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Explaining macroevolution from microevolution is a key issue in contemporary evolutionary theory. A recurrent macroevolutionary pattern is that some niche‐related traits consistently evolve slower than others, so called niche conservatism. Despite a growing amount of data, the underlying evolutionary processes are not fully understood. I here analyse adaptive radiations in an individual‐based eco‐evolutionary model. I find a coevolutionary mechanism – evolutionary niche monopolisation – as a possibly important generator of niche conservatism. A single lineage of a radiating clade can monopolise, and later diversify within, a substantial part of the available niche space – much larger than what can be explained by limiting similarity. This leads to niche conservatism, since no species evolves into or out of the monopolised region. The region can in this sense also be described as an adaptive zone. The model indicates that evolutionary niche monopolisation is operative in a large part of parameter space, underlining its possible importance. The mechanism is driven by competitive interactions and differences in niche widths in alternative niche dimensions. I discuss plausible examples of evolutionary niche monopolisation in well‐studied natural systems. [ABSTRACT FROM AUTHOR]

In organisms with complex life cycles living in seasonal environments, the synchronisation of phenological events is important from the ecological and evolutionary perspectives. Life history transitions should be synchronised to a greater degree at northern latitudes. We quantified hatching and emergence timing and synchrony in the obligate univoltine damselfly Lestes sponsa along a latitudinal gradient covering its entire north-south range in Europe. In our first experiment, populations from different latitudes were grown in separate climate chambers simulating temperature and photoperiod conditions occurring at their sites of origin. Northern populations expressed early and high synchronous hatching and emergence, central populations intermediate, and southern populations late and low synchronous hatching and emergence. This pattern was expressed at both population and full-sibling family levels, indicating stronger selection for timing and synchronisation in the north compared to the south. In our second experiment, populations from all latitudes were reared in conditions simulating an average temperature and photoperiod over the latitudinal gradient. Interestingly, the pattern of timing and synchronisation was reversed with respect to latitude when compared to the pattern shown in the first experiment, indicating the importance of environmental factors in shaping phenological events. Our results indicate strong selection for timing and synchronisation of life history events at northern latitudes, caused by time constraints. Our results also show that it is important to use as natural conditions as possible in experiments on life history shifts in organisms with complex life cycles in order to achieve a correct understanding of these shifts. [ABSTRACT FROM AUTHOR]

In nature species richness and composition, as well as the functioning of individual species, all covary along environmental gradients, making it difficult to tease apart their effects on ecosystem function. Here we use a novel extension of the Price equation to partition the causes of functional variation between any two sites sharing at least one species in common. We use the extension to separate effects of species loss from those of species gain; species gain is analogous to migration in evolution. Previous theoretical and empirical studies of biodiversity and ecosystem function fail to distinguish effects of species gain from those of species loss, and so are conceptually incomplete. Application of this approach to data on total plant biomass along an experimental N enrichment gradient leads to novel empirical insights and reveals subtle effects. For instance, effects of species gain are non-negligible even though enrichment leads to loss of many species and gain of few, and non-random gain of high-biomass species reduces the biomass of the persisting species. We also discuss the interpretation of this new approach, which provides a highly-general partitioning of the factors affecting ecosystem function. [ABSTRACT FROM AUTHOR]

Identifying factors which allow the evolution and persistence of cooperative interactions between species is a fundamental issue in evolutionary ecology. Various hypotheses have been suggested which generally focus on mechanisms that allow cooperative genotypes in different species to maintain interactions over space and time. Here, we emphasise the fact that even within mutualisms (interactions with net positive fitness effects for both partners), there may still be inherent costs, such as the occasional predation by ants upon aphids. Individuals engaged in mutualisms benefit from minimising these costs as long as it is not at the expense of breaking the interspecific interaction, which offers a net positive benefit. The most common and obvious defence traits to minimise interspecific interaction costs are resistance traits, which act to reduce encounter rate between two organisms. Tolerance traits, in contrast, minimise fitness costs to the actor, but without reducing encounter rate. Given that, by definition, it is beneficial to remain in mutualistic interactions, the only viable traits to minimise costs are tolerance-based ‘defence’ strategies. Thus, we propose that tolerance traits are an important factor promoting stability in mutualisms. Furthermore, because resistance traits tend to propagate coevolutionary arms races between antagonists, whilst tolerance traits do not, we also suggest that tolerance-based defence strategies may be important in facilitating the transition from antagonistic interactions into mutualisms. For example, the mutualism between ants and aphids has been suggested to have evolved from parasitism. We describe how phenotypic plasticity in honeydew production may be a tolerance trait that has prevented escalation into an antagonistic arms race and instead led to mutualistic coevolution. [ABSTRACT FROM AUTHOR]

Individuals who reproduce asexually have a two-fold advantage over their sexually-reproducing counterparts as they are able to reproduce twice as fast. Explaining why sexual reproduction is favoured over asexual reproduction therefore remains an important challenge in evolutionary biology. Various mechanisms involving resistance to parasites, adaptation to novel environments and helping to purge the genome of deleterious mutations have all been proposed as potential mechanisms which could promote the evolution of sex. A recent article has suggested that spiteful males may help to reduce the two-fold advantage of asexual females. Here I discuss this idea, and further ask whether punishment of asexual females by sexual females could be one way in which sexual reproduction could be maintained in groups of animals; in light of recent research on the repression of competition, it could be possible that asexual females which reproduce faster than their sexual counterparts will be punished for using group resources. It may therefore be possible that the behaviour of sexual individuals towards asexual females could have fitness consequences which could potentially reduce the two-fold advantage they gain from reproducing parthenogenetically. [ABSTRACT FROM AUTHOR]

We report data on leaf litter production and decomposition from a manipulative biodiversity experiment with trees in tropical Panama, which has been designed to explore the relationship between tree diversity and ecosystem functioning. A total of 24 plots (2025 m2) were established in 2001 using six native tree species, with 1-, 3-, and 6-species mixtures. We estimated litter production during the dry season 2005 with litter traps; decomposition was assessed with a litter bag approach during the following wet season. Litter production during the course of the dry season was highly variable among the tree species. Tree diversity significantly affected litter production, and the majority of the intermediate diverse mixtures had higher litter yields than expected based on yields in monoculture. In contrast, high diverse mixtures did not show such overyielding in litter production. Litter decomposition rates were also highly species-specific, and were related to various measures of litter quality (C/N, lignin/N, fibre content). We found no overall effect of litter diversity if the entire litter mixtures were analyzed, i.e. mixing species resulted in pure additive effects and observed decomposition rates were not different from expected rates. However, the individual species changed their decomposition pattern depending on the diversity of the litter mixture, i.e. there were species-specific responses to mixing litter. The analysis of temporal C and N dynamics within litter mixtures gave only limited evidence for nutrient transfer among litters of different quality. At this early stage of our tree diversity experiment, there are no coherent and general effects of tree species richness on both litter production and decomposition. Within the scope of the biodiversity-ecosystem functioning relationship, our results therefore highlight the process-specific effects diversity may have. Additionally, species-specific effects on ecosystem processes and their temporal dynamics are important, but such effects may change along the gradient of tree diversity. [ABSTRACT FROM AUTHOR]

Species diversity is thought to stabilize functioning of plant communities. An alternative view is that stability depends more on dynamics of dominant species than on diversity. We compared inter-annual variability (inverse of stability) of aboveground biomass in paired restored and remnant tallgrass prairies at two locations in central Texas, USA. Data from these two locations were used to test the hypothesis that greater richness and evenness in remnant than restored prairies would reduce variability in aboveground biomass in response to natural variation in rainfall. Restored prairies were chosen to be similar to paired remnant prairies in characteristics other than species diversity that affect temporal variability in biomass. Variability was measured as the coefficient of variation among years (square root of variance/mean; CV), where variance in community biomass equals the sum of variances of individual plant species plus the summed covariances between species pairs. Species diversity over five years was greater by a factor of 2 or more in remnant than restored prairies because richness and evenness were greater in remnant than restored prairies. Still, the CV of community biomass during spring and CV of annual biomass production did not differ consistently between prairie types. Neither the sum of species covariances nor total community biomass differed between prairies. Biomass varied relatively little in restored compared to remnant prairies because biomass of the dominant species in restored prairies (the grass Schizachyrium scoparium) varied less than did biomass of other dominant and sub-dominant species. In these grasslands, biomass response to natural variation in precipitation depended as much on characteristics of a dominant grass as on differences in diversity. [ABSTRACT FROM AUTHOR]

Why are many human populations presently ‘imploding’ with below-replacement fertility? Why are more and more young adults in these societies choosing to remain single and/or childless? Based on first principles of evolutionary theory, predictions can be derived for changes over time in the relative frequency distributions of four traits in humans proposed as the most direct determinants of the propensity to mate and reproduce: attractions to sex, legacy, leisure and parenting. In the past, high fitness was most profoundly determined by strong sex drive and strong legacy drive, especially in males. Female fertility was largely controlled by dominant males, who were then free to engage in attractions to both leisure and legacy through ‘memes’ (as well as through genes, or offspring) without any penalty on fitness. Natural selection in the past, therefore, neither strongly favoured nor strongly disfavoured any particular intrinsic female inclinations or preferences that might affect offspring production. The recent, widespread, and continuing rise in the empowerment of women, however, defines a dramatically different contemporary selection regime, where women are now free to indulge in their evolved attractions to leisure and legacy through memes inherited from predecessors, both of which represent compelling distractions from parenthood. The implications for the future survival of marriage and parenthood as cultural institutions look dismal in the short term, but promising in the long term. [ABSTRACT FROM AUTHOR]

This work details theory in which selection favors generalists in a more variable environment. Specifically, in a two-host-one-parasitoid model, temporal variation in host abundances alters the optimal searching strategy and leads to the evolution of more generalist parasitoid strategies. Consistent with empirical observations, parasitoids learn host/plant odors, and use them as a cue to search for oviposition sites. The amount of unsuccessful search time required before a parasitoid alters its searching cues (the “giving-up time”) is modeled in order to understand the evolutionarily optimal giving-up times under a variety of conditions. When host abundances vary across time, a generalist parasitoid strategy evolves with short giving-up times as it is likely that the host initially favored by a parasitoid will now have a low abundance. In contrast, when populations reach stable dynamics across time, giving-up times typically evolve to longer times, i.e. parasitoids remain specialized longer. The effect of temporal fluctuations is consistent across variation caused by endogenous population interactions and, to some degree, by environmental stochasticity. The conclusions are robust in that there is a strong degree of concordance between the results of a stochastic, individual-based model and a deterministic, numerical model. As an extension, spatial variation in hosts that leads to unequal tradeoffs between generalist parasitoids and specialist parasitoids may also result in the evolution of reduced giving-up times. [ABSTRACT FROM AUTHOR]

The optimal probability and distance of dispersal largely depend on the risk to end up in unsuitable habitat. This risk is highest close to the habitat's edge and consequently, optimal dispersal probability and distance should decline towards the habitat's border. This selection should lead to the emergence of spatial gradients in dispersal strategies. However, gene flow caused by dispersal itself is counteracting local adaptation. Using an individual based model we investigate the evolution of local adaptations of dispersal probability and distance within a single, circular, habitat patch. We compare evolved dispersal probabilities and distances for six different dispersal kernels (two negative exponential kernels, two skewed kernels, nearest neighbour dispersal and global dispersal) in patches of different size. For all kernels a positive correlation between patch size and dispersal probability emerges. However, a minimum patch size is necessary to allow for local adaptation of dispersal strategies within patches. Beyond this minimum patch area the difference in mean dispersal distance between center and edge increases linearly with patch radius, but the intensity of local adaptation depends on the dispersal kernel. Except for global and nearest neighbour dispersal, the evolved spatial pattern are qualitatively similar for both, mean dispersal probability and distance. We conclude, that inspite of the gene-flow originating from dispersal local adaptation of dispersal strategies is possible if a habitat is of sufficient size. This presumably holds for any realistic type of dispersal kernel. [ABSTRACT FROM AUTHOR]

Although the concept of specialization has played a central role in the development of ecological and evolutionary theory, important questions about specialization remain largely unanswered. We argue that the traditional division of specialization into evolutionary and ecological factors may be less useful than considering specialization as three components, which may not be mutually exclusive: ecological, behavioral, and functional. Many ecologists assume that these different aspects of specialization are necessarily correlated. However, this assumption has rarely been tested, but could be examined by using a phylogenetic approach. We argue that (1) ecologists should measure these different aspects of specialization within their respective organisms by placing measures of specialization on a standardized scale, and (2) should employ phylogenetic approaches for understanding how these components evolve. We argue that this approach will provide a more coordinated understanding of how specialization evolves. [ABSTRACT FROM AUTHOR]

Despite considerable theoretical advances in the evolutionary biology of host–parasite systems, our knowledge of host–parasite coevolution in natural systems is often limited. Among the reasons for the lag of experimental insight behind theory is that the parasite's virulence is not a simple trait that is controlled by the parasite's genes. Rather, virulence can be expressed in several traits due to the subtle interactions between the host and the parasite. Furthermore, the host might evolve tolerance to the parasite if there is sufficient genetic variance to reduce the detrimental effect of the parasite on these traits. We studied the traits underlying virulence and the genetic potential to evolve tolerance to infection in the host–parasite system Aedes aegypti –Brachiola algerae . We reared the mosquitoes in a half-sib design, exposed half of the individuals in each full-sib family to the parasite and measured several life history traits – juvenile mortality, age at pupation and adult size – of infected and uninfected individuals. Virulence was due in large part to a delay of the mosquito's age at pupation by about 10%. Although this imposes strong selection pressure on the mosquito to resist the parasite, all of the mosquitoes were infected, implying a lack of resistance. Furthermore, although additive genetic variance was present for other traits, we found no indication of additive genetic variation for the age at pupation, nor for the delay of pupation due to infection, implying no potential for the evolution of tolerance. Overall, the results suggest that in this host–parasite system, the host has little evolutionary control over the expression of the parasite's virulence. [ABSTRACT FROM AUTHOR]

The relative importance of genetic and environmental factors for explaining differences in trait distributions between populations is one of the major issues in evolutionary biology. In ectotherms, temperature can have a major impact on morphology, physiology, and life history strategies, and has often been inferred to explain differences between populations. In species with active thermoregulation, however, the ambient temperature may not be as important as the opportunity for thermoregulation. We studied growth in juvenile common lizards (Lacerta vivipara ) originating from two environments differing in such thermal opportunity (i.e. day length). The populations differed significantly in their norms of reaction, with lizards experiencing long days in the wild showing a steeper increase in growth rate with increasing thermal opportunity. The environment with longer days also has a lower mean temperature in the wild, and in accordance with evolutionary predictions, lizards from this population had higher endurance at low temperatures. Both populations showed genetic variation in degree of phenotypic plasticity in growth rate as evident from the extensive crossing in norms of reaction. [ABSTRACT FROM AUTHOR]

In figs (Ficus , Moraceae) there are two breeding systems: monoecy is the ancestral condition but approximately half the 750 odd species are functionally dioecious. Three hypotheses have been proposed for the evolution of dioecy in figs, invoking seasonality (Kjellberg et al. 1987), the reduction of non-pollinating wasp species (Kerdelhue and Rasplus 1996), and the persistence of pollinator populations within small groups of trees (Kameyama et al. 1999). However, there are two major problems with these ideas. Firstly, dioecy has probably evolved only twice (Weiblen 2000), which severely limits our ability to test between alternative hypotheses. Secondly, it is very simple to suggest ways in which dioecy can evolve from monoecy (Charnov 1982). To illustrate this problem, and enlarge on some recent progress in our understanding of functionally dioecious figs, we are proposing a few more hypotheses. [ABSTRACT FROM AUTHOR]

In fire-prone ecosystems, many plants possess traits that enhance their relative flammability and ecologists have suggested increased flammability could result from natural selection. To date, theoretical models addressing the evolution of flammable characteristics assume that flammable plants realize some direct fitness advantage. In this paper, we explore the idea that enhanced flammability can increase in frequency in a population without any direct fitness benefit to the flammable type. In our model, flammability evolves due to an association between an allele that promotes flammability and alleles at unlinked loci that give high fitness. In analogy to genetic hitchhiking, in which a deleterious allele can invade due to a genetic linkage, we call this process “genetic niche-hiking,” because the association results from localized niche construction. Specifically, flammable plants sacrifice themselves and their neighbors to produce local fire-cleared gaps (the constructed niche) in which their offspring are able to continually track an ever-changing environment. Niche-hiking requires that mating, dispersal and niche construction all occur locally (i.e. the population is spatially structured), such that offspring are likely to experience the niches their parents construct. Using a spatially-explicit lattice-based simulation, we find that increased flammability can evolve despite the “self-killing” cost of such a trait. Genetic niche-hiking may also be applicable to the evolution of other traits in spatially structured ecological systems such as plant disease susceptibility and forest tree characteristics that influence gap production. [ABSTRACT FROM AUTHOR]

Ngongas provide a metaphor for some of the opportunities and challenges facing the science of ecology and evolution. Ngongas, the traditional healers of the Shona culture, Zimbabwe, fail in the delivery of quality health by today's standards. Their outdated worldview makes most health related issues seem more complicated and more multi-factorial than when viewed through the worldviews of modern medicine. With the wrong worldview, one can work very hard, be very bright and dedicated, and still be ineffective. With the right worldview, one can work much less hard and still be extremely effective. As ecologists, we should be opinionated and possess clearly articulated worldviews for filtering and interpreting information. As ecologists we are also a bit like ngongas – we often fail to provide answers for society's ecological questions and problems, and we excuse ourselves with a belief that ecological systems are too complex and have too many factors. Unlike ngongas, this invites us to pay a lot of attention to promoting and assessing competing worldviews. We should be open-minded to the anomalies in our worldview and the successes of alternative viewpoints. As an admitted ecological ngonga, I discuss the worldview I use in my own research: the Optimization Research Program, a Darwinian research program that uses game theory to conceptualize and understand ecological systems. I use it illustrate how worldviews can synthesize disparate ideas. (I use kin selection and reciprocal altruism as examples.) I use it to show how new ideas and predictions can be generated. (I use root competition in plants and the possibility that increased crop yield may be forthcoming from knowledge of this game.). [ABSTRACT FROM AUTHOR]

Birds, especially those on islands, have contributed disproportionately to the development of theories that explain the origin and maintenance of organic diversity. This essay surveys the major ideas of how bird species are thought to evolve, speciate and multiply in adaptive radiations on islands. I begin with the nineteenth century view that speciation is the result of natural selection slowly causing divergence of geographically separated populations. I then trace the course of discovery and deeper understanding during the twentieth century, from the influence of Mendelian genetics at the beginning to the modern, molecular era at the end. The dominant and pivotal evolutionary question has been how new species are formed. An initial, nearly exclusive, attention to adaptive processes has been replaced by a search for additional reasons for allopatric divergence, coupled with the conditions that permit coexistence in sympatry after allopatric divergence has occurred. Recent studies have shown that pre-mating barriers to gene exchange caused by natural selection and sexual selection often arise before post-mating genetic incompatibilities have evolved. Therefore behavioural and ecological factors play a predominant role in this stage of speciation. Imprinting and imprinting-like processes of early learning contribute to pre-mating barriers by constraining mate choice. Ecological factors determine coexistence potential and the fates of hybrids. Genetical factors at all stages, from the founding of a new population to the evolution of genetic incompatibilities, are still poorly known. But molecular genetics is now helping us to reconstruct evolutionary history, thereby transforming ideas about systematic relationships, colonization routes and both the tempo and mode of evolution. As patterns of evolution within radiations become better known, more attention needs to be given to the task of explaining the sequential build-up of avian communities; by evolutionary... [ABSTRACT FROM AUTHOR]

The colonization cycle hypothesis predicts that adults of stream-dwelling insects preferentially disperse in the upstream direction in order to compensate for larval drift. Upstream biased dispersal has indeed been shown in many, albeit not all, natural populations. Based on a recently published analysis, we develop a simple stochastic model for the competition of genotypes with different dispersal strategies in a stream habitat. By means of an invasion analysis, we show that exact compensation of larval drift by upstream biased adult dispersal is an evolutionarily stable strategy. Exact compensation means that, on average, the net movement of individuals from birth to the time of reproduction is zero. At the population level, we show that, in general, upstream biased dispersal is not necessary for persistence, unless the reproductive rate is very low. Under all conditions, however, populations of exact compensators attain highest sizes or persistence times, respectively. Although selection pressure towards exact compensation is arguably very general in populations subject to stream drift, trade-offs or constraints might change the outcome of selection. Therefore, the analysis presented in this paper has to be viewed as a null model for optimal dispersal behavior in stream habitats. [ABSTRACT FROM AUTHOR]

The parental investment in angiosperms comprises the endosperm, a nutrient reserve that is used during seed development. The endosperm contains genes from both parents. The most common endosperm form is the 3n Polygonum -type with more maternal genetic influence than paternal, i.e. with two maternal nuclei and one paternal nucleus. The evolutionary original state is thought to be a diploid endosperm with equal influence of the parents. We focus on the evolution of the triploid endosperm and show that a gene for triploid endosperm would have an initial advantage in a population of diploid endosperm type plants, and increase to fixation. We assume that endosperm amount is controlled by endosperm genes. Then a gene causing triploid endosperm will increase the influence of the mother plant on parental investment. The production of endosperm with two copies of the maternal genes will modify the inheritance of endosperm amount and cause an increased production of seeds. [ABSTRACT FROM AUTHOR]

Discusses the integrating theory of clutch size and body size evolution for parasitoids. Characteristics of parasitoids; Information about the theory of clutch size evolution; Overview of the Lack model of clutch size evolution.

The ciid beetles Octotemnus glabriculus and Cis boleti exploit different developmental stages of fruit bodies of their preferred host fungus Coriolus versicolor . Larvae of the smaller beetle, O. glabriculus , mainly use young, expanding, fruit bodies; adults of O. glabriculus are predominantly found in young fruit bodies. By contrast, adults and larvae of the larger beetle, C. boleti , are prevalent in fully developed fruit bodies of C. versicolor . Because fruit bodies of most genets emerge during spring and early summer and mature by autumn, O. glabriculus and C. boleti breed in separated seasons. Adults and larvae of O. glabriculus are abundant in spring and early summer. By contrast, the number of adults and larvae of C. boleti increases gradually from late spring to summer and peaks in autumn. We conducted a field experiment that suggests that the phenological dynamics of C. versicolor fruit bodies drive the separation of breeding seasons between O. glabriculus and C. boleti . Additionally, laboratory experiments revealed that O. glabriculus and C. boleti have differential behavioural responses to odour compounds from young and mature fruit bodies of C. versicolor . We conclude that age-related changes in the chemical composition of fruit bodies may allow O. glabriculus and C. boleti to discriminate among C. versicolor , thus providing a mechanism for the partitioning of the resource. [ABSTRACT FROM AUTHOR]

The phenotypic composition of social groups can affect the foraging efficiency and predation risk of their individual members. In shoaling fishes, active choice of shoal companions on the basis of phenotypic characters such as body length has been reported. In this study, we investigated the phenotypic composition of free-ranging, multi-species fish shoals in a temperate freshwater lake. We collected 21 entire shoals, each consisting of up to four species and comprising 3622 fish in total. Shoals were strongly assorted by species, body length, parasite prevalence and parasite load. Using a mark-recapture procedure involving 785 banded killifish (Fundulus diaphanus ), we found no evidence for shoal fidelity or site fidelity, as individuals from different shoals mixed extensively after just 24 h and ranged all over the study site irrespective of initial capture locations. Killifish showed strong changes in shoal-size choice over 24 h, and no evidence was found that shoals might break up into sub-units of individuals that are more phenotypically assorted than their original shoals. The mechanisms by which assortative groups may arise and the consequences of low group fidelity for the evolution of cooperative behaviour are discussed. [ABSTRACT FROM AUTHOR]

The influence of size-selective oviposition behaviour by parasitoids on the evolution of life-history timing in their hosts was examined using an optimization model of a two-stage life history similar to a genetic algorithm. Host populations with varying durations of early-larval development were subjected to selection in scenarios where parasitoids had fixed preferences for oviposition on late-stage larvae, or those where parasitoid attack was dependent on the relative frequencies of the two life stages present in the population. Fixed preference for oviposition on late-stage larvae caused positive directional selection on the duration of early-larval development. Surviving individuals remained for as long as possible in the first stage of development in order to avoid parasitoid attack. Frequency-dependent parasitoid attack, in contrast, caused maintenance of variation in the duration of early-larval development. The influence of the fitness payoffs of different life stages on the plasticity of size-selective oviposition behaviour is discussed, as are possible implications of the model results for parasitoid-host population dynamics. [ABSTRACT FROM AUTHOR]

Animals and plants defend themselves against a variable community of biological enemies. We argue that the effectiveness of allocation to defence (the success of defence per unit allocation) may be expected to decrease as the diversity of attack types increases, and asked how the optimal allocation to defence covaries with the effectiveness of defence. Variation in effectiveness links optimal defence to coevolutionary processes; the prime characteristic of coevolutionary interactions is that they promote and maintain genetic variation in both hosts and their enemies, leading to variation in the effectiveness of defence. We present a simple model suggesting that as effectiveness decreases, the fitness benefit of defence disappears. In other words, when effectiveness is low, the optimal strategy is to tolerate damage. As effectiveness increases, the optimal allocation flips rapidly from no-defence (tolerance) to high allocation to defence, and then decreases at a decelerating pace as effectiveness increases. We conclude that diversifying coevolution, as it covaries with the effectiveness of defence, constrains the evolution of optimal defence strategies and may be a very important component in determining the optimal allocation to defence and variation in the success of defence as it is seen in the wild. [ABSTRACT FROM AUTHOR]

Resistance to an environmental stress such as elevated temperature can almost always be measured in several different ways. A number of recent studies in Drosophila show that lethal and non-lethal measures of resistance to heat stress are genetically independent. In contrast, data reported here suggest that there are interspecific correlations between measures of thermal stress resistance. Together, these results show that studies of intraspecific variation in stress resistance must demonstrate the ecological relevance of the measures of stress resistance chosen for study. In contrast, interspecific comparisons could be based on any of several measures of resistance. It would be interesting to determine if this pattern holds for resistance to other major environmental stresses. [ABSTRACT FROM AUTHOR]

It is generally believed that most homopteran-eating insects avoid ant-tended colonies of Homoptera, due to the ant aggression they encounter there. However, because homopteran colonies which are ant-tended often persist for longer than untended colonies, some homopteran-eaters may utilise ant-tended Homoptera when untended colonies are scarce. Furthermore, a few homopteran-eaters are myrmecophilous, habitually coexisting with ants. To investigate these phenomena, a study was made of aphids and aphidophagous coccinellids (ladybirds) on Scots pine, Pinus sylvestris , growing in areas foraged and unforaged by the wood ant Formica rufa . The non-tended aphid Schizolachnus pineti exhibited a marked population decline in late summer but persisted in both areas at very low density. Facultatively tended Cinara aphids exhibited higher population densities when associated with F. rufa , and remaining colonies of these aphids were only found associated with ants in late summer. Coccinellids exhibited considerable interspecific variability in their level of association with F. rufa , and there was some evidence of an increase in certain species’ frequencies of occurrence with the ant when Cinara aphids were all ant-tended, in late summer. Coexistence with ants appears to be associated with either an intolerance of low aphid densities, in Coccinella septempunctata and Harmonia quadripunctata , or with extreme dietary specialisation, in Myzia oblongoguttata . Similar factors to those which bring C. septempunctata into contact with ants were probably of importance in the initial stages of the evolution of myrmecophily of its congener, Coccinella magnifica . [ABSTRACT FROM AUTHOR]

Interspecific hybridization is common in water fleas of the Daphnia galeata species complex (e.g. D. galeata , D. cucullata , D. hyalina and their interspecific hybrids). We studied the effect of fish on the life histories of five taxa of this species complex originating from the Plußsee, northern Germany. Using four clones per taxon, we found that fish kairomones reduce size at birth and size at maturity significantly. For size at maturity larger taxa showed a significantly stronger reaction to fish kairomones than the smaller taxa. With respect to the intrinsic rate of increase, r , we compared two predation regimes (positive size selective and not selective). We found that under the fish predation regime most clones had a higher r when cultured with fish kairomones, leading to a higher r , and a stronger reaction for the smaller taxa. We conclude that fish predation might be an important factor influencing the co-occurrence of Daphnia parental taxa with their hybrids. [ABSTRACT FROM AUTHOR]

Predators are usually thought to be rarer (in the sense of having lower population densities) than non-predators. Recent analyses have suggested that this is not the case because a decline in species richness compensates for the well-known decline in number of individuals with increasing trophic rank. I show that a variety of invertebrate communities contain more species of predators than would be expected from the number of predatory individuals. This is not due to differences in dominance or taxonomic resolution between predatory and non-predatory guilds, and implies that predators are indeed relatively rare. I suggest that patterns of energy flow and body size make it likely that there will be a higher proportion of predatory species than individuals in a community, provided that predators have moderately specialized diets. [ABSTRACT FROM AUTHOR]

Cataglyphis is a fairly homogeneous ant genus which is widespread over the arid regions of the Old World. All Cataglyphis species are thermal specialists which are adapted to extreme environments where they forage at nearly lethal temperatures. This study focusses on two Cataglyphis species which differ considerably in their physical caste systems. These species have developed two alternative mechanisms facing extreme heat. In C. velox , foraging at high surface temperatures is clearly dependent on size: large C. velox workers forage at midday and are able to withstand higher temperatures than small workers. On the other hand, C. rosenhaueri has not developed great physical specialization, but the workers of this species have achieved physiological (such as low cuticular transpiration and metabolic rate), and behavioural adaptations (such as raising their abdomen to protect the vital organs contained in it from high temperatures) to tolerate thermal stress. The result is that small C. rosenhaueri workers may withstand extreme heat conditions in a similar way to large C. velox workers, and much better than small C. velox workers. The different mechanisms used by these two species to withstand extreme heat could reflect fundamental patterns of independent evolution. In some situations, selection may act to promote a relatively narrow size range of adult workers, all of them able to withstand thermal extremes, while in others it may act by producing different worker sizes with different tolerance to environmental conditions. [ABSTRACT FROM AUTHOR]

In this century, evolutionary biology has been successful in using fitness as a measure for evolutionary success. Although fitness has the intuitive association of 'something which is better', a restricted definition in the form of 'the number of successful offspring' is often used in biology. This definition is convenient because it is, in principle, easy to count offspring. Yet, the explanatory and predictive capacity of such fitness measurements are limited and the method islimited in its use to reproducing entities. To better understand traditional fitness measurements, we suggest that more attention be paidto underlying factors. For this purpose we advocate to focus on 'resource dominance'. The idea is that any unit of evolution which is good at dominating the resources needed for maintenance, growth and/or reproduction will also show a high evolutionary success. We show that a high resource dominance can be reached via different strategies. Additionally, we discuss how the capacity for resource dominance is related to the complexity and efficiency of the evolving unit [ABSTRACT FROM AUTHOR]

While many species of animals form only social or foraging intraspecific groups, a variety of taxa of marine invertebrates may further establish tissue unions (natural chimeras) between conspecifics. A model system is the tunicate Botryllus schlosseri, in which histocompatibility is governed by shared alleles on a single, highly polymorphic fusibility locus. Proposed benefits that have been attributed to natural chimerism have not been confirmed in this model system. Furthermore, laboratory studies on bichimeras, each consisting of two partners,have shown that fusion is usually followed by morphological elimination (resorption) of one partner. However, freely circulating stem cells from the 'subordinate' partner in resorption may parasitize the soma and germline of the 'winner.' Here we show that multi-partner chimeras form more stable and vigorous entities than bichimeras. Multi-chimeras grow faster, reach larger sizes, do not fragment, reduce incidence of morphological resorption and show other characteristics of more equilibriated entities. It is proposed that in these multi-partnerentitites, the different intraspecific conflicts alleviate each other, generating an improved entity where natural selection may act on 'group' level instead of on each individual genome. The state of multi-chimerism incurs, however, some costs such as a one quarter reduction in the potential maximum chimerical size as compared to bichimeras. [ABSTRACT FROM AUTHOR]

Net-spinning caddisflies construct capture nets with a wide range ofmesh sizes, although the ancestral net-spinner presumably spun large-meshed nets while inhabiting high current velocity microhabitats. Thorp proposed that the evolutionary diversification of mesh sizes resulted from the competitive displacement of some net-spinners into lower flow microhabitats where less water and therefore less food passed through their nets. As a result, smaller meshes were selectively advantageous because they captured smaller, more abundant food items. Although competition is often strongest between conspecifics, Thorp did not present a mechanism by which reproductive isolation, and ultimately speciation, would be achieved between the competitively inferior and superior individuals. One such mechanism may have been temporal isolation. Because the initially large-meshed nets of the competitivelyinferior caddisflies would have been inefficient in the lower flow microhabitats, these individuals would have theoretically grown slowerand emerged as adults later than the competitively superior individuals, which in turn would have led to reduced gene flow between the two populations. Although temporal isolation may not have been their final reproductive isolating mechanism, it may have opened the door to the evolution of other isolating mechanisms. [ABSTRACT FROM AUTHOR]

During the last decade, the study of fluctuating asymmetry (FA) in relation to different fitness aspects has become a popular issue in evolutionary biology. GA is suggested to be important as an index of selective advantage in both sexual and natural selection. Evidence for these hypotheses is obtained from field studies in which FA is found to be negatively correlated with various fitness components. In this paper, we demonstrate that the apparent significance of FA may dependon assumptions of linearity between size of a trait and FA, and between trait size and fitness. By means of numerical examples, we show that even small, and probably common, deviations from the assumed linearity can bias the statistical analyses to such an extent that it casts serious doubt on the validity of the postulated causal relationship between FA and fitness. We recommend reanalyses of existing data and emphatically suggest that the underlying statistical assumptions are critically evaluated in future studies of FA, especially concerninglinear relationships between the variables. [ABSTRACT FROM AUTHOR]

Correlations between breeding date and the life history or energy stores of individuals might stem from a combination of two different mechanisms. The conventional view is that individual size and conditioninfluence breeding date (i.e., condition dependence). A different view is that heritable maturation schedules allow temporally separated population components to adapt to selective regimes associated with particular breeding times (i.e., adaptation-by-time). Considering eachof these hypotheses, we examined a population of sockeye salmon (Oncorhynchus nerka) for evidence of correlations among life history traits (age, body size, egg size, and reproductive life span), patterns of energy allocation (somatic energy stores and gonadal investment), and breeding date. Life history traits were measured for 705 individually tagged adult salmon monitored from the onset of breeding until death. Energy stores were quantified for 60 fish collected when they entered the stream and 46 fish collected at death. Multiple regression models revealed that most of the variation could be explained by simple linear relationships among traits; older fish were larger; and larger fish had larger gonads, larger eggs, and more available energy when they started breeding. Condition-dependence did not appear relevant to breeding date; fish that started breeding early were similar in size and did not have more stored energy than fish that would breed later. Similarly, adaptation-by-time had little influence on variationin body size or egg size (predicted relationships were significant but very weak). In contrast, adaptation-by-time appeared very important to variation in reproductive life span and patterns of energy allocation. Early-breeding fish lived considerably longer than late-breeding fish (females, R2 = 0.525; males, R2 = 0.533). This pattern arose because late-breeding fish expended more energybefore breeding, and because late-breeding females invested more energy [ABSTRACT FROM AUTHOR]

Adaptive explanations for the evolution of within-clutch variation in propagule size have focused primarily on the roles of environmentalheterogeneity and variation in offspring quality. However, parental optimism theory suggests a third alternative: role-dependent tradeoffs when a portion of the brood, designated the emarginali offspring, is held in reserve to replace members of the ecorei brood. If developmental risk is correlated with offspring phenotype, within-brood diversification is favored n e.g., parents may gamble on large core offspring which take longer to develop and may be at greater risk of early mortality but which enjoy greater post-hatch fitness when a smaller marginal offspring is held in reserve. Whether the core offspring should be larger or smaller than the marginal offspring is contingent upon the precise form of relationship between developmental risk and offspring phenotype. [ABSTRACT FROM AUTHOR]

Current animal abundance (population density) and the number of species in a genus are shown to be inversely related to the deviation from the regression line of metabolic rate on body mass in mammals. Sucha regression line between key characters is interpreted as a emountain ridgei on the adaptive landscape, the more deviant a species is from the line the higher its probability to become an evolutionary dead-end. [ABSTRACT FROM AUTHOR]

The Gaia theory of Lovelock has had a very unequal treatment by conventional ecologists, ranging from being ignored to qualified support.I suggest that the phenomena described by Gaia are really capable ofexplanation by conventional ecological ideas using a gene/individualunit of selection. This is analogous to the situation in ecology during the 1960's when many apparent 'for the good of the species' adaptations were explained at lower levels of selection. [ABSTRACT FROM AUTHOR]

Costs of reproduction are traditionally defined as reductions in thepotential for future fitness contributions, induced by current reproductive investments, i.e. postbreeding costs. However, preparation for breeding and parental care may expose parents to survival costs already before offspring independence. Such prebreeding survival costs have only rarely been considered in the context of life history evolution. Theoretical analyses show that pre and postbreeding costs have differential effects on fitness, and a distinction between these two systems of costs may therefore be of crucial importance in analyses ofoptimal parental investment. With prebreeding costs, optimal parental investment is lower than with postbreeding costs, ceteris paribus. We illustrate this in a simple analysis of optimal clutch size, and provide general conditions for optimal clutch size with prebreeding and postbreeding costs, respectively. Through their differential effects on fitness, systems of costs may also influence and direct the evolution of behavioural and physiological patterns associated with parental investments. The distinction between pre and postbreeding costs provides new perspectives on costs of reproduction that may prove fruitful in the future development of life history theory. It also allowsanalyses of optimal life histories under reproductive costs in semelparous organisms. As predation may be an important mechanism behind prebreeding survival costs, we emphasize the role of predation and antipredatory behaviour in the evolution of life histories and breeding behaviour. [ABSTRACT FROM AUTHOR]

We discuss ways in which the social environment an individual inhabits may have influenced, and continues to influence, the life history patterns it follows. We argue that the social context in which a lifehistory is expressed has the potential to influence the evolution ofthat life history in four distinct ways, and that these may not always have been fully considered in classic life history theory. First, conflicts of interest are frequent when non-clonal organisms interact, and this is particularly true for reproduction, where conflicts between the sexes and between parents and offspring have numerous potential impacts on life histories. Second, the occurrence of frequency- and density-dependent selection arising from social interactions can have important consequences for predicting the evolutionary trajectories of life histories. Third, research in sexual selection has often been free of a life-history perspective; combining the two has the potential to offer interesting insights into the process of sexual selection and the evolution of sexual ornamentation. Finally, we illustrate some of the more dramatic outcomes of these factors for life history evolution by discussing the evolution and maintenance of within population polymorphisms in life histories. An appreciation of the rolesof social interactions may provide a solution to some long-standing questions in life-history research, such as how high levels of additive genetic variance for traits closely related to fitness are maintained. [ABSTRACT FROM AUTHOR]

We discuss the mechanistic ('proximate') and selective ('ultimate') factors that cause life history trade-offs such as the trade-off between offspring quality and quantity and that between current and future reproduction. We point out that key concepts in life history theoryand evolutionary biology such as negative genetic correlations and trade-offs can be understood in terms of physiological mechanisms. More specifically, negative genetic correlations causing trade-offs between life history traits are caused by hormones and their regulatory genes that have multiple and opposing, pleiotropic effects on two or more traits. Consequently, investigations of these mechanisms is a natural complement to the classical population genetical approach. We critically dissect the argument, put forward by other workers, that experimental manipulations of organismal physiology or other phenotypic traits do not reveal genetic life history trade-offs, but only reflects phenotypic plasticity. We reject this idea and discuss the proposed alternative approach: laboratory selection experiments. We point out some problems associated with laboratory selection experiments and suggest that experimental manipulations of life history traits and secondary sexual characters in natural populations is an important and invaluable empirical method if one aims to get a better understandingof life history evolution in the wild. Finally, we discuss the proximate-ultimate dichotomy, the physiology of phylogenetic constraints, the role of physiological mechanisms in population differentiation inlife history trade-offs and the importance of phenotypic plasticity in shaping such trade-offs. We suggest that life history plasticity in many cases may stem from the same endocrine mechanisms (gonadotropin, gonadal steroids, and adrenal glucocorticoids) as those involved in life history trade-offs. In particular, attempts to dichotomize life history trade-offs and life history plasticity may be unrealistic as t [ABSTRACT FROM AUTHOR]

Fluctuations in essential resources cause a strong selection pressure on the ability to adjust parental investment accordingly. In the dog family, Canidae, variance in female prebirth investment is adjustedby litter size. The arctic fox, Alopex lagopus , is a small canid living on the northern tundras of the world. It has the largest known litter size in the order Carnivora, up to 18 young, and litter size ishighly variable. We have analysed data from arctic fox populations throughout the species circumpolar range. In some areas, arctic foxes feed on strongly fluctuating populations of small rodents. In contrast, they have more stable food resources at bird cliffs and along coast lines. Food availability determines arctic fox litter and population sizes. A comparison between fluctuating and stable arctic fox populations showed that fluctuations are associated with large litter sizes. There were significant differences in litter size means, maxima and variances, as well as in placental scar count means. We have discussed five hypotheses on the determination of variation in litter size:one energetic, one genetic (based on density variation), one diet-determined, one based on reproductive allocation and one based on differences in reaction norms. Our findings suggest that litter size in the arctic fox is determined by the combined effect of immediate resource levels and the degree of resource predictability. We describe reaction norms that suggest how litter sizes result from adaptive plasticity within each of two genetic strategies where, according to the jackpot hypothesis, populations with unpredictable food resources generally have larger litter sizes. Within each genetic strategy, or reaction norm, litter sizes are adjusted through a number of plastic traits. These traits are influenced by nutritional limitations and include reduced ovulation rates, prenatal losses, and litter size reduction during the lactation period. [ABSTRACT FROM AUTHOR]

The concept of a cost of reproduction has associations with a variety of other ideas in population biology, among them trade-offs and antagonistic pleiotropy. Within the concept of a cost of reproduction, avariety of components can be distinguished, such as physiological and ecological costs of reproduction. This article concentrates on the former type of cost. Empirically, a variety of approaches can be, andhave been, used to analyze the cost of reproduction. Three are distinguished: genetic, environmental, and physiological. These are discussed first in general terms and second with specific reference to research on laboratory evolution in Drosophila. The latter discussion serves to focus and to instantiate the general analysis of the field. Emerging themes from this area of research include the environmental plasticity of costs of reproduction, the difficulties facing simple trade-off theories, and the critical role of physiological integration in determining costs at the organismal level. [ABSTRACT FROM AUTHOR]

Recent advances in understanding mammalian reproductive patterns mirror general developments in phylogenetic, physiological, and ecological research. We provide an overview of the diverse approaches used instudying mammalian litter size and discuss recent advances in evolutionary ecological theory and experimental data concerning reproductive patterns. Extensively documented natural history studies and breeding data from work in applied zoology and animal husbandry constitute a rich source of information for comparative studies. Such studies have helped identify fundamental differences in reproductive 'strategies' depending on whether taxa produce altricial or precocial young andwhether a given reproductive effort results in a single offspring ora litter. On the other hand, research on ecological and physiological correlates of litter size is providing new insights about reproductive energetics, the relationship between resource availability and litter size, and optimality in varying environments in general. Continuing experimental approaches are broadening our understanding of the mechanistic bases of patterns of mammalian litter size, but such studies, especially those involving manipulation of litter size, must carefully consider the potential biases introduced by experimentally induced non-congruence between pre- and postnatal litter sizes. For example, without accounting for the effects of placental hormones on mammogenesis and lactation, postnatal manipulations of litter size might be based on faulty assumptions implicit in such manipulations. To conclude, despite substantial advances in these diverse approaches, our degree of understanding of optimal litter size in mammals remains unclear because of inconsistencies in data often used in comparative studies and because of physiological and ecological constraints on littersize adjustments that hinder or bias experimental approaches. These areas should be addressed in future studies. [ABSTRACT FROM AUTHOR]

We used the self-thinning law to model constraints on the performance of desert annual plants in competitive situations which were also influenced by many other biotic and abiotic factors. We then applied the model to field data for two desert annual species measured over 384 permanent quadrats. Based on the self-thinning law, competition forlimiting resources from conspecifics and heterospecifics places an upper boundary on the survivorship of desert annuals. Simple trade-offmodels are also developed to define constraint lines for another measure of performance, density under interspecific competition. Similarconstraint lines can be potentially fitted to field data where stillother factors also influence plant performance. Compared with most conventional methods, such as linear regression and multivariate techniques, models of constraint lines are better able to characterize theeffects of the major limiting processes in complex systems in which many other factors contribute to the observed variation. This approach offers a powerful tool in dealing with complex ecological patterns;it should be generally applicable to many other systems. [ABSTRACT FROM AUTHOR]

Life-history traits and secondary sexual characters often demonstrate condition-dependence, and reproductive success thus ultimately appears to be determined by condition. Here we test the hypothesis that anti-parasite defense is condition-dependent and thus ultimately limits fitness. Animal hosts defend themselves against parasites by an efficient immune system that changes its activity level depending on level of infection. Since immune function is costly, as demonstrated by several field studies, we predicted that large immune defence organs should be maintained when the costs of an elevated immune response were reduced, or when the benefits were increased. Hence, the size of immune defence organs was predicted to increase in response to diseasedue to increased benefits of investment in immune function, and the size was predicted to increase in response to high body condition because of reduced costs of investment in immune function. A comparativestudy of birds demonstrated that the size of the spleen was significantly increased among individuals suffering from parasitic infectionsand signs of disease as compared to healthy individuals. Furthermore, we found evidence for a positive association between spleen size and body condition. These findings are consistent with the hypothesisedcost of immune function and hence a cost of anti-parasite defence. [ABSTRACT FROM AUTHOR]

Phenotypic plasticity has primarily been studied in the context of adaptation to environmental fluctuations and its underlying genetic mechanisms. However, its impact on competition and coexistence has rarely been addressed. In this paper we analyzed the ecological and evolutionary implications of phenotypic plasticity in diel vertical migration of competing, physiologically distinguishable, Daphnia populations. We developed a model that evaluates the competitive strength of daphnids which are food limited and which face predation. Competitive strength is assessed by a population's ability to invade and defend a previously established site. Phenotypically plastic individuals were assumed to choose an evolutionarily stable migration strategy at any time during the invasion process. We found plasticity to increase a population's competitive ability and in special cases to support coexistence of similar populations. Including community structure in analyzing the adaptive value of phenotypic plasticity reveals aspects thatare not directly inferable from isolated population models. Hence, selection for plasticity may be more widespread than generally expected. [ABSTRACT FROM AUTHOR]

Seed dormancy is hypothesized to be a risk-spreading strategy that maximizes plant fitness. By spreading germination time of offspring among years, plants may not only vary the environment to which their offspring are exposed, but may also reduce potential fitness losses dueto sib competition. The sibling competition hypothesis predicts thatlarge families whose offspring are more likely to experience siblingcompetition should stagger germination over several seasons (i.e. have greater dormancy) to a greater extent than small families. This should be observed as a lower initial probability of germination for seeds from large families than those from small families. We examined the relationship between family size and germination behavior in the desert mustard Lesquerella fendleri. In a single population in centralNew Mexico, USA, we estimated family size and germination fraction of 189 randomly selected maternal plants. Although a median test was not supportive, the randomization test showed marginal support for thesibling competition hypothesis. An additional study of twelve large and twelve small families in each of three other populations also showed a trend (significant in one population) for larger families to have lower germination fractions than small families. These results suggest a degree of support for the hypothesis, given that so many selective forces interact to generate patterns of seed dormancy. Thus, we have not ruled out the hypothesis that large families reduce the likelihood of sib competition in good years by producing seed crops with initially low germination. Sibling competition as well as environmental heterogeneity may have influenced the evolution of seed dormancy in this system. [ABSTRACT FROM AUTHOR]

Models of the evolution of aposematic coloration assume that the first brightly coloured morphs to appear in a cryptic population of chemically defended insects would have attracted the attention of predators, putting them at a disadvantage by comparison with their less conspicuous relatives. However, if birds show a generalised avoidance response (neophobia) towards novel prey, this could allow conspicuous morphs to survive long enough to enable aposematic selection to take over. The present experiment attempted to evaluate this possibility by pre-training individually identifiable wild blackbirds (n = 19) and robins (n = 12) with pastry baits of a particular colour and then giving them a choice between familiar-coloured and novel-coloured baits. The number of trials taken for each bird to contact the novel colour of bait for the first time, and to eat it on a regular basis, was recorded. Birds were extremely variable in their responses: some accepted novel-coloured baits immediately while others took more than a hundred exposures before eating them regularly. Green baits were avoided for longer than red or yellow. The results suggest (a) that a new aposematically coloured morph would not necessarily suffer a higher predation rate than its cryptic ancestors, and (b) that colours that are normally considered to be aposematic on the basis of laboratory experiments may not be especially aversive to wild birds. [ABSTRACT FROM AUTHOR]

We describe patterns of variation in the body size of tapeworms (Cestoda) parasitizing rodents and shrews. Tapeworms display considerableinterspecific variation in body size. Tapeworms also occur only in certain, species-specific parts of intestine. Empirical data on tapeworms suggest that these two phenomena are related: the biggest tapeworm species tend to occur in midgut. We put forward a hypothesis that the observed variation in body size and age-at-maturity reflects adaptation to specific levels of mortality and amount of nutrients in intestinal environment. We construct a simple energy allocation model in which we assume that optimal life-histories maximize the expected reproductive success (R0). Under realistic intestinal gradients of mortality and amount of nutrients, the predicted patterns of variation in body size resemble those observed in tapeworms of small mammals. [ABSTRACT FROM AUTHOR]