular biology. A unique feature of molecular biology is that by and large its practitioners are still young. The field is populated by post-war babies, a majority of whom may have been born after Watson and Crick developed their model for DNA structure. As one of those who tends to take molecular biology for granted, the struggles involved in having DNA accepted as the genetic material seem the substance of fiction rather than fact. The zeroing in on the truth and its ultimate acceptance by the scientific community provides a study in the workings of science that should fascinate many. This book also provides a good view of the acceleration of modern science as diverse fields such as physical chemistry, microbiology, enzymology and analytical chemistry merge to create the basis for the blossoming of what has become molecular biology. While the rigours required to establish the chemical nature of the transforming principle provide the main course, there are many side dishes. Described for example, is perhaps the first demonstration of the power of microbial genetics, the use of strain R36 by McCleod in 1934. This strain showed no background reversion, yet was an highly efficient recipient often yielding transformation in 100% of the cells. A particularly fascinating bit of technology was the creative conversion of an air driven cream separator spinning at 30,000 rev./min into a continuous flow centrifuge and the subsequent need for biological containment to prevent the release of pathogens in an invisible aerosol. Equally interesting is McCarty's response to the discovery that subcellular fractions could be sedimented in an early model of an analytical centrifuge. This merging of science and technology is taken for granted by this generation of molecular biologist, but it was not always the case.