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PubMed2006

Host-parasitoid dynamics of a generalized Thompson model.

A discrete-time host-parasitoid model including host-density dependence and a generalized Thompson escape function is analyzed. This model assumes that parasitoids are egg-limited but not search-limited, and is proven to exhibit five types of dynamics: host failure in which the host goes extinct in the parasitoid's presence or absence, unconditional parasitoid failure in which the parasitoid always goes extinct while the host persists, conditional parasitoid failure in the host and the parasitoid go extinct or coexist depending on the initial host-parasitoid ratio, parasitoid driven extinction in which the parasitoid invariably drives the host to extinction, and coexistence in which the host and parasitoid coexist about a global attractor. The latter two dynamics only occur when the parasitoid's maximal rate of growth exceeds the host's maximal rate of growth. Moreover, coexistence requires parasitism events to be sufficiently aggregated. Small additive noise is proven to alter the dynamical outcomes in two ways. The addition of noise to parasitoid driven extinction results in random outbreaks of the host and parasitoid with varying intensity. Additive noise converts conditional parasitoid failure to unconditional parasitoid failure. Implications for classical biological control are discussed.

Animals↗

PubMed2000

Using the WWW to supply the molecular biology lab.

The World Wide Web is changing the face of business today and will continue to reshape it in the coming years. Nonvalue-added processes are being eliminated as organizations reengineer their internal ways of doing business in a continual effort to reduce costs and remain competitive. Current electronic market followers claim that gateways, or portals, of information will be the wave of the future. This does not mean that individual suppliers' websites will become extinct. It means that content will continue to change and adapt so that sites will continue to be more useful. If gateways do turn out to be the user's choice for product information presentation in the future, expect more consolidated sites devoted to specific product categories, such as chemicals, molecular biology supplies, and the like.

Commerce↗

PubMed2005

Evolutionary, biological origins of morality: implications for research with human embryonic stem cells.

Medical research with human embryonic stem cells, despite its enormous potential to reduce human suffering, is banned in many countries and heavily restricted in others. "Moral reasons" are invoked to justify bans and restrictions on this promising research. Rather surprisingly, while those moral reasons have been extensively discussed and hotly debated in several papers, not a single article on the moral aspects of that research has attempted to answer this fundamental question: What is morality? Considering that a scientifically objective definition of morality is essential to determine whether those moral reasons are justified or groundless, this article focuses on the evolutionary origins of morality and its biological basis. Morality arose as a selectively advantageous product of evolution and preceded all religions and philosophies by millions of years. For the 99% of humankind's evolution, morality was axiomatically aimed at reducing the sufferings of the social members, because pains and afflictions, as expressions of diseases and impairments, tended to hasten the extinction of the small ancestral groups, which characteristically consisted of a few tens of members. Had the therapeutic use of human embryos been available in remote times, our ancestors would have deemed it unquestionably immoral to save amorphous and microscopic agglomerates of insensitive cells representing neither parental nor social investment, at the expense of the lives of the suffering members of their little communities. Unless we venture the untenable thesis that the unlikelihood of extinction of our immense societies entitles us to overturn the meaning of morality, we cannot but conclude that bans and restrictions on research with human embryonic stem cells are patently immoral.

Altruism↗

PubMed2006

Evolutionary games and population dynamics: maintenance of cooperation in public goods games.

The emergence and abundance of cooperation in nature poses a tenacious and challenging puzzle to evolutionary biology. Cooperative behaviour seems to contradict Darwinian evolution because altruistic individuals increase the fitness of other members of the population at a cost to themselves. Thus, in the absence of supporting mechanisms, cooperation should decrease and vanish, as predicted by classical models for cooperation in evolutionary game theory, such as the Prisoner's Dilemma and public goods games. Traditional approaches to studying the problem of cooperation assume constant population sizes and thus neglect the ecology of the interacting individuals. Here, we incorporate ecological dynamics into evolutionary games and reveal a new mechanism for maintaining cooperation. In public goods games, cooperation can gain a foothold if the population density depends on the average population payoff. Decreasing population densities, due to defection leading to small payoffs, results in smaller interaction group sizes in which cooperation can be favoured. This feedback between ecological dynamics and game dynamics can generate stable coexistence of cooperators and defectors in public goods games. However, this mechanism fails for pairwise Prisoner's Dilemma interactions and the population is driven to extinction. Our model represents natural extension of replicator dynamics to populations of varying densities.

Animals↗

PubMed2003

Population genetic structure of the toad Bufo woodhousii: an empirical assessment of the effects of haplotype extinction on nested cladistic analysis.

Nested cladistic analysis (NCA) is increasingly being used to infer historical population-level processes, including population fragmentation, range expansion and long-distance colonization. However, the effects on interpretation of NCA inferences of stochastic extinction of haplotypes due to genetic drift (lineage sorting), or of haplotype loss via localized biotic or climatic influences, have not been thoroughly explored. We provide empirical evidence suggesting that NCA may misinterpret population history when haplotypes or haplotype groups from one clade are replaced by those of another clade. We do so by using NCA to analyse mitochondrial sequences from the toad Bufo woodhousii from 45 locations spanning the Great Plains and southwestern USA. Portions of this region were glaciated and/or desertified in the late Pleistocene and early Holocene, and hence uninhabitable for plains-dwelling organisms. Although NCA inferences of isolation-by-distance and gradual range expansion in B. woodhousii are compatible with expectations based on climatic data and toad biology, NCA also detected several instances of long-distance movement. Such movement seems unlikely, given the low vagility of this species. We conclude that inferences of long-distance colonization likely result from extinction of haplotypes in intervening areas. We suggest using additional methods to look for congruent inferences, and amending the NCA inference key, to help avoid misinterpretations resulting from haplotype extinction.

Animals↗

PubMed1993

A selectionist approach to reinforcement.

We describe a principle of reinforcement that draws upon experimental analyses of both behavior and the neurosciences. Some of the implications of this principle for the interpretation of behavior are explored using computer simulations of adaptive neural networks. The simulations indicate that a single reinforcement principle, implemented in a biologically plausible neural network, is competent to produce as its cumulative product networks that can mediate a substantial number of the phenomena generated by respondent and operant contingencies. These include acquisition, extinction, reacquisition, conditioned reinforcement, and stimulus-control phenomena such as blocking and stimulus discrimination. The characteristics of the environment-behavior relations selected by the action of reinforcement on the connectivity of the network are consistent with behavior-analytic formulations: Operants are not elicited but, instead, the network permits them to be guided by the environment. Moreover, the guidance of behavior is context dependent, with the pathways activated by a stimulus determined in part by what other stimuli are acting on the network at that moment. In keeping with a selectionist approach to complexity, the cumulative effects of relatively simple reinforcement processes give promise of simulating the complex behavior of living organisms when acting upon adaptive neural networks.

Behavior↗

PubMed1976

The premise of equipotentiality in human classical conditioning: conditioned electrodermal responses to potentially phobic stimuli.

The premise of equipotentiality, which has been widely adhered to among learning theorists, states that the laws of learning should not vary with the use of particular stimuli, responses, or reinforcements. This premise has recently been challenged by some data originating within the learning tradition itself, for example, studies on the effects of verbal stimuli in eye-lid conditioning. More importantly, however, the premise of equipotentiality is incompatible with data from experiments carried out within a biological-ethological framework. The results of such studies indicate that a given species is prepared to associate certain stimuli, responses, and reinforcers but not others. In an attempt to examine the validity of this premise in human classical conditioning, we investigated the effect of pictures of potentially phobic objects as conditioned stimuli (CSs) for electrodermal responses, since it has been suggested that phobias may be instances of biologically prepared learning. Three experiments are reported, all of them involving a long interstimulus interval differential conditioning paradigm with different pictures as CSs and electric shock as the unconditioned stimulus (UCS). In Experiment 1 we established that different pictures are differentially effective as CSs. A groupconditioned to potentially phobic stimuli, snakes or spiders, showed greater resistance to extinction than a group conditioned to fear-irrelevant pictorial stimuli, that is, flowers or mushrooms. A third group conditioned to "representative laboratory stimuli," circles or triangles, fell in between thses two groups. Experuce similar effects to those observed with phobic and fear-irrelevant stimuli in Experiment 1. In Experiment 3 superior resistance to extinction for phobic stimuli was demonstrated when the UCS was an electric shock, but not when it was a tone to which the subject produced reaction times. Thus, the effect appears specific for aversive UCSs, and CS-UCS "belongingness" has been demonstrated. It was concluded that our data do challenge the premise of equipotentiality in human conditioning. There are several learning-theory accounts that could accommodate at least some aspects of the data, but they seem to be best explained in terms of biologically orsiented constructs, such as preparedness.

Acoustic Stimulation↗

PubMed1979

Deterministic single-locus density-dependent selection.

Density-regulated selection is considered for a single, multiallele gene locus and separated generations. Characteristics resulting from the basic assumption that the average population fitness decreases with increasing density are derived. Under this assumption, it proves to be necessary to distinguish between regions of allelic frequencies which imply limited population growth, unlimited growth, or ultimate extinction when the population stays in the respective region. Particular attention is given to the investigation of the region of limited growth and the 'carrying capacity function' theta defined on it. Relationships between theta and the average fitness (adaptive surface) in the non-density dependent model are explained. Besides stability properties of equilibrium points, more general characteristics concerning the asymptotic behavior of population trajectories are treated. In this context, the problems of sudden loss of alleles and of population extinction as a result of large fluctuations in density are discussed.

Alleles↗

PubMed1994

Abnormal classical eye-blink conditioning in autism.

Cerebellar and limbic system pathologies have been reported in persons with autism. Because these brain areas are involved centrally in the acquisition and performance in classical eye-blink conditioning, this study evaluated conditioning in 11 persons with autism. Compared to matched controls, persons with autism learned the task faster but performed short-latency, high-amplitude conditioned responses. In addition, differences in learning the extinction rates systematically varied with age thus suggesting a developmental conditioning abnormality in autism. The observed pattern of eye-blink conditioning may indicate that persons with autism have the ability to rapidly associate paired stimuli but, depending on processing of certain contextual information, have impairments in modulating the timing and topography of the learned responses. This abnormality may relate to deviant cerebellar-hippocampal interactions. The classical eye-blink conditioning paradigm may provide a useful model for understanding the biological and behavioral bases of autism.

Adolescent↗

PubMed1990

Learning and unlearning fear: a clinical and evolutionary perspective.

Many fears, phobias and rituals seem to arise from prepared phylogenetic mechanisms which favor old over new evolutionary dangers and affect the rules of aversive learning which govern the acquisition of fear. Recent developments in several forms of aversive learning (sensitization, conditioning, extinction, observational learning) can improve them as paradigms of the acquisition, spread and maintenance of normal and clinical fears. The most reliable treatment for phobias and rituals is exposure, whose effects closely parallel the habituation of normal defensive responses and the extinction of conditioned fear and avoidance in animals. Habituation during exposure is usually slow and step by step, and generalizes little, but once attained tends to endure. Conditioned fear extinction and fear habituation have similar courses and may depend on similar neural processes. To be reduced, avoidance has to be prevented or the safety intervals that it heralds must be given up. Some phobias may result less from enhanced acquisition than from insufficient exposure to attain habituation. Finally, the review discusses the limits of habituation and the instability of fear extinction in relation to the long-term efficacy of exposure therapy.

Animals↗

PubMed2002

Simulating species loss following perturbation: assessing the effects on process rates.

We removed stream-living macroinvertebrate shredder species in the sequences in which they are predicted to disappear, in response to two common types of anthropogenic disturbances: acidification and organic pollution, and analysed the effects on leaf breakdown rates. The experiment was performed in field microcosms using three shredder species. Species identity significantly affected leaf breakdown rates, while species richness per se was non-significant. The simulated sequential species loss showed large effects on leaf breakdown rates, with observed rates being significantly higher than expected from single-species treatments in two, out of four, two-species, and in all four three-species treatments. The invertebrates used in this study were taxonomically distinct (Insecta: Plecoptera and Trichoptera; Crustacea: Amphipoda), and of different sizes, hence a high degree of complementarity was probably present. A method to study the effects of species loss, characteristic of perturbation type, could be more useful than a random approach when investigating the impact of perturbation. Our results may have general applicability for investigations on the effects of diversity loss on ecosystem functioning in any ecosystem exposed to human perturbations, given that the order of extinction is known or can easily be assessed.

Amphipoda↗

PubMed2006

The astronomical pulse of global extinction events.

The linkage between astronomical cycles and the periodicity of mass extinctions is reviewed and discussed. In particular, the apparent 26 million year cycle of global extinctions may be related to the motion of the solar system around the galaxy, especially perpendicular to the galactic plane. The potential relevance of Milankovitch cycles is also explored in the light of current evidence for the possible causes of extinction events over a geological timescale.

Animals↗

PubMed2006

Quantifying the extinction vortex.

We developed a database of 10 wild vertebrate populations whose declines to extinction were monitored over at least 12 years. We quantitatively characterized the final declines of these well-monitored populations and tested key theoretical predictions about the process of extinction, obtaining two primary results. First, we found evidence of logarithmic scaling of time-to-extinction as a function of population size for each of the 10 populations. Second, two lines of evidence suggested that these extinction-bound populations collectively exhibited dynamics akin to those theoretically proposed to occur in extinction vortices. Specifically, retrospective analyses suggested that a population size of n individuals within a decade of extinction was somehow less valuable to persistence than the same population size was earlier. Likewise, both year-to-year rates of decline and year-to-year variability increased as the time-to-extinction decreased. Together, these results provide key empirical insights into extinction dynamics, an important topic that has received extensive theoretical attention.

Animals↗

PubMed2004

Avian extinction and mammalian introductions on oceanic islands.

The arrival of humans on oceanic islands has precipitated a wave of extinctions among the islands' native birds. Nevertheless, the magnitude of this extinction event varies markedly between avifaunas. We show that the probability that a bird species has been extirpated from each of 220 oceanic islands is positively correlated with the number of exotic predatory mammal species established on those islands after European colonization and that the effect of these predators is greater on island endemic species. In contrast, the proportions of currently threatened species are independent of the numbers of exotic mammalian predator species, suggesting that the principal threat to island birds has changed through time as species susceptible to exotic predators have been driven extinct.

Animals↗

PubMed2006

Maximal yields from multispecies fisheries systems: rules for systems with multiple trophic levels.

Increasing centralization of the control of fisheries combined with increased knowledge of food-web relationships is likely to lead to attempts to maximize economic yield from entire food webs. With the exception of predator-prey systems, we lack any analysis of the nature of such yield-maximizing strategies. We use simple food-web models to investigate the nature of yield- or profit-maximizing exploitation of communities including two types of three-species food webs and a variety of six-species systems with as many as five trophic levels. These models show that, for most webs, relatively few species are harvested at equilibrium and that a significant fraction of the species is lost from the web. These extinctions occur for two reasons: (1) indirect effects due to harvesting of species that had positive effects on the extinct species, and (2) intentional eradication of species that are not themselves valuable, but have negative effects on more valuable species. In most cases, the yield-maximizing harvest involves taking only species from one trophic level. In no case was an unharvested top predator part of the yield-maximizing strategy. Analyses reveal that the existence of direct density dependence in consumers has a large effect on the nature of the optimal harvest policy, typically resulting in harvest of a larger number of species. A constraint that all species must be retained in the system (a "constraint of biodiversity conservation") usually increases the number of species and trophic levels harvested at the yield-maximizing policy. The reduction in total yield caused by such a constraint is modest for most food webs but can be over 90% in some cases. Independent harvesting of species within the web can also cause extinctions but is less likely to do so.

Animals↗

PubMed1988

[Characteristics of learning in ontogeny].

The rate of learning in young animals and in adults is different, it is higher in ontogenesis. The mechanism of learning in young animals is also different. In the ontogenesis it is similar to imprinting. Probably the mechanisms of the infantile amnesia develops according to the model of dissociated when the association is a certain chemical composition of brain which is typical of this age level. The information more important from the biological point of view is less dissipated and better restored that testifies the low effectiveness of reproductive system and not low memory trace consolidation. The difficulties in extinction and differentiation in experiments with rats are connected with reflex automatisation but not with the deficit in inhibition.

Aging↗

PubMed2001

Consequences of a biological invasion reveal the importance of mutualism for plant communities.

Seed-dispersal mutualisms have a fundamental role in regenerating natural communities. Interest in the importance of seed dispersal to plant communities has been heightened by worldwide declines in animal dispersers. One view, the 'keystone mutualist hypothesis', predicts that these human-caused losses will trigger a cascade of linked extinctions throughout the community. Implicitly, this view holds that mutualisms, such as seed dispersal, are crucial ecological interactions that maintain the structure and diversity of natural communities. Although many studies suggest the importance of mutualism, empirical evidence for community-level impacts of mutualists has remained anecdotal, and the central role of mutualism, relative to other species interactions, has long been debated in the theoretical literature. Here I report the community-level consequences of a biological invasion that disrupts important seed-dispersal mutualisms. I show that invasion of South African shrublands by the Argentine ant (Linepithema humile) leads to a shift in composition of the plant community, owing to a disproportionate reduction in the densities of large-seeded plants. This study suggests that the preservation of mutualistic interactions may be essential for maintaining natural communities.

Africa, Southern↗

PubMed1999

Evolutionary and preservational constraints on origins of biologic groups: divergence times of eutherian mammals.

Some molecular clock estimates of divergence times of taxonomic groups undergoing evolutionary radiation are much older than the groups' first observed fossil record. Mathematical models of branching evolution are used to estimate the maximal rate of fossil preservation consistent with a postulated missing history, given the sum of species durations implied by early origins under a range of species origination and extinction rates. The plausibility of postulated divergence times depends on origination, extinction, and preservation rates estimated from the fossil record. For eutherian mammals, this approach suggests that it is unlikely that many modern orders arose much earlier than their oldest fossil records.

Animals↗