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Showing papers in "Philosophical Transactions of the Royal Society B in 1994"


Journal ArticleDOI
TL;DR: The importance of using 'reference' sites to assess the true richness and composition of species assemblages, to measure ecologically significant ratios between unrelated taxa, toMeasure taxon/sub-taxon (hierarchical) ratios, and to 'calibrate' standardized sampling methods is discussed.
Abstract: Both the magnitude and the urgency of the task of assessing global biodiversity require that we make the most of what we know through the use of estimation and extrapolation. Likewise, future biodiversity inventories need to be designed around the use of effective sampling and estimation procedures, especially for 'hyperdiverse' groups of terrestrial organisms, such as arthropods, nematodes, fungi, and microorganisms. The challenge of estimating patterns of species richness from samples can be separated into (i) the problem of estimating local species richness, and (ii) the problem of estimating the distinctness, or complementarity, of species assemblages. These concepts apply on a wide range of spatial, temporal, and functional scales. Local richness can be estimated by extrapolating species accumulation curves, fitting parametric distributions of relative abundance, or using non-parametric techniques based on the distribution of individuals among species or of species among samples. We present several of these methods and examine their effectiveness for an example data set. We present a simple measure of complementarity, with some biogeographic examples, and outline the difficult problem of estimating complementarity from samples. Finally, we discuss the importance of using 'reference' sites (or sub-sites) to assess the true richness and composition of species assemblages, to measure ecologically significant ratios between unrelated taxa, to measure taxon/sub-taxon (hierarchical) ratios, and to 'calibrate' standardized sampling methods. This information can then be applied to the rapid, approximate assessment of species richness and faunal or floral composition at 'comparative' sites.

4,245 citations


Journal ArticleDOI
TL;DR: This work derives probability models for phylogenies reconstructed from contemporary taxa, allowing real data to be compared with specified null models of evolution, and lineage birth and death rates to be estimated.
Abstract: Phylogenies reconstructed from contemporary taxa do not contain information about lineages that have gone extinct. We derive probability models for such phylogenies, allowing real data to be compared with specified null models of evolution, and lineage birth and death rates to be estimated.

777 citations


Journal ArticleDOI
TL;DR: A coalescent approach to provide recursions for the probabilities of particular sample configurations is used, and a Monte Carlo method by which the solutions to such recursions can be approximated is described.
Abstract: We develop a sampling theory for genes sampled from a population evolving with deterministically varying size. We use a coalescent approach to provide recursions for the probabilities of particular sample configurations, and describe a Monte Carlo method by which the solutions to such recursions can be approximated. We focus on infinite-alleles, infinite-sites and finite-sites models. This approach may be used to find maximum likelihood estimates of parameters of genetic interest, and to test hypotheses about the varying environment. The methods are illustrated with data from the mitochondrial control region sampled from a North American Indian tribe.

575 citations


Journal ArticleDOI
TL;DR: Analysis of case studies suggests that plants often compensate for high risk in one of the three categories by low risk in another, and some systems, including elements of the Cape flora and lowland tropical rain forest, lack compensatory traits and the risk of plant extinction from failed mutualism is high.
Abstract: There is a voluminous literature on pollination and dispersal, very little of which deals with the consequences of reproductive failure and its most extreme consequence: extinction. The risk of plant extinctions can be assessed by considering the probability of dispersal or pollinator failure, reproductive dependence on the mutualism and demographic dependence on seeds. Traits for ranking species rapidly according to these three criteria are indicated. Analysis of case studies suggests that plants often compensate for high risk in one of the three categories by low risk in another. For example, self-incompatible plants with rare specialist pollinators often propagate vegetatively. Some systems, including elements of the Cape flora and lowland tropical rain forest, lack compensatory traits and the risk of plant extinction from failed mutualism is high. 'What escapes the eye, however, is a much more insidious kind of extincnction: the extinction of ecological interactions' Janzen (1974).

520 citations


Journal ArticleDOI
M. Bruce, A. Chree, I. McConnell, James Foster, G Pearson1, H Fraser 
TL;DR: The BSE agent has retained its identity when passaged through a range of species and the 'donor' species has little specific influence on disease characteristics in mice, adding to evidence for an agent-specific informational molecule.
Abstract: Transmissions of bovine spongiform encephalopathy (BSE) from seven unrelated cattle sources have given remarkably uniform disease characteristics in mice, differing from over twenty previous and contemporary transmissions of sheep and goat scrapie. Transmissions to mice of spongiform encephalopathy from six species (including sheep and goats) which have been experimentally or naturally infected with BSE have given similar results to direct BSE transmissions from cattle. Therefore the BSE agent has retained its identity when passaged through a range of species and the 'donor' species has little specific influence on disease characteristics in mice, adding to evidence for an agent-specific informational molecule. On transmission of BSE or scrapie to mice the incubation periods are long compared with subsequent mouse-to-mouse passages (the 'species barrier'). Contributing factors include a low efficiency of infection on interspecies transmission, the apparent failure of intracerebrally injected 'foreign' inoculum to establish infection directly in mouse brain and the selection of variant strains of agent which replicate most readily in the new host species.

486 citations


Journal ArticleDOI
TL;DR: This paper reviews the basic mathematical literature and three influential examples from the ecological literature are considered and placed within the general framework, which is shown to be a powerful one for the study of spatial ecological interactions.
Abstract: Spatial pattern, how it arises and how it is maintained, are central foci for ecological theory. In recent years, some attention has shifted from continuum models to spatially discrete analogues, which allow easy treatment of local stochastic effects and of non-local spatial influences. Many of these fall within the area of mathematics known as `interacting particle systems9, which provides a body of results that facilitate the interpretation of the suite of simulation models that have been considered, and point towards future analyses. In this paper we review the basic mathematical literature. Three influential examples from the ecological literature are considered and placed within the general framework, which is shown to be a powerful one for the study of spatial ecological interactions.

476 citations


Journal ArticleDOI
TL;DR: This article used molecular phylogenies to reject null models of the way we think evolution occurred, including patterns of lineage extinction, and they can also be used to provide maximum likelihood estimates of parameters associated with lineage birth and death rates.
Abstract: Molecular phylogenies can be used to reject null models of the way we think evolution occurred, including patterns of lineage extinction. They can also be used to provide maximum likelihood estimates of parameters associated with lineage birth and death rates. We illustrate: (i) how molecular phylogenies provide information about the extent to which particular clades are likely to be under threat from extinction; (ii) how cursory analyses of molecular phylogenies can lead to incorrect conclusions about the evolutionary processes that have been at work; and (iii) how different evolutionary processes leave distinctive marks on the structure of reconstructed phylogenies.

417 citations


Book ChapterDOI
TL;DR: It seems possible that the interaction between parasitic infection, nutrition and reproductive effort can be an important mechanism in the ultimate shaping of life-history variation in avian populations.
Abstract: Reproductive effort can have profound effects on subsequent performance. Field experiments on the collared flycatcher (Ficedula albicollis) have demonstrated a number of trade-offs between life-history traits at different ages. The mechanism by which reproductive effort is mediated into future reproductive performance remains obscure. Anti-parasite adaptations such as cell-mediated immunity may probably also be costly. Hence the possibility exists of a trade-off between reproductive effort and the ability to resist parasitic infection. Serological tests on unmanipulated collared flycatchers show that pre-breeding nutritional status correlates positively with reproductive success and negatively with susceptibility to parasitism (viruses, bacteria and protozoan parasites). Both immune response and several indicators of infectious disease correlate negatively with reproductive success. Similar relations are found between secondary sexual characters and infection parameters. For brood-size-manipulated birds there was a significant interaction between experimentally increased reproductive effort and parasitic infection rate with regard to both current and future fecundity. It seems possible that the interaction between parasitic infection, nutrition and reproductive effort can be an important mechanism in the ultimate shaping of life-history variation in avian populations.

405 citations


Journal ArticleDOI
TL;DR: The distribution of the immunoreactivity to nitric oxide synthase has been examined from rostral to caudal areas of the rat central nervous system using light microscopy.
Abstract: The distribution of the immunoreactivity to nitric oxide synthase has been examined from rostral to caudal areas of the rat central nervous system using light microscopy. Endogenous nitric oxide synthase was located using a specific polyclonal antiserum, produced against affinity purified nitric oxide synthase from whole rat brain, following the avidin-biotin peroxidase procedure. Immunoreactive cell bodies and processes showed a widespread distribution in the brain. In the telencephalon, immunoreactive structures were distributed in all areas of the cerebral cortex, the ventral endopiriform nucleus and claustrum, the main and accessory olfactory bulb, the anterior and posterior olfactory nuclei, the precommisural hippocampus, the taenia tecta, the nucleus accumbens, the stria terminalis, the caudate putamen, the olfactory tubercle and islands of Calleja, septum, globus pallidus and substantia innominata, hippocampus and amygdala. In the diencephalon, the immunoreactivity was largely found in both the hypothalamus and thalamus. In the hypothalamus, immunoreactive cell bodies were characteristically located in the perivascular-neurosecretory systems and mamillary bodies. In addition, immunoreactive nerve fibres were detected in the median eminence of the infundibular stem. The mesencephalon showed nitric oxide synthase immunoreactivity in the ventral tegmental area, the interpeduncular nucleus, the rostral linear nucleus of the raphe and the dorsal raphe nucleus. Immunoreactive structures were also found in the nuclei of the central grey, the peripeduncular nucleus and substantia nigra pars lateralis, the geniculate nucleus and in the superior and inferior colliculi. The pons displayed immunoreactive structures principally in the pedunculopontine and laterodorsal tegmental nuclei, the ventral tegmental nucleus, the reticulotegmental pontine nucleus, the parabrachial nucleus and locus coeruleus. In the medulla oblongata, immunoreactive neurons and processes were detected in the principal sensory trigeminal nucleus, the trapezoid body, the raphe magnus, the pontine reticular nuclei, the supragenual nucleus, the prepositus hypoglossal nucleus, the medial and spinal vestibular nuclei, the dorsal cochlear nucleus, the medullary reticular field, the nucleus of the solitary tract, the gracile and cuneate nuclei, the dorsal nucleus of the vagus nerve and the oral, interpolar and caudal parts of the spinal trigeminal nucleus. In the cerebellum, the stellate and basket cells showed immunoreactivity, which was also seen in the basket terminal fibres of the Purkinje cell layer. Isolated immunoreactive Purkinje cells were found in the vermis and parafloccular regions of the cerebellum. In the granular layer of the cerebellum, the granular cells and glomeruli were also immunoreactive. Numerous positive varicose nerve fibres and occasional neurons were also found in the lateral and interposed cerebellar nuclei.(ABSTRACT TRUNCATED AT 400 WORDS)

377 citations


Journal ArticleDOI
TL;DR: An index is suggested by which the results of selecting an indicator taxon can be more accurately communicated and based on the number of criteria that are successfully tested for the proposed indicator and their priority.
Abstract: Introducing greater objectivity to selection of indicator taxa produces results that are likely to reduce uncertainty, be more efficiently obtained and more clearly communicated. Seven criteria are presented that can be used to objectively test the claim that a given taxon is an ideal indicator: (i) well known and stable taxonomy; (ii) well known natural history; (iii) readily surveyed and manipulated; (iv) higher taxa broadly distributed geographically and over a breadth of habitat types; (v) lower taxa specialized and sensitive to habitat changes; (vi) patterns of biodiversity reflected in other related and unrelated taxa; and (vii) potential economic importance. These criteria have different priorities depending on which of two general categories of biodiversity the indicator taxon is to be used. Monitoring places an emphasis on sensitivity to habitat change, and inventory places an emphasis on systematics. An index is suggested by which the results of selecting an indicator taxon can be more accurately communicated. This index is based on the number of criteria that are successfully tested for the proposed indicator and their priority.

337 citations


Journal ArticleDOI
TL;DR: Work is described which identifies the molecular basis for differences in the survival potential of stem cells in the crypts of the colon and small intestine, and describes the attenuation of cell survival as a key component in strategies for the drug treatment of disseminated cancers.
Abstract: The major disseminated cancers remain stubbornly resistant to systemic therapy. Drug-resistant tumours include both slow and fast growing types, with the carcinomas constituting the major problem. Strategies for drug discovery have, in the past, been focused on attempts to design antiproliferative agents, largely targeted to interfere with DNA integrity and replication. The malignant phenotype might be characterized by the emergence of cell populations with a greater survival potential: a lower proclivity to undergo apoptosis. This idea provides a possible explanation of the genesis and progression of cancer and of the inherent resistance of tumour cells to engage apoptosis. Work is described which identifies the molecular basis for differences in the survival potential of stem cells in the crypts of the colon and small intestine. The advantageous survival of colonic stem cells, provided by expression of bcl-2 and a muted p53 response to DNA damage, allows damaged cells to survive. Continued expression of bcl-2 renders tumour cells resistant to drug-induced DNA damage by a mechanism different from classical mechanisms of drug resistance. The attenuation of cell survival is described as a key component in strategies for the drug treatment of disseminated cancers.

Journal ArticleDOI
TL;DR: An alternative published model for relating features to a particular form of phylogenetic pattern is considered, and shown to make unwarranted assumptions, and a possible alternative definition of the underlying units of diversity is examined.
Abstract: Biodiversity can be explored at a number of different levels and in principle may be separately quantified at each. Phylogenetic pattern has the potential to quantify and estimate biodiversity at the finest scale, that is, variation among species in features or attributes. This scale is an important one for conservation, as it should form the basis for prioritizing conservation efforts at the species level. Further, recent published objections to differentially weighting species are answered by defining option value at this feature-level. Unfortunately, there has been no consensus on exactly how phylogeny can be used to value species, possibly because proper consideration of the link between pattern and underlying features generally has been unresolved. `Phylogenetic diversity' (PD) represents just one of several approaches that do consider diversity at the feature-level explicitly. These alternative approaches are discussed in the context of a general framework for using pattern to quantify diversity at a level below that of the original objects. The pattern framework highlights that estimation of biodiversity at a lower level using pattern will require decisions about the nature of the units of diversity, the kind of pattern to be used, the model relating unit items to pattern, and finally how this implies a pattern-based measure reflecting biodiversity. An alternative published model for relating features to a particular form of phylogenetic pattern is considered, and shown to make unwarranted assumptions. A possible alternative definition of the underlying units of diversity is examined, which may represent a different form of option value, also quantifiable using phylogeny. A possible alternative pattern to a phylogenetic tree for the prediction of feature diversity is also discussed. The appeal of these alternative approaches depends on the goals of conservation; in addition, justification for prioritizing or weighting requires that any practical approach avoid arbitrary, unwarranted, assumptions.

Journal ArticleDOI
TL;DR: Approaches to extinction analysis and prediction based on morphological variety or biodisparity should be explored as an adjunct or alternative to taxon inventories or phylogenetic metrics.
Abstract: Direct comparison of ancient extinctions to the present-day situation is difficult, because quantitative palaeontological data come primarily from marine invertebrates, fossilized species are usually drawn from the more abundant and widespread taxa, and time resolution is rarely better than 10 $^{3}$ -10 $^{4}$ years. A growing array of techniques permits quantitative error estimates on some of these potential biases, and allows calculation of species extinction intensities from genus-level data, which are more robust. Extensive as today's species losses probably are, they have yet to equal any of the Big Five mass extinctions. Background extinction patterns are potential sources of insight regarding present-day biotic losses; over 90% of past species extinction has occurred at times other than the Big Five mass extinctions. Mean durations of fossil species vary by more than an order of magnitude even within clades, rendering uninformative any global average for background extinction. Taxon-specific variation is evidently related to intrinsic biotic factors such as geographic range and population size. Approaches to extinction analysis and prediction based on morphological variety or biodisparity should be explored as an adjunct or alternative to taxon inventories or phylogenetic metrics. Rebounds from mass extinctions are geologically rapid but ecologically slow; biodiversity recovery and the re-establishment of some communities typically requires 5-10 million years.

Journal ArticleDOI
TL;DR: Phenomena reviewed include a positive correlation between local abundance and size of geographic range, putting some populations in ‘double jeopardy’; smaller average range sizes in the tropics; metapopulation dynamic processes that can spread risk, but only if the spatial correlation between fluctuations of conspecific populations is low; and phylogenetic constraints on range sizes and abundance.
Abstract: Although the causes of population extinction, locally and globally, are many, varied and idiosyncratic when individual species are considered, there are nevertheless some basic general constraints and ground rules that make some species more extinction-prone than others. This paper reviews their implications for the ultimate and proximate causes of extinction. Phenomena reviewed include: a positive correlation between local abundance and size of geographic range, putting some populations in ‘double jeopardy’; smaller average range sizes in the tropics; metapopulation dynamic processes that can spread risk, but only if the spatial correlation between fluctuations of conspecific populations is low; the influence of body size and trophic position; and phylogenetic constraints on range sizes and abundance.

Journal ArticleDOI
TL;DR: Granulocyte apoptosis, with its potential for modulation by external mediators, is likely to play a key dynamic role in the control of the 'tissue load' of granulocytes at inflamed sites.
Abstract: We have described a novel pathway available for the clearance of extravasated granulocytes from inflamed tissues whereby aging granulocytes undergo apoptosis, a process which leads to their phagocytosis by inflammatory macrophages. By contrast with necrosis, which may also be seen at inflamed sites, apoptosis represents a granulocyte fate which by a number of mechanisms would tend to limit inflammatory tissue injury and promote resolution rather than progression of inflammation: (i) apoptosis is responsible for macrophage recognition of senescent neutrophils with intact cell membranes which exclude vital dyes and retain their potentially histotoxic granule contents; (ii) the apoptotic neutrophil loses its ability to secrete granule enzymes on deliberate external stimulation; (iii) the macrophage possesses a huge phagocytic capacity for apoptotic neutrophils which it rapidly ingests and degrades without disgorging neutrophil contents; and (iv) the macrophage utilizes a novel phagocytic recognition mechanism which fails to trigger the release of pro-inflammatory macrophage mediators during the phagocytosis of apoptotic neutrophils. Preliminary characterization of the recognition mechanism implicates the integrin α v β3 (vitronectin receptor) and CD36 (thrombospondin receptor) on the macrophage surface. Macrophage phagocytosis of apoptotic neutrophils is greatly influenced by the microenvironmental pH and by the presence of cationic molecules. Moreover, it can be specifically modulated by external cytokines and intracellular second messenger systems. By controlling the functional longevity of neutrophil and eosinophil granulocytes and their subsequent removal by macrophages, granulocyte apoptosis, with its potential for modulation by external mediators, is likely to play a key dynamic role in the control of the ‘tissue load’ of granulocytes at inflamed sites. Further elucidation of the mechanisms and control of apoptosis in granulocytes is likely to shed new light on the pathophysiology of inflammation and suggest new approaches to the therapy of inflammatory diseases.

Journal ArticleDOI
TL;DR: The evening emergence time among echolocating bats appears to be a function of dietary specializations and foraging strategy, and is probably also affected by the ability to avoid predation.
Abstract: We hypothesized that interspecific differences in evening emergence time among echolocating bats are subject to natural selection through effects of variation in food availability and predation risk, both of which are related to flight technique and foraging strategy. We predicted that bats that feed on small aerial insects emerge relatively early to get access to the peak in flight activity of small dipterans at dusk. By emerging well before dark, however, they expose themselves to increased risks of predation and/or harassment from raptorial or insectivorous birds which may still be active. Bats that can feed independently of the dusk peak of dipterans, i.e. those that are adapted to feed on moths, on flightless or diurnal prey or on plants, would be expected to emerge later, thus minimizing the predation risk. We tested these predictions by analysis of two data bases: one including European bats only and another including a worldwide sample. The predictions were largely supported. The evening emergence time appears to be a function of dietary specializations and foraging strategy, and is probably also affected by the ability to avoid predation.

Journal ArticleDOI
TL;DR: Analysis of patterns of diversity on land and in the sea at higher taxonomic levels and on the possible causes and consequences of these patterns finds some estimates of impending extinction rates.
Abstract: This paper begins with a brief survey of the numbers of species that have been named and recorded on land and in the sea. It then assesses how many species there may be in total, giving particular attention to recent suggestions for dramatic upward revision of species totals for tropical invertebrates, fungi, benthic macrofauna, and microorganisms in general. Against this background, I comment on patterns of diversity on land and in the sea at higher taxonomic levels - orders through phyla - and on the possible causes and consequences of these patterns. I conclude with some estimates of impending extinction rates.

Journal ArticleDOI
TL;DR: In this article, the Ioannina 249 sedimentary core was used to study the vegetation history of the western part of Greece through a series of glacial-interglacial Qaternary cycles.
Abstract: A core recovered from a thick sedimentary sequence in the Ioannina basin, on the western flank of the Pindus M ountain Range, northwest Greece, presents the opportunity to observe multiple changes in vegetational communities at one locality through a series of glacial-interglacial Q uaternary cycles. The Ioannina 249 record adds to the knowledge of vegetation history of areas of increased topographical variability and precipitation of the western Balkans and provides a complete stratigraphical record that can be compared with that of other long terrestrial sequences and with the marine record. Pollen analytical results are presented as percentages and concentrations, the former providing information on the composition and structure of vegetation, while the latter is considered here to be a reliable indication of vegetation density when changes differing by an order of magnitude are documented. The record shows an hierarchical order of variation in the response of vegetation to environmental change. Higher order of magnitude changes are alternations between forest and open vegetation communities, a reflection of major climatic shifts from interglacial to glacial modes. Superimposed on these oscillations is a lower order variability associated with vegetation changes within interglacial and glacial periods. During forest periods a succession is recorded with Quercus and Ulmus/Zelkova expanding early, followed by Carpinus betulus and also Ostrya carpinifolia/Carpinus orientalis , and finally Abies often accompanied by Fagus . Although individual periods may be characterized by dominance of one or more taxa, the underlying pattern of differential expansion is usually distinct and consistent. Nine forested intervals are distinguished and are assigned local names to facilitate longdistance comparisons and correlations. During open vegetation periods a series of changes is also observed from transitional steppe—forest or forest-steppe vegetation, through grassland steppe communities, culminating in a discontinuous desert-steppe vegetation. In addition to the two ends of the spectrum (forest and desert—steppe), attention is drawn to the intermediate phases representing ‘average’ Quaternary conditions. The Ioannina record is correlated with that of other long sequences from Europe and variation in the response of vegetation with site characteristics is considered. A strategy for long-distance correlations relying on the primary structure of vegetation and relative stratigraphical position of individual periods is described. The last interglacial period followed by two interstadials is recorded in much the same fashion in all records. Correlation of earlier periods was also in general agreement although only two continuous records that extend beyond the last interglacial are at present available for comparison. To minimize elements of circularity, similarities in the behaviour of individual taxa during particular periods are not part of the correlation criteria so that if their chronostratigraphical equivalence is independently corroborated their significance can be examined. On this basis, the importance of Carpinus betulus and the almost complete absence of Fagus on a subcontinental scale during the last interglacial are noted. Possible effects of climate, competition and disease are discussed. Cross-correlation with the deepsea oxygen isotope record provides a tentative chronology for the Ioannina record. Based on this, the sequence down to a depth of 162.75 m is considered to represent a record of approximately the past 423 000 years. Aspects of land-sea correlations are discussed in the light of the Ioannina 249 record and the importance of long sequences in the development of European Quaternary stratigraphy is emphasized.

Journal ArticleDOI
TL;DR: Two issues are discussed: that the frontal eye of amphioxus is sufficiently similar in structure and organization to the paired eyes of vertebrates to indicate homology; and that its position at the extreme anterior end of the cord, together with structural and histochemical evidence, suggests it may derive from the apical organ.
Abstract: The anterior end of the dorsal nerve cord of amphioxus is described at the 3-4 gill slit stage based on serial transmission electron microscopy and three-dimensional reconstruction, with special attention to structures that are potential landmarks for comparing amphioxus with other chordates. The larval nerve cord is divisible, at approximately the level of the first somite, into a short anterior region, the cerebral vesicle (c.v.), and an extended posterior region that is thought to include homologues of the vertebrate hindbrain and spinal cord. The c.v., in turn, has an anterior part with a tubular neural canal and a posterior part with a keyhole-shaped neural canal similar to that found in the rest of the cord. The junction between these two parts of the c.v. is marked by a cluster of infundibular cells. The anterior c.v., whose cells have cilia that point anteriorly, includes (i) a structure we call the frontal eye, consisting of a pigment spot and transverse rows of putative receptor and nerve cells, and (ii) several small ventral commissures bridging the major nerve tracts that run ventrolaterally along either side of the nerve cord. The posterior c.v., in contrast, contains cells whose cilia point posteriorly, and includes (i) the beginnings of the floorplate, which continues posteriorly through the rest of the nerve cord, (ii) the dorsal lamellar body, made up of cells with cilia that expand into flattened lamellae, and (iii) a large ventral commissure that incorporates fibres arising from cells of the lamellar body. Where probable homologues of c.v. structures can be identified in vertebrate brain, they are found in the diencephalon, which suggests the c.v. and the vertebrate diencephalon are, to a degree, homologous. On structural evidence, the frontal eye and the lamellar body are both ciliary photoreceptors, contrasting with the microvillar ocelli (organs of Hesse) distributed along most of the rest of the nerve cord. In young larvae the lamellar body is large and conspicuous. We suggest it functions as a high sensitivity, non-directional photoreceptor. The frontal eye is smaller, but organizationally more complex. We suggest it functions as a low sensitivity directional photoreceptor. The frontal eye is potentially of considerable interest from an evolutionary standpoint. Two issues are discussed: (i) that it is sufficiently similar in structure and organization to the paired eyes of vertebrates to indicate homology; and (ii) that its position at the extreme anterior end of the cord, together with structural and histochemical evidence, suggests it may derive from the apical organ, an evolutionarily ancient structure marking the embryonic anterior pole in diverse invertebrate phyla, whose homologue in chordates has hitherto not been identified.

Journal ArticleDOI
TL;DR: A framework for understanding multiplequeening in leptothoracines almost certainly results from high costs to single queens of dispersal and colony foundation (high ecological constraints), which select for queens to seek adoption in their natal colony.
Abstract: Multiple-queen societies of ants are key subjects in the study of communal breeding. Societies of leptothoracine ants may be obligately monogynous (contain a single reproductive queen), functionally monogynous (only one of several mated queens lays eggs), or facultatively polygynous (some colonies contain more than one egg-laying, mated queen). This paper presents a framework for understanding these diverse social systems as a function of leptothoracine ecology. The framework is derived from a synthesis of empirical information - in particular, a link between the social system and the degree of habitat patchiness - with three bodies of theory. These are ecological constraints theory, ESS (evolutionarily stable strategy) models of dispersal, and kin selection models predicting the stable reproductive skew (allocation of reproduction). In contrast to several previous ecological hypotheses, multiplequeening in leptothoracines almost certainly results from high costs to single queens of dispersal and colony foundation (high ecological constraints), which select for queens to seek adoption in their natal colony. Factors raising these costs include nest-site limitation, cold climate, and habitat patchiness. ESS models suggest that high dispersal costs lead to a larger stable fraction of non-dispersers and hence to higher relatedness between queens. Skew models predict that high ecological constraints and high relatedness promote high skew (one or a few individuals dominate reproduction) and high within-colony aggression. Therefore, (i) extensive habitats with moderate costs of solitary colony foundation should promote multiple-queening with high dispersal levels, moderate queen relatedness, low reproductive skew, and low queen aggression. By contrast, (ii) patchy habitats should induce multiple-queening with less dispersal, higher queen relatedness, higher skew, and higher aggression. In addition, (iii) habitats with small or widely spaced nest-sites, or with low costs of founding colonies alone, should lead to universal dispersal without multiple-queening. These associations of traits occur in facultatively polygynous, functionally monogynous and obligately monogynous leptothoracines respectively. Therefore, the framework in this paper explains a substantial amount of the social and ecological diversity of leptothoracine ants.

Journal ArticleDOI
TL;DR: The principle of employing simple ratios for extrapolation is also applicable to other speciose groups and other settings, and the importance of appropriate rigour in assembling the species datasets that form the basis of estimates is stressed.
Abstract: The overall dimensions of global species richness remain very imprecisely known and the manner in which this richness is distributed only sketchily understood. This lamentable state of affairs is largely due to an inadequate appreciation of the contributions made by the most speciose groups. The most reliable, practical and cost-effective means of documenting patterns and estimating species richness in these groups is the use of a piecemeal, step by step, approach, eschewing the use of first principles, empirical relationships that are not directly amenable to calibration, diversity indices and \`short cuts' that take no account of the effects of scale. Instead, simple ratios of species richness from taxon to taxon, focal group to more inclusive group, site to site, sample to inventory, and across spatial scales provide a basis for extrapolation. Essential features of this approach are the calibration of ratios, ensuring that like is compared with like, and the fullest use of \`hands on' knowledge of the groups in question and the settings in which they are found. The choice and use of focal groups for extrapolation to larger groups and the choice and use of sampling methods to obtain reliable sample data from which to extrapolate to site inventories are considered in some detail. The way that the interplay between patchy distributions, method of sampling and sample `dimensions' influences the reliability and precision of estimates is also discussed. The importance of appropriate rigour in assembling the species datasets that form the basis of estimates, including care in the choice and use of sampling regimes and accuracy in species recognition and sorting, is stressed. Although species richness patterns in terrestrial arthropods are used here as examples, the principle of employing simple ratios for extrapolation is also applicable to other speciose groups and other settings.

Journal ArticleDOI
TL;DR: The development of phonological awareness is related to the acquisition of literacy, which, in turn, brings about further refinements in phonological skills, which affect a variety of other skills.
Abstract: In this paper we consider the nature and consequences of the development of phonological skills in children. We begin with evidence for developmental refinements in phonological processes. These developments, in turn, affect a variety of other skills. We consider two particular examples: the relationship between the development of speech skills and verbal short-term memory and the development of children's phonological awareness. The development of phonological awareness is related to the acquisition of literacy, which, in turn, brings about further refinements in phonological skills.

Journal ArticleDOI
TL;DR: The amount of normal cell death in mammalian development is still underestimated and the possibility that the death programme, like mitosis, is orchestrated by a cytosolic regulator that acts on multiple organelles in parallel is raised.
Abstract: We draw the following tentative conclusions from our studies on programmed cell death (PCD): (i) the amount of normal cell death in mammalian development is still underestimated; (ii) most mammalian cells constitutively express the proteins required to undergo PCD; (iii) the death programme operates by default when a mammalian cell is deprived of signals from other cells; (iv) many normal cell deaths may occur because cells fail to obtain the extracellular signals they need to suppress the death programme; and (v) neither the nucleus nor mitochondrial respiration is required for PCD (or Bcl-2 protection from PCD), raising the possibility that the death programme, like mitosis, is orchestrated by a cytosolic regulator that acts on multiple organelles in parallel.

Journal ArticleDOI
TL;DR: In this article, a review of the different kinds of temporal variability, the different ways in which it can be measured, the design of appropriate sampling schemes, methods of analysing variability, and patterns in temporal variability are presented.
Abstract: From first principles, the temporal variability of a time series of abundances can be defined as the average deviation of values from a mean value on a proportional scale. In this paper we review: (i) the different kinds of temporal variability; (ii) the different ways in which it can be measured; (iii) the design of appropriate sampling schemes; (iv) methods of analysing variability; and (v) patterns in temporal variability. We emphasize that some commonly applied measures are not appropriate, that several do not measure the desired feature of time series, and the importance of considerations of trend and sampling error. A number of suggestions are made for the improvement of the basis for comparative analyses of levels of variability, and some of the potential pitfalls are identified. Given the serious faults in many previous analyses of ecological patterns in the temporal variability of animal abundances, emphasis is laid on the theoretical basis for different patterns, and hence a set of hypotheses for testing is generated.

Journal ArticleDOI
TL;DR: The mathematical formulation is used to analyse a non-reproductive form of evolution in which various learning rules compete and evolve and is given a very tentative economic application which has interesting ESAS and phenotype dynamics.
Abstract: We extend the ideas of evolutionary dynamics and stability to a very broad class of biological and other dynamical systems. We simultaneously develop the general mathematical theory and a discussion of some illustrative examples. After developing an appropriate formulation for the dynamics, we define the notion of an evolutionary stable attractor (ESA) and give some samples of ESAS with simple and complex dynamics. We discuss the relationship between our theory and that for ESSS in classical linear evolutionary game theory by considering some dynamical extensions. We then introduce and develop our main mathematical tool, the invasion exponent. This allows analytical and numerical analysis of relatively complex situations, such as the coevolution of multiple species with chaotic population dynamics. Using this, we introduce the notion of differential selective pressure which for generic systems is nonlinear and characterizes internal ESAS. We use this to analytically determine the ESAS in our previous examples. Then we introduce the phenotype dynamics which describe how a population with a distribution of phenotypes changes in time with or without mutations. We discuss the relation between the asymptotic states of this and the ESAS. Finally, we use our mathematical formulation to analyse a non-reproductive form of evolution in which various learning rules compete and evolve. We give a very tentative economic application which has interesting ESAS and phenotype dynamics.

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TL;DR: Constancy of species and communities of species can be demonstrated to be the norm for at least the last million or so years (= generations), and the enigma of how such constancy was sustained in the face of large-scale climatic fluctuations remains a puzzle.
Abstract: The extinction of species of small invertebrates is difficult to recognize. However, in deposits that date from the past few million years, insect fossils are remarkably common and provide objective data on the history of the organisms that constitute the biotic communities of the present day. It might have been expected that the great climatic oscillations of the glacial-interglacial cycles should have caused widespread extinctions, if their effects on the large vertebrates is taken as our model. Yet the record of Quaternary fossil insects shows no high extinction rates during this period. Constancy of species and communities of species can be demonstrated to be the norm for at least the last million or so years (= generations). The enigma of how such constancy was sustained in the face of large-scale climatic fluctuations remains a puzzle though several possible solutions are suggested. These solutions carry implications for our estimates of present and future extinction rates.

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TL;DR: It is concluded that accurate assessment of the taxon component of biodiversity of the neotropics must be based at the species level rather than at that of higher taxa, and that species data are a valuable tool for conservation planning.
Abstract: The neotropical region as a whole is much more species diverse than the palaeotropics but no more diverse at the family and only slightly at the generic level. Only 6.4% of neotropical species belong to plant families that are confined to the region and there is a greater generic diversity in Africa than in the neotropics indicating that species are much better indicators of total diversity. The striking difference in species make up between the northern Andean region and Amazonia is important to recognize in conservation planning. In the northern Andean region, herbs and shrubs with Andean-centred distributions predominate whereas in Amazonia, trees and shrubs with Amazonian-centred distributions predominate. The analysis of species distributions show both centres of endemism and centres of diversity which are not apparent when only higher taxa are considered. The local demography of species varies considerably within the lowland rainforest regions and this is discussed using data from quantitative inventories. It is concluded that accurate assessment of the taxon component of biodiversity of the neotropics must be based at the species level rather than at that of higher taxa, and that species data are a valuable tool for conservation planning.

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TL;DR: It is suggested that inbreeding avoidance may be the most important function of MHC-based mating preferences and therefore the fundamental selective force diversifying MHC genes in species with such mating patterns.
Abstract: In house mice, and probably most mammals, major histocompatibility complex (MHC) gene products influence both immune recognition and individual odours in an allele-specific fashion. Although it is generally assumed that some form of pathogen-driven balancing selection is responsible for the unprecedented genetic diversity of MHC genes, the MHC-based mating preferences observed in house mice are sufficient to account for the genetic diversity of MHC genes found in this and other vertebrates. These MHC disassortative mating preferences are completely consistent with the conventional view that pathogen-driven MHC heterozygote advantage operates on MHC genes. This is because such matings preferentially produce MHC-heterozygours progeny, which could enjoy enhanced disease resistance. However, such matings could also function to avoid genome-wide inbreeding. To discriminate between these two hypotheses we measured the fitness consequences of both experimentally manipulated levels of inbreeding and MHC homozygosity and heterozygosity in semi-natural populations of wild-derived house mice. We were able to measure a fitness decline associated with inbreeding, but were unable to detect fitness declines associated with MHC homozygosity. These data suggest that inbreeding avoidance may be the most important function of MHC-based mating preferences and therefore the fundamental selective force diversifying MHC genes in species with such mating patterns. Although controversial, this conclusion is consistent with the majority of the data from the inbreeding and immunological literature.

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TL;DR: Phylogenetic analysis by several methods confirms earlier suggestions based on ultrastructure that thraustochytrids constitute a deeply divergent branch of the phylum Heterokonta, which is currently classified in the kingdom Chromista.
Abstract: Thraustochytrids are a neglected group of non-photosynthetic marine protists characterized by the presence of a sagenogenetosome, an ectoplasmic net and a cell wall composed of non-cellulosic scales. Though originally classified as fungi, they are ultrastructurally closest to the labyrinthulids, which have often been treated as protozoa, but are now usually considered to be more closely related to the heterokont algae. In order to clarify their controversial taxonomic position and evolutionary relationships, we have cloned and sequenced the 18s rRNA genes of the thraustochytrids Thraustochytrium kinnei and Ulkenia profunda. Phylogenetic analysis by several methods confirms earlier suggestions based on ultrastructure that thraustochytrids constitute a deeply divergent branch of the phylum Heterokonta, which is currently classified in the kingdom Chromista. The phylum Heterokonta also includes numerous chromophyte algae (e.g. brown algae and diatoms) as well as the oomycetes. Heterokont 18s rRNA sequences can be distinguished from those of all other eukaryotes by having an AU not a UA base pair at the very base of helix 47 in the V9 region. We also tabulate signature sequences that can be used to characterize 18s rRNA sequences from each of the major heterokont taxa.

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TL;DR: To what extent numbers of species are an adequate measure of biological diversity, either locally or globally, is asked; both for evolutionary understanding and for practical applications, biodiversity may often be better quantified at lower or higher levels.
Abstract: This paper begins by asking to what extent numbers of species are an adequate measure of biological diversity, either locally or globally; both for evolutionary understanding and for practical applications, biodiversity may often be better quantified at lower or higher levels, from genes to ecosystems. The subsequent discussion, however, focuses on species, and discusses questions that arise in estimating how many species there have ever been, how many there currently are in various taxonomic groups, and how we may quantify the differing degrees of 'independent evolutionary history' or 'taxonomic distinctiveness' in different species or groups. I conclude with opinions about how the practical task of identifying and recording species diversity might be better managed.