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Abstract
This article describes how empirical discoveries in the 1930s-1950s regarding population variation for chromosomal inversions affected Theodosius Dobzhansky and Richard Goldschmidt. A significant fraction of the empirical work I discuss was done by Dobzhansky and his coworkers; Goldschmidt was an astute interpreter, with strong and unusual commitments. I argue that both belong to a mechanistic tradition in genetics, concerned with the effects of chromosomal organization and systems on the inheritance patterns of species. Their different trajectories illustrate how scientists' commitments affect how they interpret new evidence and adjust to it. Dobzhansky was moved to revised views about selection, while Goldschmidt moved his attention to different genetic phenomena. However different, there are significant connections between the two that enrich our understanding of their views. I focus on two: the role of developmental considerations in Dobzhansky's thought and the role of neutrality and drift in Goldschmidt's evolutionary account. Dobzhansky's struggle with chromosomal variation is not solely about competing schools of thought within the selectionist camp, as insightfully articulated by John Beatty, but also a story of competition between selectionist thinking and developmental perspectives. In contraposition, Goldschmidt emphasized the role of low penetrance mutations that spread neutrally and pointed out that drift could result from developmental canalization. This account adds to the dominant story about Goldschmidt's resistance to the splitting of development from genetics, as told by Garland Allen and Michael Dietrich. The story I tell illustrates how developmental thinking and genetic thinking conflicted and influenced researchers with different convictions about the significance of chromosomal organization.
During the 1960s and 1970s population geneticists pushed beyond models of single genes to grapple with the effect on evolution of multiple genes associated by linkage. The resulting models of multiple interacting loci suggested that blocks of genes, maybe even entire chromosomes or the genome itself, should be treated as a unit. In this context, Richard Lewontin wrote his famous 1974 book The Genetic Basis of Evolutionary Change, which concludes with an argument for considering the entire genome as the unit of selection as a result of linkage. Why did Lewontin and others devote so much intellectual energy to the "complications of linkage" in the 1960s and 1970s? We argue that this attention to linkage should be understood in the context of research on chromosomal inversions and co-adapted gene complexes that occupied mid-century evolutionary genetics. For Lewontin, the complications of linkage were an extension of this chromosomal focus expressed in the new language of models for linkage disequilibrium.
The dichotomy between 'typological thinking' and 'population thinking' features in a range of debates in contemporary and historical biology. The origins of this dichotomy are often traced to Ernst Mayr, who is said to have coined it in the 1950s as a rhetorical device that could be used to shield the Modern Synthesis from attacks by the opponents of population biology. In this two-part essay I argue that the origins of the typology/population dichotomy are considerably more complicated and more interesting than is commonly thought. In this first part, I will argue that Mayr's dichotomy was based on two distinct type/population contrasts that had been articulated much earlier by George Gaylord Simpson and Theodosius Dobzhansky. Their distinctions made eminent sense in their own, isolated contexts. In the second part, I will show how Mayr conflated these type/population distinctions and blended in some of his own, unrelated concerns with 'types' of a rather different sort. Although Mayr told his early critics that he was merely making "a temporary oversimplification," he ended up burdening the history and philosophy of biology with a troubled dichotomy.
Richard Goldschmidt's research on homeotic mutants from 1940 until his death in 1958 represents one of the first serious efforts to integrate genetics, development, and evolution. Using two different models, Goldschmidt tried to show how different views of genetic structure and gene action could provide a mechanism for rapid speciation. Developmental systems were emphasized in one model and a hierarchy of genetic structures in the other. While Goldschmidt tried to find a balance between development and genetics, critics, such as Sewall Wright, urged him and eventually helped him incorporate population dynamics into his models as well. As such, the history of Goldschmidt's research on homeotic mutants highlights the continuing challenge of producing a balanced and integrated developmental evolutionary genetics.
Richard Goldschmidt was one of the most controversial biologists of the mid-twentieth century. Rather than fade from view, Goldschmidt's work and reputation has persisted in the biological community long after he has. Goldschmidt's longevity is due in large part to how he was represented by Stephen J. Gould. When viewed from the perspective of the biographer, Gould's revival of Goldschmidt as an evolutionary heretic in the 1970s and 1980s represents a selective reinvention of Goldschmidt that provides a contrast to other kinds of biographical commemorations by scientists.
Based on the FISHER-MULLER theory of the evolution of recombination, an argument can be constructed predicting that a recessive allele favoring recombination will be favored, if there are either favorable or deleterious mutants occurring at other loci. In this case there is no clear distinction between individual and group selection. Computer simulation of populations segregating for recessive or dominant recombination alleles showed selection favoring recombination, except in the case of a dominant recombination allele with deleterious background mutants. The relationship of this work to parallel investigations by WILLIAMS and by STROBECK, MAYNARD SMITH, and CHARLESWORTH is explored. All seem to rely on the same phenomenon. There seems no reason to assume that the evolution of recombination must have occurred by group selection.
Historians, philosophers, sociologists, and biologists explore the history of the idea that embryological development and evolution are linked.
Although we now know that ontogeny (individual development) does not actually recapitulate phylogeny (evolutionary transformation), contrary to Ernst Haeckel's famous dictum, the relationship between embryological development and evolution remains the subject of intense scientific interest. In the 1990s a new field, evolutionary developmental biology (or evo-devo), was hailed as the synthesis of developmental and evolutionary biology. In From Embryology to Evo-Devo, historians, philosophers, sociologists, and biologists offer diverse perspectives on the history of efforts to understand the links between development and evolution. After examining events in the history of early twentieth century embryology and developmental genetics—including the fate of Haeckel's law and its various reformulations, the ideas of William Bateson, and Richard Goldschmidt's idiosyncratic synthesis of ontogeny and phylogeny—the contributors explore additional topics ranging from the history of comparative embryology in America to a philosophical-historical analysis of different research styles. Finally, three major figures in theoretical biology—Brian Hall, Gerd Müller, and Günter Wagner—reflect on the past and future of evo-devo, particularly on the interdisciplinary nature of the field. The sum is an exciting interdisciplinary exploration of developmental evolution.
The controversy over the evolutionary advantage of recombination initially discovered by Fisher and by Muller is reviewed. Those authors whose models had finite-population effects found an advantage of recombination, and those whose models had infinite populations found none. The advantage of recombination is that it breaks down random linkage disequilibrium generated by genetic drift. Hill and Robertson found that the average effect of this randomly-generated linkage disequilibrium was to cause linked loci to interfere with each other's response to selection, even where there was no gene interaction between the loci. This effect is shown to be identical to the original argument of Fisher and Muller. It also predicts the "ratchet mechanism" discovered by Muller, who pointed out that deleterious mutants would more readily increase in a population without recombination. Computer simulations of substitution of favorable mutants and of the long-term increase of deleterious mutants verified the essential correctness of the original Fisher-Muller argument and the reality of the Muller ratchet mechanism. It is argued that these constitute an intrinsic advantage of recombination capable of accounting for its persistence in the face of selection for tighter linkage between interacting polymorphisms, and possibly capable of accounting for its origin.
Nothing in biology makes sense except in the light of evolution. This statement is an apt motto for the lifework of Theodosius Dobzhansky, whom Stephen Jay Gould has called "the greatest evolutionary geneticist of our times." Between 1937 and 1975, the year of his death, Dobzhansky and twenty-two of his collaborators published forty-three papers in a series called "The Genetics of Natural Populations." Taken as a whole, the series is perhaps the most important single corpus in modern evolutionary genetics. Dobzhansky's Genetics of Natural Populations, I-XLIII reproduces these forty-three articles. Because three of the four editor's of this volume are former students and long-time collaborators of Dobzhansky, they are able to set these important papers in critical perspective. The editors briefly evaluate the totality of Dobzhansky's work and summarize the views expressed in the series-including the historic development of Dobzhansky's ideas on genetic variation. Critical comments illuminate the relationship of the papers to each other and to Dobzhansky's other work. In addition, the editors discuss the role of Dobzhansky's interaction with Alfred Sturtevant and Sewall Wright. In particular, the book features Dobzhansky's pioneering field studies of Drosophila pseudoobscura, which were critical in the formulation of some of his most important conclusions about the genetic structure of populations and, more broadly, about the way evolution works.
The terms ‘genotype’, ‘phenotype’ and ‘gene’ originally had a different meaning from that in the Modern Synthesis. These terms were coined in the first decade of the twentieth century by the Danish plant physiologist Wilhelm Johannsen. His bean selection experiment and his theoretical analysis of the difference between genotype and phenotype were important inputs to the formation of genetics as a well-defined special discipline. This paper shows how Johannsen's holistic genotype theory provided a platform for criticism of narrowly genocentric versions of the chromosome theory of heredity that came to dominate genetics in the middle decades of the twentieth century. Johannsen came to recognize the epoch-making importance of the work done by the Drosophila group, but he continued to insist on the incompleteness of the chromosome theory. Genes of the kind that they mapped on the chromosomes could only give a partial explanation of biological heredity and evolution.
This paper applies the conceptual toolkit of Evolutionary Developmental Biology (evo-devo) to the evolution of the genome and the role of the genome in organism development. This challenges both the Modern Evolutionary Synthesis, the dominant view in evolutionary theory for much of the 20th century, and the typically unreflective analysis of heredity by evo-devo. First, the history of the marginalization of applying system-thinking to the genome is described. Next, the suggested framework is presented. Finally, its application to the evolution of genome modularity, the evolution of induced mutations, the junk DNA versus ENCODE debate, the role of drift in genome evolution, and the relationship between genome dynamics and symbiosis with microorganisms are briefly discussed.
Darwin’s theory of the origin of species by means of natural selection was essentially dependent on his views about inheritance. The character had to be inherited, to some extent at least, if the mechanism of natural selection was to produce evolutionary changes. Only hereditary variability would work. But Darwin’s theory of heredity was primitive. As the Danish plant physiologist Wilhelm Johannsen wrote, Darwin’s theory of pangenesis was more or less the same as that which Hippocrates had held 22 centuries earlier [p. 54].1 With the cytological discoveries of the late 19th century, and the efforts to give plant and animal breeding a scientific basis, a new era began. Botanists and plant breeders in particular played a central role in the decades around 1900, when the founding of genetics took place.
The chromosome theory as outlined by Thomas Hunt Morgan and his co-workers in The Mechanism of Mendelian Heredity2 was a major benchmark in the early history of genetics. It can be seen as setting the course that led to the discovery of DNA structure and the beginnings of molecular genetics four decades later. It is worth asking if similar status should be given to Johannsen’s 1910 lecture on ‘The genotype conception of heredity’.3 This lecture summed up his experimental and theoretical achievements, including a sharp analysis of the concepts of ‘genotype’ and ‘gene’. He had himself coined these terms two years earlier in his magisterial textbook Elemente der Exakten Erblichkeitslehre (Elements of an Exact Theory of Heredity).4
Genotype is the basic concept in Johannsen’s 1910 lecture. The stability of the genotype is what makes a science of heredity possible. The concept of ‘gene’ is derivative. It represents an experimentally identifiable difference between genotypes. From this holistic perspective he developed his criticism of the particulate gene concept that became the basis of mid-20th century Neo-Darwinism, the ‘New Synthesis’ of genetics and evolution. Johannsen in his later writings praised highly the discoveries of Drosophila genetics as fundamental novelties5 (consecutive editions were extensively reworked and thus demonstrate both the continuity and the change in Johannsen’s genotype theory), but still insisted that there was more to heredity and to the explanation of evolution than could be explained by such particulate genes.6,7
The controversy over the evolutionary advantage of recombination initially discovered by Fisher and by Muller is reviewed. Those authors whose models had finite-population effects found an advantage of recombination, and those whose models had infinite populations found none. The advantage of recombination is that it breaks down random linkage disequilibrium generated by genetic drift. Hill and Robertson found that the average effect of this randomly-generated linkage disequilibrium was to cause linked loci to interfere with each other's response to selection, even where there was no gene interaction between the loci. This effect is shown to be identical to the original argument of Fisher and Muller. It also predicts the ''ratchet mechanism'' discovered by Muller, who pointed out that deleterious mutants would more readily increase in a population without recombination. Computer simulations of substitution of favorable mutants and of the long-term increase of deleterious mutants verified the essential correctness of the original Fisher-Muller argument and the reality of the Muller ratchet mechanism. It is argued that these constitute an intrinsic advantage of recombination capable of accounting for its persistence in the face of selection for tighter linkage between interacting polymorphisms, and possibly capable of accounting for its origin.
SPONTANEOUS chromosomal rearrangements are considered to be rare in Drosophila. (Pale and blond are the known cases.) During the past few years I have found two new cases of considerable interest, the detailed analysis of which was delayed by external circumstances. Both cases occurred in pedigree and closely watched cultures, the abnormal broods being among numerous identical normal ones.
The use of 'race' as a proxy for population structure in the genetic mapping of complex traits has provoked controversy about its legitimacy as a category for biomedical research, given its social and political connotations. The controversy has reignited debates among scientists and philosophers of science about whether there is a legitimate biological concept of race. This paper examines the genetic race concept as it developed historically in the work of Theodosius Dobzhansky from the 1930s to 1950s. Dobzhansky's definitions of race changed over this time from races as 'arrays of forms' or 'clusters' in 1933-1939, to races as genetically distinct geographical populations in 1940-1946, to races as genetically distinct 'Mendelian populations' in 1947-1955. Dobzhansky responded to nominalist challenges by appealing to the biological reality of race as a process. This response came into tension with the object ontology of race that was implied by Dobzhansky's increasingly holistic treatment of Mendelian populations, a tension, the paper argues, he failed to appreciate or resolve.
Edward O. Wilson's recent decision to abandon kin selection theory has sent shockwaves throughout the biological sciences. Over the past two years, more than a hundred biologists have signed letters protesting his reversal. Making sense of Wilson's decision and the controversy it has spawned requires familiarity with the historical record. This entails not only examining the conditions under which kin selection theory first emerged, but also the organicist tradition against which it rebelled. In similar fashion, one must not only examine Wilson's long career, but also those thinkers who influenced him most, especially his intellectual grandfather, William Morton Wheeler (1865-1937). Wilson belongs to a long line of organicists, biologists whose research highlighted integration and coordination, many of whom struggled over the exact same biological riddles that have long defined Wilson's career. Drawing inspiration (and sometimes ideas) from these intellectual forebears, Wilson is confident that he has finally identified the origin of the social impulse.
The occurrence of natural selection demands (i) that there exists genetical heterogeneity, and (2) that unlike genotypes leave different average numbers of progeny. It is now known that both of these conditions are fulfilled, and all the available facts of evolution are in accord with the genetical theory of variation and selection.
Species must, on this view, differ in the same way as, but to a greater extent than, varieties or individuals of the same species.
The application of this criterion leads us to the conclusion that species differences are polygenic, i.e. depend on quantitative characters whose variation is controlled by many genes. These genes have individual effects which are both similar to one another and small when compared with non‐heritable fluctuation. Other kinds of heritable difference are ancillary to polygenic variation in speciation.
Each individual polygene is inherited in the same way as the familiar major mutants of the laboratory. As, however, there are many polygenes affecting a given character, the aggregate type of inheritance is distinct from that of the major mutants. Polygenically controlled differences are quantitative rather than qualitative and do not lead to the sharp segregation shown by the more familiar genetical differences. Polygenic characters, such as stature in man, can show any degree of expression between wide limits. Many genotypes may have the same phenotype. Thus polygenic theory relates continuous phenotypical variation to discontinuous genotypical variation, the biometrical to the genetical.
These special properties of polygenic behaviour lead us to a new and clearer understanding of the action of natural selection in producing adaptive and evolutionary changes.
Very fine adaptation of the phenotype to environment is made possible by the existence of such a wide range of phenotypic expression. The frequency distribution of the individual phenotypes found in a population may approximate to a normal curve. It may, however, also be skew, to an extent determined by the dominance and interaction relations of the polygenes and by the scale on which the character is measured. The central, most frequent, phenotype must closely approximate to the optimum for the prevailing environment. Departure from this central type will thus mean poorer adaptation and loss of fitness.
The phenotype is produced by the genotype acting as a whole. Since polygenes have effects similar to one another, a given phenotype may correspond to various genotypes some containing one and some another allelomorph of a given polygene. As a consequence neither allelomorph will have an unconditional advantage over the other, in the way that major mutants do. Rather the advantage of any allelomorph of a polygene will be conditioned by the other polygenes present. Fisher's theory of dominance then leads us to expect that, in wild populations, equal numbers of polygenes will show dominance of the allelomorphs leading to increased and decreased expression of the character. Artificial selection disturbs this equality. The existing evidence is in keeping with these expectations.
The existence of polygenic variation free in the phenotype must lead to some individuals departing from the optimum and so showing reduced fitness. Variation is to this extent disadvantageous, but it is also essential for prospective adaptive and evolutionary change. The polygenic variability necessary for prospective change need not, however, exist as free phenotypic variation which will affect fitness. It may be hidden in the genotype under the cloak of phenotypic constancy, when it will have no effect in lowering fitness. Such hidden, or potential, variability is released, and shown freely by the phenotype, as a result of segregation from heterozygotes. Free variability may pass into the potential state by means of crossing between unlike individuals. Some potential variability will exist as differences between homozygous individuals. Such homozygotic variability can be freed by segregation only after intercrossing has rendered it hetero‐zygotic.
If most of the variability in a population is potential, high current fitness can be combined with the possibility of great, though slow, change under selection. In such cases the response of the organism to selection will largely depend on the fixation of variability as it passes from the undetectable potential to the detectable free state. Thus selection may superficially appear to create its own free directional variability.
The frequency of recombination between polygenes affecting a character will control the rate of variability release. Consequently the effective recombination frequency is itself an adaptive character and will be subject to selective action. The evolution of genetic systems is largely the history of this selective control of effective recombination.
Control of recombination is almost wholly achieved within chromosomes, so that the storage of variability must depend on intrachromosome adjustment. Natural selection will tend to build up balanced combinations of polygenes within each of the chromosomes. These combinations will have the properties of close adaptation to the optimum, great variability storage and slow variability release.
Combinations are characterized by two kinds of balance, that of the individual combination, as shown in homozygotes (internal balance), and that of pairs of combinations when working together in heterozygotes (relational balance). Dominance permits the adjustment of these balances independently of one another. The theory of polygenic balance shows how polymorphism and clines can be maintained.
Heterosis is due to a particular kind of poor relational balance brought about by artificial selection. The concept of heterosis is now extended to include all types of such unbalance, natural and artificial. Poor relational balance encourages isolation, and so heterosis, in this broad sense, stimulates the rise of isolation mechanisms and hybrid sterility.
The store of polygenic variability, steadily depleted by random fluctuations in allelomorph frequency and by response to selection, is replenished by new mutations. Since all polygenes affecting a given character have much the same effect, the phenotypical properties of a population may be stable or nearly so even though the genotype is fluid. Fixation, mutation, segregation and recombination cause a genotypic flux to exist under the cloak of a phenotypic stability, itself maintained by the action of the same natural selection, which, under new conditions, would lead to new adaptation.
The mechanical relations of unlike combinations, whose constituent polygenes are intermingled along the same chromosome, are sufficient to account for the degeneration of unused organs.
The breeding, or mating, system of a species determines the frequency of heterozygosity, upon which the rate of release of potential variability depends. Inbreeding gives homozygosity and high immediate fitness; but it freezes potential variability in the homozygotic state and so reduces the chance of prospective adaptation. Outbreeding has the reverse effect and sacrifices some fitness to flexibility.
The breeding system is thus an adaptive character. It will be subject to selective change towards more closely controlled inbreeding or outbreeding. A controlled compromise between inbreeding and outbreeding may also occur. The strength and direction of control is probably polygenically determined, though the actual controlling mechanism may depend upon a major switch gene for its direct action.
A change from outbreeding to inbreeding increases local adaptation and so leads to heterosis and isolation. It also freezes potential variability and lowers the chance of prospective adaptation. Thus a species which shows such a change to inbreeding will break up into a swarm of small, locally fit, but inflexible, new species. As a consequence of their inflexibility, most of these must perish when environmental changes set in.
The classical/balance controversy continued along these lines throughout the first half of the sixties. Then, at about the same time that the classical position lost its leading advocate, the balance position received striking new support from Harry Harris, and independently from Dobzhansky's former student Lewontin, and Lewontin's research partner, Jack Hubby.80 These developments served more to reorient the controversy than to end it — and the resulting “neoclassical”/balance controversy is different enough to be grist for another mill.
Social policy considerations no longer play a role in keeping the dispute alive. This particular respect in which the issues have changed is, as Diane Paul suggests in her contribution to this volume, as striking as any other.82 There is, however, little danger of our forgetting that this was once much more than just a narrowly technical controversy — the additional social policy issues were far too blatant.
However, although blatant, they were by no means the only, or even the most important, issues. In choosing to concentrate on the social policy considerations, I do not mean to suggest that the empirical issues were irrelevant, or simple and straightforward, or otherwise uninteresting. That is by no means the case. What I have tried to show is that there was much more to the classical/balance stalemate than just the empirical underdetermination of the theoretical issues, and that the empirical issues cannot be treated adequately without taking into account the social policy considerations that were involved. Dobzhansky and Muller both appealed to the dangers of misguided social policy that might have resulted from prematurely resolving their controversy in the other's favor. They called for high empirical standards on those grounds, more than once seeking to forestall the resolution of their dispute in this way.
The most important issue on which H. J. Muller and Th. Dobzhansky differed was a question that had puzzled plant and animal breeders during most of the first half of the twentieth century. Breeders were concerned with maximizing performance. Dobzhansky was concerned with the potential of the species for further evolution -- and so was Muller, but his immediate concern was the impact on future generations of a radiation-induced increase in the mutation rate. All three concerns centered on how much of the genetic variability in the population depends on overdominance, that is, gene loci where the phenotype of a heterozygote is outside the range of the phenotypes of the two corresponding homozygotes. Heterosis, the greater vigor and performance of heterozygotes, attracted so much attention that in the summer of 1950 Iowa State College held a five-week symposium on the subject.l It will be helpful to make the vocabulary clear at the outset with an illustrative example:
Richard Goldschmidt's research on homeotic mutants from 1940 until his death in 1958 represents one of the first serious efforts to integrate genetics, development, and evolution. Using two different models, Goldschmidt tried to show how different views of genetic structure and gene action could provide a mechanism for rapid speciation. Developmental systems were emphasized in one model and a hierarchy of genetic structures in the other. While Goldschmidt tried to find a balance between development and genetics, critics, such as Sewall Wright, urged him and eventually helped him incorporate population dynamics into his models as well. As such, the history of Goldschmidt's research on homeotic mutants highlights the continuing challenge of producing a balanced and integrated developmental evolutionary genetics.
This paper describes the historical background and early formation of Wilhelm Johannsen's distinction between genotype and phenotype. It is argued that contrary to a widely accepted interpretation (For instance, W. Provine, 1971. The Origins of Theoretical Population Genetics. Chicago: The University of Chicago Press; Mayr, 1973; F. B. Churchill, 1974. Journal of the History of Biology 7: 5-30; E. Mayr, 1982. The Growth of Biological Thought, Cambridge: Harvard University Press; J. Sapp, 2003. Genesis. The Evolution of Biology. New York: Oxford University Press) his concepts referred primarily to properties of individual organisms and not to statistical averages. Johannsen's concept of genotype was derived from the idea of species in the tradition of biological systematics from Linnaeus to de Vries: An individual belonged to a group - species, subspecies, elementary species - by representing a certain underlying type (S. Müller-Wille and V. Orel, 2007. Annals of Science 64: 171-215). Johannsen sharpened this idea theoretically in the light of recent biological discoveries, not least those of cytology. He tested and confirmed it experimentally combining the methods of biometry, as developed by Francis Galton, with the individual selection method and pedigree analysis, as developed for instance by Louis Vilmorin. The term "genotype" was introduced in W. Johannsen's 1909 (Elemente der Exakten Erblichkeitslehre. Jena: Gustav Fischer) treatise, but the idea of a stable underlying biological "type" distinct from observable properties was the core idea of his classical bean selection experiment published 6 years earlier (W. Johannsen, 1903. Ueber Erblichkeit in Populationen und reinen Linien. Eine Beitrag zur Beleuchtung schwebender Selektionsfragen, Jena: Gustav Fischer, pp. 58-59). The individual ontological foundation of population analysis was a self-evident presupposition in Johannsen's studies of heredity in populations from their start in the early 1890s till his death in 1927. The claim that there was a "substantial but cautious modification of Johannsen's phenotype-genotype distinction" (Churchill, 1974, p. 24) from a statistical to an individual ontological perspective derives from a misreading of the 1903 and 1909 texts. The immediate purpose of this paper is to correct this reading of the 1903 monograph by showing how its problems and results grow out of Johannsen's earlier work in heredity and plant breeding. Johannsen presented his famous selection experiment as the culmination of a line of criticism of orthodox Darwinism by William Bateson, Hugo de Vries, and others (Johannsen, 1903). They had argued that evolution is based on stepwise rather than continuous change in heredity. Johannsen's paradigmatic experiment showed how stepwise variation in heredity could be operationally distinguished from the observable, continuous morphological variation. To test Galton's law of partial regression, Johannsen deliberately chose pure lines of self-fertilizing plants, a pure line being the descendants in successive generations of one single individual. Such a population could be assumed to be highly homogeneous with respect to hereditary type, and Johannsen found that selection produced no change in this type. Galton, he explained, had experimented with populations composed of a number of stable hereditary types. The partial regression which Galton found was simply an effect of selection between types, increasing the proportion of some types at the expense of others.
The common fruit fly, Drosophila, has long been one of the most productive of all laboratory animals. From 1910 to 1940, the center of Drosophila culture in America was the school of Thomas Hunt Morgan and his students Alfred Sturtevant and Calvin Bridges. They first created "standard" flies through inbreeding and by organizing a network for exchanging stocks of flies that spread their practices around the world. In Lords of the Fly, Robert E. Kohler argues that fly laboratories are a special kind of ecological niche in which the wild fruit fly is transformed into an artificial animal with a distinctive natural history. He shows that the fly was essentially a laboratory tool whose startling productivity opened many new lines of genetic research. Kohler also explores the moral economy of the "Drosophilists": the rules for regulating access to research tools, allocating credit for achievements, and transferring authority from one generation of scientists to the next. By closely examining the Drosophilists' culture and customs, Kohler reveals essential features of how experimental scientists do their work.
The controversy over the evolutionary advantage of recombination initially discovered by Fisher and by Muller is reviewed. Those authors whose models had finite-population effects found an advantage of recombination, and those whose models had infinite populations found none. The advantage of recombination is that it breaks down random linkage disequilibrium generated by genetic drift. Hill and Robertson found that the average effect of this randomly-generated linkage disequilibrium was to cause linked loci to interfere with each other's response to selection, even where there was no gene interaction between the loci. This effect is shown to be identical to the original argument of Fisher and Muller. It also predicts the "ratchet mechanism" discovered by Muller, who pointed out that deleterious mutants would more readily increase in a population without recombination. Computer simulations of substitution of favorable mutants and of the long-term increase of deleterious mutants verified the essential correctness of the original Fisher-Muller argument and the reality of the Muller ratchet mechanism. It is argued that these constitute an intrinsic advantage of recombination capable of accounting for its persistence in the face of selection for tighter linkage between interacting polymorphisms, and possibly capable of accounting for its origin.
PrefaceAcknowledgmentsCh. 1The Exegesis of Unifying Biology3Ch. 2A "Moving Target": Historical Background on the Evolutionary Synthesis19Ch. 3Rethinking the Evolutionary Synthesis: Historiographic Questions and Perspectives Explored45Ch. 4The New Contextualism: Science as Discourse and Culture73Ch. 5The Narrative of Unifying Biology: The Evolutionary Synthesis and Evolutionary Biology97Ch. 6Reproblematizing the Evolutionary Synthesis191Ch. 7Epilogue211Select Bibliography217Index223