Showing papers in "The Quarterly Review of Biology in 1993"

Genetic conflicts in human pregnancy.

Abstract: Pregnancy has commonly been viewed as a cooperative interaction between a mother and her fetus. The effects of natural selection on genes expressed in fetuses, however, may be opposed by the effects of natural selection on genes expressed in mothers. In this sense, a genetic conflict can be said to exist between maternal and fetal genes. Fetal genes will be selected to increase the transfer of nutrients to their fetus, and maternal genes will be selected to limit transfers in excess of some maternal optimum. Thus a process of evolutionary escalation is predicted in which fetal actions are opposed by maternal countermeasures. The phenomenon of genomic imprinting means that a similar conflict exists within fetal cells between genes that are expressed when maternally derived, and genes that are expressed when paternally derived. During implantation, fetally derived cells (trophoblast) invade the maternal endometrium and remodel the endometrial spiral arteries into low-resistance vessels that are unable to constrict. This invasion has three consequences. First, the fetus gains direct access to its mother's arterial blood. Therefore, a mother cannot reduce the nutrient content of blood reaching the placenta without reducing the nutrient supply to her own tissues. Second, the volume of blood reaching the placenta becomes largely independent of control by the local maternal vasculature. Third, the placenta is able to release hormones and other substances directly into the maternal circulation. Placental hormones, including human chorionic gonadotropin (hCG) and human placental lactogen (hPL), are predicted to manipulate maternal physiology for fetal benefit. For example, hPL is proposed to act on maternal prolactin receptors to increase maternal resistance to insulin. If unopposed, the effect of hPL would be to maintain higher blood glucose levels for longer periods after meals. This action, however, is countered by increased maternal production of insulin. Gestational diabetes develops if the mother is unable to mount an adequate response to fetal manipulation. Similarly, fetal genes are predicted to enhance the flow of maternal blood through the placenta by increasing maternal blood pressure. Preeclampsia can be interpreted as an attempt by a poorly nourished fetus to increase its supply of nutrients by increasing the resistance of its mother's peripheral circulation.

Adaptation and the Goals of Evolutionary Research

Abstract: Adaptation is and has always been a sloppery concept. Even among contemporary evolutionary biologists there is no consensus about how to identify adaptive traits. Most definition of adaptation incorporate elements of history. Among these, some require that adaptive traits be built by natural selection for their current roles. Others take a phylogenetic perspective and require that adaptations be derived relative to traits of antecedent taxa. Still other definitions require evidence of functional design in the biological machinery underlying the trait, implying the trait, implying a history of selective fine-tuning. The historical definition are most useful for addressing questions about evolutionary history like, "What was the phylogenetic trajectory of a phenotype with current utility?," or "How does a trait's current use relate to its original function?" Historical definitions are less useful for addressing questions about phenotype existence like, "Why do certain phenotypes predominate over others in n...

A critical assessment of levins's the strategy of model building in population biology (1966)

Abstract: Richard Levins's 1966 article "The strategy of model building in population biology" is an extremely influential analysis of the nature of scientific model building His claims that model building involves a necessary trade-off among generality, realism and precision, and that truths about nature can be revealed by finding "robust theorems" are important and deserve careful scrutiny We clarify the concepts of generality, realism and precision and argue that there is no necessary conflict among them We also examine the idea of robustness and conclude that it lacks proper definition and that its bearing on the question of whether a proposition is true is highly problematic Accordingly, we believe that neither of Levins's claims should be accepted

The concept of the gene: short history and present status

Abstract: The concept of the gene is and has always been a continuously evolving one. In order to provide a structure for understanding the concept, its history is divided into classical, neoclassical, and modern periods. The classical view prevailed into the 1930s, and conceived the gene as an indivisible unit of genetic transmission, recombination, mutation, and function. The discovery of intragenic recombination in the early 1940s and the establishment of DNA as the physical basis of inheritance led to the neoclassical concept of the gene, which prevailed until the 1970s. In this view the gene (or cistron, as it was called then) was subdivided into its constituent parts, mutons and recons, identified as nucleotides. Each cistron was believed to be responsible for the synthesis of a single mRNA and hence for one polypeptide. This colinearity hypothesis prevailed from 1955 to the 1970s. Starting from the early 1970s, DNA technologies have led to the modern period of gene conceptualization, wherein none of the classical or neoclassical criteria are sufficient to define a gene. Modern discoveries include those of repeated genes, split genes and alternative splicing, assembled genes, overlapping genes, transposable genes, complex promoters, multiple polyadenylation sites, polyprotein genes, editing of the primary transcript, and nested genes. We are currently left with a rather abstract, open, and generalized concept of the gene, even though our comprehension of the structure and organization of the genetic material has greatly increased.

A Response to Orzack and Sober: Formal Analysis and the Fluidity of Science

Abstract: JN THE BOOKSTORES of my youth it was common to find titles that included the term \"foundations of science.\" These were not books about science at all but about logic, heirs of the Russell-Whitehead program to derive mathematics from logic, and science from mathematics. The hope persisted that objectivity could be achieved by analytical methods: clear definitions, unambiguous categories, sharp measurement, and the discovery of algorithms that could substitute for the caprice of human judgment. But the program as a whole has been a failure, as indeed it had to be. The foundations of science are to be found in history and sociology, not formal analysis. Orzack and Sober's critique of my 1966 paper falls within the tradition of formal analysis, and our disagreements fall along the axis from formal analytical to dialectical views of the scientific process (Levins and Lewontin, 1985). Formal analysisfreezes moments of a process into things. My essay was concerned with model building as a process, embedded in the larger processes of scientific investigation. It looked at the decisions that population biologists were making at the time (the middle 1960s) in order to solve different kinds of problems. It was concerned with two difficulties: one was that attempts to optimize different criteria for satisfactory models interfere with each other, and the other was that all models are partly false. I identified three tendencies in model building in population biology, each of which involved the sacrifice of one of the desiderata: generality, realism and precision. Other criteria were mentioned (manageability and understandability) but not pursued. Nor did I include in the listing strategies that sacrificed two criteria for the third. It is clear from the context hat the listing of three strategies was not intended to be exhaustive of all possible population biology practice or of science as a whole. What was important was the notion of trade-offs in model building. Orzack and Sober reify the discussion of model building into a discussion of models, and turn the trends I noted into \"Levins's taxonomy of models\" or \"Levins's trichotomy.\" Formal analysis prefersfixed d finitions f objects, free of their context, inorder to allowfor unambiguous measurement a dranking. But in science definitions evolve with the problem. The formal mode of thought prefers sharply separated disjunct categories and has indeed encouraged the long list of false dichotomies that have plagued our biology: heredity versus environment, physical versus biotic control of population abundance, internal versus external determination, random versus determin-

Menstruation as a defense against pathogens transported by sperm.

Abstract: Sperm are vectors of disease. During mammalian insemination bacteria from the male and female genitalia regularly cling to sperm tails and are transported to the uterus. I propose that menstruation functions to protect the uterus and oviducts from colonization by pathogens. Menstrual blood exerts mechanical pressure on uterine tissue, forcing it to shed, and delivers large numbers of immune cells throughout the uterine cavity, directly combating pathogens. The mechanisms of menstruation show evidence of adaptive design. Spiral arteries that open to the lining of the uterus trigger menstruation by abruptly constricting, which deprives the local tissue of blood, and then abruptly dilating, which causes blood to force loose the necrotic tissue. Menstrual blood flows easily, unlike blood at most wound sites, because it lacks the normal level of clotting factors. Overt (externally visible) or covert (not externally visible) menstruation has been documented in many species of primate, including Old World monkeys and apes, New World monkeys, and prosimians, as well as in various species of bat and insectivore. The antipathogen hypothesis predicts that: (1) menstruation (overt or covert) is either universal or nearly so among mammalian species; (2) if the latter, then the existence of menstruation among species varies inversely with the probability of becoming pregnant per estrous cycle (menstruation would be especially adaptive in species with significantly less than 100% probability of becoming pregnant per estrous cycle); (3) among menstruating species, the average degree of menstrual bleeding for a given species is a function of the factors affecting menstruation's costs and benefits--in particular, the degree of bleeding is positively correlated with the average body size and sexually transmitted pathogen load of that species (profuse bleeding would be especially adaptive in large-bodied species with either promiscuous breeding systems or continuous sexual receptivity); and (4) other forms of normal uterine bleeding--proestrous, periovulatory, implantation, and postpartum--also have an antipathogen function. The hypothesis presented in this article has implications for the diagnosis, treatment and prevention of uterine infection and, therefore, for the prevention of pathogen-induced infertility. The uterus appears to be designed to increase its bleeding if it detects infection: Human uteri that become infected (or otherwise inflamed) bleed more profusely, bleed on more days per cycle, and often bleed intermittently throughout the cycle. Thus artificially curtailing infection-induced uterine bleeding may be contraindicated.

Sensory and Hormonal Control of Clutch Size in Birds

Abstract: The control of clutch size is examined in several taxa of birds and the concept that species can be classified as determinate or indeterminate layers is reevaluated. Determinate layers are defined as species in which extrinsic factors perceived by the female, such as eggs present in the nest, are not involved in determining either the number of largeyolky follicles produced by the ovary or the number offollicles ovulated (e.g., albatrosses, auks and pigeons). In some species, the number of largeyolkyfollicles produced is unrelated to extrinsicfactors, but contact between thefemale and her eggs usually reduces the number offollicles ovulated and hence the clutch size. These species are classified as semideterminate layers (penguins). Among indeterminate layers, both the number of largeyolky follicles and the number offollicles ovulated depend upon extrinsic factors. Three categories of indeterminate layers are recognized, depending upon the nature of the stimulus that stops egg laying. The first is tactile contact between the female's brood patch and the eggs; it is the most common. This category may be subdivided according to the number of eggs required to stimulate the brood patch. In some species egg removal leaving no egg in the nest is requiredfor the female to lay extra eggs (contact with a Single egg sufficient: type S, woodpeckers, plovers); others may lay extra eggs even if an egg is left in the nest (contact with Multiple eggs required: type M, kestrels, hawks, grebes). The second category of stimuli causing laying to cease, observed in parasitic birds, is probably the lack of suitable host nests. These visual cues trigger either reabsorption of all largeyolky follicles (Discontinuous production of largeyolky follicles: type D, parasitic cuckoos) or simply reabsortion of one or two largeyolkyfollicles (Continuous production of largeyolky follicles: type C, parasitic cowbirds). The third category, used by megapodes, could be thermal information derivedfrom their nest mounds. Each category can be discriminated by observations on egg laying and appropriate egg removal experiments. Anatomical data from laying females are also a valuable source of information. The endocrine mechanism of clutch-size determination is still poorly understood; it has been linked to the onset of incubation behavior and in particular to a rise in prolactin levels. I argue that the rise in prolactin levels associated with incubation behavior might play a role in the determination of clutch size in tactile indeterminate layers, but not in determinate layers. Inhibition of gonadotrophin secretion constitutes another hypothesis that might be relevant whatever the mode of clutch-size control. Finally, a new hypothesis is proposed to account for the evolution of the control of clutch size in birds that relates determinate and indeterminate laying patterns tofood availability at the time of eggformation, and variability of the best time for raisingyoung.

Will the Real Charles Darwin Please Stand Up

Abstract: C HARLES Robert Darwin is a mystery man. He is one of the three or four most famous scientists of all time. He was acknowledged as such during his lifetime, and so (unlike Mendel, for example) there were lots of people around to record their meetings and reactions and thoughts. Letters to and from Darwin are really remarkably well preserved in an ongoing series edited by Burkhardt and Smith (1985 ff.). The crucial notebooks at times of discovery have survived in extremely good shape (Barrett et al., 1987). In the hundred years or so since his death, the words have poured forth in a never-ending yet everincreasing torrent. And there must be somebody reading the stuff. Publishers are prepared to put up advances of a magnitude undreamed of in usual academic circles. I understand that there is one biography in preparation, financed by a check for ?100,000. Yet, I repeat that Charles Darwin is a mystery man. Was he a great scientist, really great I mean, of the caliber of Albert Einstein, that everyone accepts as having been a genius? Or was he perhaps like some of the prominent figures of molecular biology smart and ambitious, but lucky in having been the person around when important conceptual moves and empirical discoveries were there to be made? Was he even a bit thick, a man who hit on his theory but really had no idea of what he had grasped? 'Yes\" answers to all of these questions can be found in the literature-answers that have been provided by professionals as often as by hack journalists. (Many articles, giving different viewpoints, together with a massive bibliography, can be found in Kohn, 1985.) Was Darwin's theory really significant, or have its merits been overplayed? Did Darwin make a really fundamental breakthrough, or would what he did have happened anyway? Did Darwin retard the progress of science? Should we think of the schoolmaster's comment on the boy's essay: \"This answer is both good and original. Regretfully, those parts which are good are not particularly original and those parts which are original are not particularly good.\" Again, we find positive responses to all of these questions. Usually the really negative comments come from those who have another hero to champion. Supporters of Alfred Russel Wallace, the codiscoverer of natural selection, tend to be viciously anti-Darwinian. This is a stance that would have much embarrassed Wallace himself, who always acknowledged Darwin's greater significance. (See, for example, Brooks, 1984.)

Haldane, Marxism, and the Conduct of Research

Abstract: Center for Population Biology and Humanities Institute, University of California Davis, California 95616 USA THE EMBRACE of Communism by John Burdon Sanderson Haldane UBS) stimulated many discussions of how science and ideology are related at various levels, and it continues to evoke comment today (Gardner, 1992; Sarkar, 1992). No one seems to have looked for the influence of ideology in Haldane's scientific work, however. Leigh (1990) pointed out that Haldane differed from R. A. Fisher and Sewall Wright in being a puzzle-solver rather than a system-builder, an observation echoed by Maynard Smith (1992). Yet it was Haldane who committed himself to one of the most sweeping of all systems, Marxism, and for a while he not only defended it but also propagandized for it copiously and well. In the process he made claims for Marxism as a tool in science claims that went beyond the mere assertion that the Marxist worldview best fit the material universe. There are at least three levels at which the influence of Haldane's Marxism on his science might be explored. First, did it make him a better biologist, in a qualitative sense? To answer this requires value judgments that are unlikely to be politically neutral. Second, did it make him a more productive biologist, in purely quantitative terms? Since the period of his allegiance to Marxism is at least approximately defined, one can address this question by examining his research output throughout his career; when only strictly research or theoretical publications are counted, the answer is an unequivocal \"no.\" Third, did Marxism detectably alter his way of doing science? This is the most interesting of these questions, and fortunately it is approachable from his own work. Before making that approach it is necessary to examine Haldane's political essays to identify his own concept of Marxism and its relationship to science, in general, and his own work, in particular.