The recombination of genes is clearly disadvantageous in a uniform and predictable environment wh... more The recombination of genes is clearly disadvantageous in a uniform and predictable environment where a single multilocus genotype is optimal. But heterogeneity or unpredictability do not necessarily imply a short-term advantage for recombination, which will be created only if the correlation between adaptively important features of the environment frequently changes sign1. Another way of expressing this conclusion is to say
Page 1. CONSISTENCY OF INDIVIDUAL LEADERSHIP POSITION IN SMALL GROUPS OF VARYING MEMBERSHIP BY GR... more Page 1. CONSISTENCY OF INDIVIDUAL LEADERSHIP POSITION IN SMALL GROUPS OF VARYING MEMBERSHIP BY GRAHAM B. BELL AND ROBERT L. FRENCH Northwestern University ECENT formulations of leadership ...
Sex is a general feature of the life cycle of eukaryotes. It is not universal, however, as many o... more Sex is a general feature of the life cycle of eukaryotes. It is not universal, however, as many organisms seem to lack sex entirely. The widespread occurrence of sex is puzzling, both because meiotic recombination can disrupt co-adapted combinations of genes, and because it halves the potential rate of reproduction in organisms with strongly differentiated male and female gametes. Most attempts to explain the maintenance of sexuality invoke differences between parents and sexual offspring. These differences may be advantageous in novel or changing environments if new gene combinations are favoured from time to time. Sex would then serve to concentrate beneficial mutations that have arisen independently into the same line of descent. But in a stable environment sex might serve to concentrate deleterious mutations, so that they will be more effectively purged from the population by selection. We have studied the effect of sex on mean fitness in experimental populations of the budding ...
Here, we provide a review of the direct effect of increasing CO2 on aquatic primary producers thr... more Here, we provide a review of the direct effect of increasing CO2 on aquatic primary producers through its function as a source of carbon, focusing our analysis on the interpretation of this increase as an increase in the availability of a resource. This provides an interesting context to evaluate ecological and evolutionary theories relating to nutrient availability and leads us to: the assessment of theories about limitation of productivity and the integration of CO2 into the co-limitation paradigm; the prediction of community composition and of change in communities from known changes in the environment; and evaluation of the potential for evolutionary adaptation in conditions that increase growth.
A study is made of the thermal behaviour of solid mixtures, containing a component A with molecul... more A study is made of the thermal behaviour of solid mixtures, containing a component A with molecules having several orientation states whose co-operative interaction leads, in pure A, to a second-order transition with a discontinuity in the specific heat and a diluting component B with molecules of the same size. Regular models are used and it can be shown from previous work on ternary assemblies that if A has two equivalent orientation states and complete equilibrium is attained, then the curve of critical temperature against the mole fraction xA of A passes through the origin. It is shown here that if A-B diffusion is forbidden in the temperature range concerned so that the distribution of A and B is fixed, though A molecules can change their orientation, then the specific heat is continuous below a critical value of xA. This effect depends on short-range ordering and a first-order quasi-chemical approximation is used. Similar results are obtained when A has three available orienta...
The central themes of community ecology-distribution, abundance, and diversity-display strongly m... more The central themes of community ecology-distribution, abundance, and diversity-display strongly marked and very general patterns. These include the log-normal distribution of abundance, the relation between range and abundance, the species-area law, and the turnover of species composition. Each pattern is the subject of a large literature that interprets it in terms of ecological processes, typically involving the sorting of differently specialized species onto heterogeneous landscapes. All of these patterns can be shown to arise, however, from neutral community models in which all individuals have identical properties, as the consequence of local dispersal alone. This implies, at the least, that functional interpretations of these patterns must be reevaluated. More fundamentally, neutral community models provide a general theory for biodiversity and conservation biology capable of predicting the fundamental processes and patterns of community ecology.
Proceedings of the Royal Society B: Biological Sciences, 2004
assemblages increased the species richness of local freshwater fish Cordillera exchange mediated ... more assemblages increased the species richness of local freshwater fish Cordillera exchange mediated by the Panama Canal − Cross Supplementary data ml
Philosophical Transactions of the Royal Society B: Biological Sciences, 2012
Populations subject to severe stress may be rescued by natural selection, but its operation is re... more Populations subject to severe stress may be rescued by natural selection, but its operation is restricted by ecological and genetic constraints. The cost of natural selection expresses the limited capacity of a population to sustain the load of mortality or sterility required for effective selection. Genostasis expresses the lack of variation that prevents many populations from adapting to stress. While the role of relative fitness in adaptation is well understood, evolutionary rescue emphasizes the need to recognize explicitly the importance of absolute fitness. Permanent adaptation requires a range of genetic variation in absolute fitness that is broad enough to provide a few extreme types capable of sustained growth under a stress that would cause extinction if they were not present. This principle implies that population size is an important determinant of rescue. The overall number of individuals exposed to selection will be greater when the population declines gradually under a constant stress, or is progressively challenged by gradually increasing stress. In gradually deteriorating environments, survival at lethal stress may be procured by prior adaptation to sublethal stress through genetic correlation. Neither the standing genetic variation of small populations nor the mutation supply of large populations, however, may be sufficient to provide evolutionary rescue for most populations.
INTRODUCTION The object of this paper is to suggest that there may be an unexpected connexion bet... more INTRODUCTION The object of this paper is to suggest that there may be an unexpected connexion between parasites and the evolution of sex, using for illustration an unfamiliar type of parasite, the selfish chromosome. The major intellectual challenge of sexuality is to ...
The flowers of the annual herb Impatiens capensis have distinct male and female phases. The male ... more The flowers of the annual herb Impatiens capensis have distinct male and female phases. The male phase lasts four times as long as the female phase, and male flowers contain about 50% more nectar than female flowers. This suggests that the bulk of allocation to the flower is designed to ensure the dispersal of pollen rather than the fertilization of ovules. Honeybees, wasps and bumble bees all land on male flowers more often than would be expected by chance, and, having landed, wasps and bumble bees are more likely to enter a male flower than a female flower. The frequency of male flowers in the diet therefore exceeds their frequency in the population. This preference, although strong and consistent, is only partial, since some female flowers are included in the diet. We propose two hypotheses to account for the observed partial preference, the first based on competition between bees for flowers, and the second asserting that the bees detect nectar levels directly without using floral gender as a cue. The results of an experiment in which the most obvious gender cue, the androecium, was removed are consistent with the second hypothesis.
The process of adaptation occurs on two timescales. In the short term, natural selection merely s... more The process of adaptation occurs on two timescales. In the short term, natural selection merely sorts the variation already present in a population, whereas in the longer term genotypes quite different from any that were initially present evolve through the cumulation of new mutations. The first process is described by the mathematical theory of population genetics. However, this theory begins by defining a fixed set of genotypes and cannot provide a satisfactory analysis of the second process because it does not permit any genuinely new type to arise. The evolutionary outcome of selection acting on novel variation arising over long periods is therefore difficult to predict. The classical problem of this kind is whether 'replaying the tape of life' would invariably lead to the familiar organisms of the modern biota 1,2 . Here we study the long-term behaviour of populations of autonomously replicating computer programs and find that the same type, introduced into the same simple environment, evolves on any given occasion along a unique trajectory towards one of many well-adapted end points.
The recombination of genes is clearly disadvantageous in a uniform and predictable environment wh... more The recombination of genes is clearly disadvantageous in a uniform and predictable environment where a single multilocus genotype is optimal. But heterogeneity or unpredictability do not necessarily imply a short-term advantage for recombination, which will be created only if the correlation between adaptively important features of the environment frequently changes sign1. Another way of expressing this conclusion is to say
Page 1. CONSISTENCY OF INDIVIDUAL LEADERSHIP POSITION IN SMALL GROUPS OF VARYING MEMBERSHIP BY GR... more Page 1. CONSISTENCY OF INDIVIDUAL LEADERSHIP POSITION IN SMALL GROUPS OF VARYING MEMBERSHIP BY GRAHAM B. BELL AND ROBERT L. FRENCH Northwestern University ECENT formulations of leadership ...
Sex is a general feature of the life cycle of eukaryotes. It is not universal, however, as many o... more Sex is a general feature of the life cycle of eukaryotes. It is not universal, however, as many organisms seem to lack sex entirely. The widespread occurrence of sex is puzzling, both because meiotic recombination can disrupt co-adapted combinations of genes, and because it halves the potential rate of reproduction in organisms with strongly differentiated male and female gametes. Most attempts to explain the maintenance of sexuality invoke differences between parents and sexual offspring. These differences may be advantageous in novel or changing environments if new gene combinations are favoured from time to time. Sex would then serve to concentrate beneficial mutations that have arisen independently into the same line of descent. But in a stable environment sex might serve to concentrate deleterious mutations, so that they will be more effectively purged from the population by selection. We have studied the effect of sex on mean fitness in experimental populations of the budding ...
Here, we provide a review of the direct effect of increasing CO2 on aquatic primary producers thr... more Here, we provide a review of the direct effect of increasing CO2 on aquatic primary producers through its function as a source of carbon, focusing our analysis on the interpretation of this increase as an increase in the availability of a resource. This provides an interesting context to evaluate ecological and evolutionary theories relating to nutrient availability and leads us to: the assessment of theories about limitation of productivity and the integration of CO2 into the co-limitation paradigm; the prediction of community composition and of change in communities from known changes in the environment; and evaluation of the potential for evolutionary adaptation in conditions that increase growth.
A study is made of the thermal behaviour of solid mixtures, containing a component A with molecul... more A study is made of the thermal behaviour of solid mixtures, containing a component A with molecules having several orientation states whose co-operative interaction leads, in pure A, to a second-order transition with a discontinuity in the specific heat and a diluting component B with molecules of the same size. Regular models are used and it can be shown from previous work on ternary assemblies that if A has two equivalent orientation states and complete equilibrium is attained, then the curve of critical temperature against the mole fraction xA of A passes through the origin. It is shown here that if A-B diffusion is forbidden in the temperature range concerned so that the distribution of A and B is fixed, though A molecules can change their orientation, then the specific heat is continuous below a critical value of xA. This effect depends on short-range ordering and a first-order quasi-chemical approximation is used. Similar results are obtained when A has three available orienta...
The central themes of community ecology-distribution, abundance, and diversity-display strongly m... more The central themes of community ecology-distribution, abundance, and diversity-display strongly marked and very general patterns. These include the log-normal distribution of abundance, the relation between range and abundance, the species-area law, and the turnover of species composition. Each pattern is the subject of a large literature that interprets it in terms of ecological processes, typically involving the sorting of differently specialized species onto heterogeneous landscapes. All of these patterns can be shown to arise, however, from neutral community models in which all individuals have identical properties, as the consequence of local dispersal alone. This implies, at the least, that functional interpretations of these patterns must be reevaluated. More fundamentally, neutral community models provide a general theory for biodiversity and conservation biology capable of predicting the fundamental processes and patterns of community ecology.
Proceedings of the Royal Society B: Biological Sciences, 2004
assemblages increased the species richness of local freshwater fish Cordillera exchange mediated ... more assemblages increased the species richness of local freshwater fish Cordillera exchange mediated by the Panama Canal − Cross Supplementary data ml
Philosophical Transactions of the Royal Society B: Biological Sciences, 2012
Populations subject to severe stress may be rescued by natural selection, but its operation is re... more Populations subject to severe stress may be rescued by natural selection, but its operation is restricted by ecological and genetic constraints. The cost of natural selection expresses the limited capacity of a population to sustain the load of mortality or sterility required for effective selection. Genostasis expresses the lack of variation that prevents many populations from adapting to stress. While the role of relative fitness in adaptation is well understood, evolutionary rescue emphasizes the need to recognize explicitly the importance of absolute fitness. Permanent adaptation requires a range of genetic variation in absolute fitness that is broad enough to provide a few extreme types capable of sustained growth under a stress that would cause extinction if they were not present. This principle implies that population size is an important determinant of rescue. The overall number of individuals exposed to selection will be greater when the population declines gradually under a constant stress, or is progressively challenged by gradually increasing stress. In gradually deteriorating environments, survival at lethal stress may be procured by prior adaptation to sublethal stress through genetic correlation. Neither the standing genetic variation of small populations nor the mutation supply of large populations, however, may be sufficient to provide evolutionary rescue for most populations.
INTRODUCTION The object of this paper is to suggest that there may be an unexpected connexion bet... more INTRODUCTION The object of this paper is to suggest that there may be an unexpected connexion between parasites and the evolution of sex, using for illustration an unfamiliar type of parasite, the selfish chromosome. The major intellectual challenge of sexuality is to ...
The flowers of the annual herb Impatiens capensis have distinct male and female phases. The male ... more The flowers of the annual herb Impatiens capensis have distinct male and female phases. The male phase lasts four times as long as the female phase, and male flowers contain about 50% more nectar than female flowers. This suggests that the bulk of allocation to the flower is designed to ensure the dispersal of pollen rather than the fertilization of ovules. Honeybees, wasps and bumble bees all land on male flowers more often than would be expected by chance, and, having landed, wasps and bumble bees are more likely to enter a male flower than a female flower. The frequency of male flowers in the diet therefore exceeds their frequency in the population. This preference, although strong and consistent, is only partial, since some female flowers are included in the diet. We propose two hypotheses to account for the observed partial preference, the first based on competition between bees for flowers, and the second asserting that the bees detect nectar levels directly without using floral gender as a cue. The results of an experiment in which the most obvious gender cue, the androecium, was removed are consistent with the second hypothesis.
The process of adaptation occurs on two timescales. In the short term, natural selection merely s... more The process of adaptation occurs on two timescales. In the short term, natural selection merely sorts the variation already present in a population, whereas in the longer term genotypes quite different from any that were initially present evolve through the cumulation of new mutations. The first process is described by the mathematical theory of population genetics. However, this theory begins by defining a fixed set of genotypes and cannot provide a satisfactory analysis of the second process because it does not permit any genuinely new type to arise. The evolutionary outcome of selection acting on novel variation arising over long periods is therefore difficult to predict. The classical problem of this kind is whether 'replaying the tape of life' would invariably lead to the familiar organisms of the modern biota 1,2 . Here we study the long-term behaviour of populations of autonomously replicating computer programs and find that the same type, introduced into the same simple environment, evolves on any given occasion along a unique trajectory towards one of many well-adapted end points.
Uploads
Papers by Graham Bell