QUESTIONS: Whether or not cooperation can be enhanced if players with a performance higher than the mean are forced to pay an additional cost in each generation? MATHEMATICAL METHODS: Analysis of replicator dynamics with mutation. The ESS distribution of cooperation level is obtained. KEY ASSUMPTIONS: Players engage in cooperative dilemma game, and at the end of each generation, those with higher performance than the mean are forced to pay additional cost. CONCLUSIONS: Without mutation, the entire population eventually conforms to a single cooperation level determined by the initial composition of the population. With mutation, there is an equilibrium distribution of cooperation level, which has a peak at an intermediate level of cooperation. Whether it is institutionalized such as tax or just a social custom, fitness adjustment based ultimately on people's emtion of "envy" is able to maintain cooperation.
A host obtains symbionts by horizontal transmission when infected from the environment or contagiously from other hosts in the same generation. In contrast, vertical transmission occurs when a host obtains its symbionts directly from its parents. Either vertical or horizontal transmission can sustain an association between a host and its symbiont.
What evolutionary forces are necessary to evolve from an ancestral state of horizontal transmission to a derived state of vertical transmission?
We explore a general model of fitness interaction, including both additive and epistatic effects, between host and symbiont genes. Recursion equations allow us to analyse the short-term behaviour of the model and to study long-term deterministic effects with numerical iterations.
Obligate interaction between a symbiont and a single host species with genetically determined horizontal and vertical transmission. No free-living symbionts or uninfected hosts and each host is infected by only a single symbiont genetic lineage (no multiple infections). No population structure.
Epistasis for fitness between host and symbiont genes, like that in a matching alleles model, is a necessary condition for the evolution of vertical from horizontal transmission. Stochastic individual-based simulations show that (1) mutation facilitates the switch to vertical transmission and (2) vertical transmission is a stable evolutionary endpoint for a matching alleles model.
QUESTION: Does water loss during drought stress represent an important physiological constraint on the evolution of flower size? ORGANISM: A genetically diverse population of Mimulus guttatus (yellow monkeyflower) originally sampled from an alpine meadow in Oregon, USA. METHODS: We grew plants of three different genotypic classes (small, medium, and large flowered) under both well-watered and drought-stress conditions and measured water use efficiency using stable carbon isotopes. RESULTS: There was no difference in water use efficiency among flower size genotypes under well-watered conditions, but the water use efficiency of small-flowered plants was substantially lower than that of medium or large genotypes under drought stress. Whether this paradoxical result is a direct effect of flower size or an indirect (i.e. pleiotropic) effect, the presence of a genetic correlation between floral and physiological traits indicates that selection of one does impact the other.
Questions: Can there be a selective explanation for suicide? Or are all suicides evolutionary mistakes, ever pruned by natural selection to the extent that the tendency to perform them is heritable? Model: A simple variant of trait group selection (where a population is divided into mutually exclusive groups, with the direct effects of behaviour limited to group-mates), employing predators as the mechanism underlying group selection. Predators evaluate groups to avoid potentially suicidal defenders (which, when present, limit a predator’s net return), thus acting as a group selection mechanism favouring groups with potentially suicidal altruists. Conclusion: The model supports contingent strong altruism (depressing one’s direct reproduction – absolute fitness – to aid others) without kin assortment. Even an extreme contingent suicidal type (destroying self for the sake of others) may either saturate a population or be polymorphic with a type avoiding such altruism. The model does not, however, support a sterile worker caste, where sterility occurs before life-history events associated with effective altruism; under random assortment, reproductive suicide must remain contingent or facultative.
Given an endogenous timescale set by invasion in a constant environment, we
introduced periodic temporal variation in competitive superiority by
alternating the species' propagation rates. By manipulating habitat size and
introduction rate, we simulated environments where successful invasion proceeds
through growth of many spatial clusters, and where invasion can occur only as a
single-cluster process. In the multi-cluster invasion regime, rapid
environmental variation produced spatial mixing of the species and
non-equilibrium coexistence. The dynamics' dominant response effectively
averaged environmental fluctuation, so that each species could avoid
competitive exclusion. Increasing the environment's half-period to match the
population-dynamic timescale let the (initially) more abundant resident
repeatedly repel the invader. Periodic transition in propagation-rate advantage
rarely interrupted the exclusion process when the more abundant species had
competitive advantage. However, at infrequent and randomly occurring times, the
rare species could invade and reverse the density pattern by rapidly eroding
the resident's preemption of space. In the single-cluster invasion regime,
environmental variation occurring faster than the population-dynamic timescale
prohibited successful invasion; the first species to reach its stationary
density (calculated for a constant environment) continued to repel the other
during long simulations. When the endogenous and exogenous timescales matched,
the species randomly reversed roles of resident and invader; the waiting times
for reversal of abundances indicate stochastic resonance. For both invasion
regimes, environmental fluctuation occurring much slower than the endogenous
dynamics produced symmetric limit cycles, alternations of the
Existing theories for the evolution of aging and death treat senescence as a side-effect of strong selection for fertility. These theories are well-developed mathematically, but fit poorly with emerging experimental data. The data suggest that aging is an adaptation, selected for its own sake. But aging contributes only negatively to fitness of the individual. What kind of selection model would permit aging to emerge as a population-level adaptation? I explore the thesis that population dynamics is inherently chaotic, and that aging is selected for its role in smoothing demographic fluctuations. The logistic equation provides a natural vehicle for this model because it has played a central role in two sciences: Population growth in a resource-limited niche has long been modeled by the differential LE; and, as a difference equation, the LE is a canonical example of the emergence of chaos. Suppose that feedback about depleted resources generally arrives too late to avoid a wave of unsupportable population growth; then logistic population dynamics is subject to chaotic fluctuations. It is my thesis that aging is an evolutionary adaptation selected for its stabilizing effect on chaotic population dynamics.
Recently, Charnov (1996) investigated the consequences of sperm competition for optimal sex allocation in simultaneous hermaphrodites. Charnov argued that the optimal sex allocation strategy can be derived on the basis of the `sperm displacement rule'; that is, the function describing the relationship between sperm production and sperm displacement. Based on three specific examples of such sperm displacement rules, Charnov claimed that the details of the relationship between ejaculate volume and sperm displacement have only a minor eVect on the optimal sex allocation strategy. We demonstrate that this `invariance principle' is less general than suggested. The optimal sex allocation strategy as a function of the dimensionless quantity # (ratio of maximum sperm volume to sperm storage volume) can have a wide variety of shapes. This is because Charnov's results depend crucially on two assumptions of questionable generality: diminishing displacement with investment in sperm and linear fitness returns with investment in eggs. We argue that deeper insights into optimal sex allocation are obtained if the allocation decision is partitioned into multiple components. Using this approach, we find a novel invariance principle: if egg survival only depends on the investment per egg (and not on clutch size), then the shape of this relationship has no eVect whatsoever on optimal allocation to eggs versus sperm.
Nearly simultaneous fertilization of an egg by two or more sperm (polyspermy) is a lethal condition in most organisms. Sperm competing for an egg face opposing selective pressures. The race to fertilize favours rapid penetration of the egg's outer protective layer; a close finish between two sperm leads to polyspermy and death. Under most conditions of sperm competition, selection favours maximal speed of penetration by sperm in spite of potentially significant mortality imposed on both sperm and eggs. Eggs, in response to sperm competition and polyspermy, are favoured to increase the dierence in arrival times between competing sperm. I model this sperm--sperm--egg conflict to study the population genetic consequences of polyspermy. To separate sperm arrival times, selection typically favours polymorphism of egg characters that influence the rate of passage by sperm through the egg's outer protective layer. In response to diverse eggs, the population of sperm characters may be favoured to diversify in a matching way or to stabilize at a point that maximizes average penetration speed. Divergence of reproductive characters by sexual selection is frequently cited as a potentially important factor in reproductive isolation and speciation. The biochemistry of fertilization characters provides a useful model system to study these processes. Keywords: co-evolution, fertilization, sexual selection, speciation.
The frequency of workers adopting dierent behaviours often depends on thresholds. If too few workers are foraging, for example, then those workers with relatively low foraging thresholds begin to collect food. The regulation of colony phenotype is controlled by frequency-dependent feedback among worker threshold values. I show that, for a given average threshold value among workers, variability in thresholds influences the frequency of workers that adopt a particular behaviour. This conclusion applies whenever the tendency of a developmental unit to adopt a particular phenotype depends on the frequency of other units with that phenotype. Social insects are unusual, however, because the distribution of thresholds among developmental units (workers) depends on the number of times the queen has mated. Keywords: foraging, multiple mating, ontogeny, regulatory network.
Developmental rates of ectotherms (y) are often linearly related to temperature (Tc in C) within some biologically relevant range of temperatures as y = (1/S)(Tc - Tb), where Tb is the estimated temperature at zero development, and the thermal constant S is the development time multiplied by the temperature above Tb (i.e. degree days above Tb). Among similar species, it has been widely shown that S and Tb are negatively related across environments, that S is positively related to body size, and that Tb is independent of body size but increases with mean environmental temperature. Here we present a model that predicts quantitatively each of these relationships by showing that the developmental rate equation (y) is a linear approximation to a universal exponential function (in Kelvin) reflecting the underlying biochemical kinetics of metabolism. The model combines the effects of body size and temperature on individual growth to explain the majority of variation in development rates among a broad assortment of aquatic ectotherms (fish, amphibians, zooplankton) at different life stages. Specifically, the model predicts that body size enters as (mass)1/4, and that Tb is about 10C below the mean developmental temperature for ectotherms in nature ("the 10C rule"). We conclude by explaining how differences in food type would affect the model.
With hundreds of species established in new localities around the world, ants are an important, widely distributed, and growing group of exotic animals. The success of many established exotic ants is hypothesized to be related to competitive advantages associated with smaller workers and larger colonies relative to co-occurring native species. To evaluate this hypothesis, ant assemblages were thoroughly sampled across the range of upland ecosystems in north-central Florida, a region with one of the most diverse exotic ant faunae in the world. Patterns of species richness, abundance, worker body size, and colony size were compared among species and ecosystems. We found that exotic ants were neither abundant nor diverse in any of the undisturbed upland ecosystems. In disturbed field sites, exotic ants accounted for about 40% of total ant abundance and 25% of species richness. A total of 94 species, including 13 exotic species and 9 endemic species, were captured. The average body size of exotic ants was not obviously different from related native species. The average colony size of exotic ants was smaller than native species, with the exception of Solenopsis invicta which had the largest colony size of all species. Introduced ants (including S. invicta) were neither speciose nor abundant in any of the native woodland ecosystems. Florida's intact, native upland ecosystems appear to be resistant to invasion of exotic ant species despite the fact that surrounding disturbed habitats host a large diversity and abundance of introduced species. The prediction that exotic species have smaller workers than related native species was not supported.
We compared host acceptance behaviour between two strains of the parasitoid Cotesia glomerata: one strain from the USA where C. glomerata was introduced from Europe 120 years ago, and one native European strain. In the USA larvae of Pieris rapae are attacked, whereas in Europe both P. rapae and P. brassicae serve as hosts. Pieris brassicae is the preferred host species, but since it is absent in the USA, it has not been available to American C. glomerata for about 350 generations. We observed clear geographic variation in host acceptance between American and European parasitoid strains: American C. glomerata rejected P. brassicae significantly more often than European parasitoids did. Early experience through development in and emergence from the less preferred host P. rapae increased acceptance of this host in European C. glomerata. Host acceptance of the preferred host was ‘hardwired’: it was high regardless of previous experience. Such strong inflexible responses to important stimuli and plastic responses to less important stimuli are observed in many other parasitoid–host systems. However, our results show that 350 generations of selection were sufficient to override this hardwiring in the American parasitoid strain.
in a predictable environment, variation in quality among breeding sites should select for mechanisms by which animals can increase their chances of ending up in a good site. In this paper, I develop an optimality model that explicitly considers the use of information by first-time breeders for selection of local breeding habitat. The model compares the expected fecundity in the first breeding season of two strategies. One strategy (natal dispersal) allows individuals to prospect (i.e. gather information) for good future breeding sites at a cost that increases with their prospecting activity. Individuals using the other strategy (natal philopatry) have complete information about their natal breeding site at the time of birth and they do not prospect. Allowing prospecting activity and age at first breeding to evolve, the model yields several qualitative predictions about how natal dispersal within a population should evolve under different environmental (prevalence of good sites, predictability of site quality among years) and demographic (pre-breeding survival rate) conditions. Natal philopatry is expected to prevail in all environments when pre-breeding survival is low, while increasing pre- breeding survival should allow natal dispersal to dominate in an increasing range of environments appearing first in unpredictable environments.
Questions: How often has dispersal of seeds by ants evolved in monocots and is the timing of origins associated with changes in the ant community or instead with the rise of forests? Are patterns in the origin of elaiosomes (the trait associated with the dispersal of seeds by ants) through time similar to those for the origins of fleshy fruits? Data studied: We estimate the timing of the origin of elaiosomes and fleshy fruits respectivelyby mapping seed morphology onto a recent phylogeny based on ndhF sequence data forthe monocots (Givnish et al., 2005). For comparison, we use fossil data on ant relativeabundance through time and phylogenetic data for the timing of the origin of seed-dispersing ant lineages. Search method: We mapped origins of both elaiosomes and fleshy fruits onto the phylogenyusing parsimony in the program Mesquite (Maddison and Maddison, 2005). We analysed therelationship between ant relative abundances, the number of origins of seed-dispersing ants, and the rate of origination of elaiosomes using randomization-based Monte Carlo regression in the program R (Cliff and Ord, 1981). Using the program Discrete (Pagel, 2006), we test whether fleshy fruits or elaiosomes and shaded forest understoreys show correlated evolution.Conclusions: Morphological features for the dispersal of seeds by ants (myrmecochory) have evolved at least twenty times within the monocots. Origins of myrmecochory are not associated with the rise of forests during the Cretaceous or with subsequent transitions of plant lineages into closed canopy habitats, nor are they contemporaneous with the origins of fleshy fruits. Instead, the origins of myrmecochory are closely associated with the rise in relative abundanceof ants (proportion of all individual insects in fossils) towards the end of the Eocene and more recently.
Background: Parents can invest in offspring through a variety of behaviours, some of which trade off against each other, such as investment in the current brood versus investment in a future one. Question: When should hymenopteran parents stop provisioning the current nest and decide whether to seal the entrance to the nest (e.g. with a number of leaf pieces)? Method and key assumptions: A dynamic state variable model. We assume that mothers alter reproductive decisions based on their perception of costs and benefits of brood cell and nest construction. Some of these construction behaviours allocate investment at one or a few offspring in a brood but others affect the entire brood. Conclusions: Several factors impact the decisions of when to cease provisioning new offspring and whether to seal the nest. Higher current nest value and greater risk of mortality increase the likelihood of both ceasing provisioning earlier and sealing the nest. The greater the benefit of sealing, either because of increased benefits or decreased negative impacts, the earlier and the more frequently it occurs.
To determine the influence of dispersal on the expression of immune traits, we conducted a reciprocal transfer experiment. Chorthippus biguttulus grasshoppers from two populations were released as juveniles into their native and transfer environments. After recapture as adults, we found that an immune trait, the amount of phagocytically active cells, was significantly reduced in the transfer environments. In contrast, adult body mass differed between the two habitats, but was not reduced in the transfer environments. The results suggest that dispersal to a new environment can reduce the expression of immune traits, while otherwise not influencing body condition. One reason for such an effect could be that the parasite community in the foreign environment might be relatively maladapted, which would lead to reduced demands for resource allocation to immune traits.
Question: What is the general quantitative relationship between adaptive phenotypic diversity, or bet-hedging, and the environmental uncertainty that selects for it? Mathematical methods: Building on the fitness set approach introduced by Levins, we develop a graphical heuristic for determining the optimal amount of diversity in a fluctuating environment. We use as our optimality criterion the expected long-term growth rate of a lineage. Key insights: Each of the phenotypes in a polyphenic popula- tion may be seen as investing a certain proportion of its reproductive effort in each of the possible environments. A bet-hedging lineage that produces the phenotypes in just the right proportions—so that the overall reproductive investment in each environment matches the environmental frequencies—grows faster on average than other lin- eages. How much faster it grows than the resident population, and thus the strength of selection towards the optimal bet-hedging strat- egy, depends on how far the residents are from the optimal investment profile. Predictions: A rigorous empirical demonstration that bet-hedging is adaptive requires a comparison of the degree of phenotypic diver- sification in similar populations subject to varying levels of environ- mental uncertainty. We confirm that bet-hedging should be observed only within a certain range of environmental variation; when the envi- ronment is more predictable than this, a phenotypic generalist would
In addition to reproductive rates and generation times, local parasite adaptation is predicted to be associated with relative host/parasite migration rates, parasite virulence and local resource levels. We tested for local parasite adaptation in a spatially structured natural host-parasite system with fluctuating host resources. Using a cross-fostering design replicated over 2 years, we exchanged chicks of the Black-legged kittiwake (Rissa tridactyla) between subpopulations of its ectoparasite, the tick Ixodes uriae. We found evidence that ticks were adapted to their local hosts; ticks had higher success (capacity to continue to the next life stage) and shorter engorgement times on sympatric birds than on allopatric birds. However, infestation levels were similar between the resident and non-resident chicks in a nest, implying that ticks are unable to distinguish between good and bad hosts and that selection acts during tick engorgement. Hosts appeared to be locally maladapted to their parasites; growth rates tended to be lower for sympatric birds in the presence of parasites. However, we found no effect of host group on the T-cell immune response of chicks. Overall, the results seemed to depend on the environmental quality. When resources were low, local maladaptation was expressed in the host, but adaptation was not shown by the parasite. In the higher quality year, evidence for local parasite adaptation was found, but the host seemed to be able to compensate for the pathogenic effects of ticks. This suggests that virulence (pathogenic effect on host) and the reciprocal effects of the interaction can fluctuate depending on host environmental conditions
Assuming a linear trade-off function between the habitat-specific fitness values in Levene's model, De Meeûs and Goudet found that an evolutionarily stable strategy always exists and that polymorphism cannot be maintained on an evolutionary time-scale. On the other hand, Kisdi and Geritz showed that, in a broad parameter region, evolution necessarily leads to polymorphism in a Levene-type model with stabilizing selection within each habitat. Here I reconcile these results by demonstrating that the convexity of the trade-off function plays an essential role in the evolution of polymorphism.
Question: Why do male frogs invest in heterospecific matings in hybridogenetic systems with large heterospecific and small conspecific females? When is a strategy to mate with larger females evolutionarily stable? Mathematical method: A continuous-time model of reproductive values with discrete classes of individuals is developed to investigate the balance between two strong selective pressures: large conspecific females are the best mates, but large females are also more likely to be heterospecific. Key assumption: Males can detect female size, but are unable to distinguish between conspecific and heterospecific females. Matings incur time costs and the mating season is limited. Therefore, males of the small parental species should evolve to ignore heterospecific females. Conclusion: The results indicate that direct benefits of male mate choice within conspecifics can counteract the selective pressure to avoid large females as mates. This trade-off can balance out in a way that makes indiscriminate mating adaptive.
Organisms in nature are both proximate (operating by rules of thumb) and adapted (the rules influenced by natural selection). We introduce new methods that can be used to study in silico versions of organisms behaving according to proximate adapted rules. Our approach goes beyond neural networks and offers an alternative to optimization methods. It is based on the idea that organisms receive signals from the environment, that the signals are modified by internal (state-dependent) factors to create feelings (which we refer to as hedonic tones), and that behavioural processes (decisions) are a response to the hedonic tones. We illustrate these ideas through a model of a fish moving in a vertically structured environment, subject to predation and competition from conspecifics. The fish in our model responds to food, light, temperature and conspecifics, without any reference to current or future fitness. We use a combination of hedonic modelling to process the response, and genetic algorithms to modify the response via natural selection, according to internal needs and evolutionary history. We show that many different combinations of genes can lead to similar fitnesses, so that this approach generates genetic diversity. We compare our results with those of a variety of empirical studies and show that our approach can lead to new links between empirical and simulation studies.
Explanations for the sexual behaviour of gonochoristic (dioecious) and hermaphroditic animal species differ. Gonochore sexual behaviours are generally understood in the context of sexual signals, whereas those of hermaphrodites have been explained by sexual conflict resolution. Specifically, the alternation or simultaneity of mating roles observed in some hermaphroditic species has been attributed to conditional reciprocity, in which an individual's donation of gametes (eggs or sperm) appears to depend on its partner's release of that same gamete type. Because it appears that individuals 'give gametes to get gametes' so as to avoid being cheated, this phenomenon is known as 'gamete trading'. Here, the observations from the original report analysing the egg trading behaviour of the sea bass Hypoplectrus nigricans are re-interpreted within the context of modern sexual signalling theory. Questions raised by the gamete trading hypothesis are resolved once the observed behaviours are viewed as sexual signals. I therefore propose that 'giving gametes to get gametes' represents conventional sexual signalling. Generalization of the hypothesis to other hermaphroditic mating systems, combined with empirical support, should contribute to a consistent theory of sexual signalling that is applicable to all animal mating systems.
Long-term data sets that quantitatively confirm basic ecological theory are rare for field populations. Highly variable recruitment often causes wide temporal variation in population age distribution and basic theory for adaptive sex ratio often predicts ‘sex ratio tracking’ to match the fluctuating age distribution. Using sex-changing shrimp as a model system, we test this in a new data set of 20 years duration. The new data support the theory, despite intense fishery exploitation that itself has greatly altered the age distribution in recent years.
Background: Forty years ago, G.C. Williams predicted that reproductive effort should be inversely related to the average adult life span across species. Aim: Use allometric life-history theory to refine that prediction. Result: Reproductive effort should be inversely proportional to average adult life span, a −1 scaling rule.
The somatic costs of reproduction are important for understanding the relationship between sexual selection and life-history evolution, and there are two main hypotheses used to explain the pattern of reproductive effort in ungulates. The terminal investment hypothesis predicts that
reproductive effort should increase with age, because the value of each offspring increases as the number of future potential offspring decreases over the lifetime of an individual. In contrast, the mating strategy-effort hypothesis predicts that reproductive effort should be highest in prime-aged males, and lower in both younger and older males, since prime-aged males are most active in trying to gain matings. We examined reproductive effort among prime-aged (5–8 years old) fallow bucks (Dama dama) by comparing mass loss during the breeding season with mating success and activities associated with mating. Males lost about 26% of their body mass during the breeding season and mating success was strongly positively related to the time spent moving and in vocal display. However, mass loss was not related to either mating success or the behaviours associated with mating success. This indicates that males of higher quality were more efficient at converting energy into reproductive success, and is consistent with our earlier results showing phenotypic quality differences between males in our study population. Mass loss was positively correlated with initial mass. Therefore, body condition at the start of the breeding season was the most important determinant of reproductive effort. Mass loss was not related to age, in that it neither increased with age nor peaked in males that are usually the most reproductively active (ages 6 and 7). Thus, for reproductive effort in prime-aged males, our results do not support either the terminal investment hypothesis or the mating strategy-effort hypothesis.
Species usually have to use indirect cues when assessing habitat quality. This means that it is possible for humans to alter habitats in a way that causes a discrepancy between the cues and the true quality of different habitats. This phenomenon is called an 'ecological trap'. Here we show that the trap may lead to a behaviourally mediated Allee effect, where population growth is reduced because of non-ideal choices of individuals. The reduction is greatest at low densities because more individuals can choose their preferred habitat when competition for breeding sites is reduced. An ecological trap may lead to multiple equilibria in population dynamics and cause deterministic extinction in habitats that are capable of sustaining a viable population. We also study the efficiency of three mechanisms that may rescue a population from this extinction trap: natural selection acting on habitat preferences and two forms of phenotypic plasticity, experience-based learning and a philopatric preference for the natal habitat. Selection is most efficient in short-lived species with large heritable variation in habitat preferences, whereas in long-lived species, plastic traits outperform genetically determined preferences. The simple philopatric strategy generally produces the most favourable outcome. It hardly differs from the optimal strategy that assumes perfect and immediate knowledge of habitat change, and is very robust to non-ideal variation in the strength of habitat preferences. We conclude that conservation biologists need to ensure that cues for habitat choice correlate with habitat quality.
The trade-off in the allocation of resources between skeletal growth and the storage of reserves has received little attention, despite relevance to all growing organisms. We explored this trade-off by manipulating food availability for juvenile Atlantic salmon, Salmo salar, so as to create the same reduction in growth and loss of energy reserves at different times of the year. The fish showed seasonal differences in their responses to the nutritional deficit when food was restored. In winter, fish restored lipid reserves, but their growth in length over the recovery period was negligible. In summer, fish allocated resources to growth in length as well as the restoration of lipid reserves; moreover, this skeletal growth was significantly faster than that of control fish that had received food ad libitum throughout. We demonstrate that current physiological and energetic models of animal growth cannot account for such seasonal variation in compensatory growth and allocation patterns, and the regulation of growth and energy reserves is a dynamic and state-dependent process. We then predict - on the basis of expected effects on fitness - how somatic allocation and catch-up growth should vary over time and in contrasting environments.
Allometric relationships of floral organs were compared in related outcrossing (herkogamous) and selfing (autonomously self-pollinated) species in the genera Mazus and Hosta to determine how floral traits have evolved under selection favouring autonomous self-pollination. Autogamous Scutellaria dependens was also examined. We measured several floral traits in the five species. Selfing species had a steeper slope of the log–log regression and correlation coefficient for the filament–stigma height relationship than related outcrossing species. In three selfing species, the filament–stigma correlation was stronger than the petal–filament and petal–stigma correlations. Outcrossing species had weaker filament–stigma correlations than the other correlations. These findings suggest that, in selfing species, the placement of the stigma close to the anthers has evolved under selection favouring autogamy and that filament–stigma correlations might have evolved together with mating-system evolution in flowering plants.
Question: Does testosterone suppress the immune system of males in a strongly sexually dimorphic and long-lived ungulate? Immunocompetence handicap hypothesis: Testosterone promotes the development of secondary sexual characteristics and simultaneously suppresses immunological defence. Organisms: Free-ranging and individually identifiable male Alpine ibex (Capra ibex). Methods: In faecal samples, measure testosterone levels (ng · g−1) and the number of parasite eggs per gram of faeces (faecal egg counts). Determine social dominance by observing the outcomes of agonistic interactions in the field. Weigh males at a salt-lick scale. Data analysis: Path analysis to examine the relationships between testosterone levels, dominance, body mass, age, and faecal egg counts. Conclusions: We found a strong positive effect of testosterone on the amount of parasite eggs in the faeces of males. The level of parasite infection did not depend on any other tested variable. Testosterone therefore has an immunosuppressive effect in male Alpine ibex, as suggested by the immunocompetence handicap hypothesis.
Life-history theory predicts that energetically costly activities, such as growth, reproduction, or predator defenses should trade off against immunity. However, the effects of predator induced phenotypes immune system are relatively unexplored. I experimentally tested the effect of natural predators on the immune system of wood frog tadpoles (Rana sylvatica) by exposing half of the tadpoles to caged dragonfly larvae predators, and half to empty cages. I then administered a standard immunoassay, the phytohemagglutinin (PHA) assay to a randomly selected group of animals from each treatment. These results reveal that exposure to predators reduces the response to PHA in larval R. sylvatica. Furthermore, predator-exposed larvae lack the typical decline in immunocompetence during metamorphosis that is found in normal amphibian larvae and have a weaker response to PHA throughout their development. Thus, predators have an effect on both immunocompetence and developmental patterns of immunity. Generally, predator exposure may facilitate parasitic infection in amphibians by reducing immune function, and thereby render amphibian populations vulnerable to co-exploitation by both predators and parasites. Master of Science School of Natural Resources and Environment University of Michigan http://deepblue.lib.umich.edu/bitstream/2027.42/62085/1/seiter thesis.doc
We measured the size of floral organs in andromonoecious Commelina communis to test the hypothesis that pollinator-mediated selection might regulate variation in the size of floral organs. We compared variation in floral organ size between C. communis perfect flowers, with fertile pistils, and C. communis staminate flowers, with sterile pistils. We hypothesized that variation in size of the sexually functionless pistil would be large. We found supporting evidence from eight C. communis populations. These results suggest that pollinator-mediated selection may have stabilized variation in the style length of perfect flowers. We also found differences in variation in length among three different types of anthers in both perfect and staminate flowers, only two of which produce fertile pollen. This is consistent with our prediction that mating-related organs should vary less in size than attraction-related organs.
Question: What conditions are required for evolution of predator-specific inducible defences? Hypotheses: (1) Prey organisms distinguish among predators to which they are exposed. (2) Prey individuals with a predator-specific defence must attain higher survivorship than those with a mismatched defensive phenotype. Organisms: Prey, anuran tadpoles (Rana pirica); biting type predator, dragonfly larvae (Aeshna nigroflava); swallowing type predator, salamander larvae (Hynobius retardatus). Methods: Rana pirica tadpoles were exposed to the predator signal in close proximity to or remote from the dragonfly larvae or the salamander larvae to determine whether the tadpoles develop predator-specific morphologies and whether they utilize predator-specific signals in the induction process. We conducted predation trials to determine whether the tadpoles with induced phenotypes were more resistant to the attack in the corresponding predator environment. Results: Rana pirica tadpoles developed predator-specific morphologies in response to exposure to two different types of predator. The tadpoles discriminated between the predators . that is, different signals were required to develop the specific phenotypes in the induction process. The survival rate of tadpoles of specific phenotypes was higher than that of tadpoles of mismatched or non-induced phenotypes when exposed to predation by the corresponding predators.
For pollinators to exert selection on floral traits, they must be able to detect and respond to differences in floral morphologies. I examined foraging preferences by the bee Apis mellifera (Apidae) visiting naturally occurring flowers of Mimulus guttatus (Scrophulariaceae). The results indicate that A. mellifera preferentially selects larger M. guttatus flowers in their foraging bouts. Since differences in flower size in M. guttatus have previously been shown to have a significant heritable component, I suggest that A. mellifera has the potential to be an important selective force in the floral morphology of M. guttatus.
choose mates on the basis of these characters. Organism: King penguins, Aptenodytes patagonicus. Field site: Colony of approximately 16,000 breeding pairs on Possession Island in the Crozet Archipelago, southern Indian Ocean. Methods: We measured foot-web swelling in males resulting from experimental injection of a novel mitogen (phytohaemagglutinin, PHA) and compared that measure of immunocompetence with colours of the beak spot and plumage. Conclusions: Breast plumage colour is a reliable indicator of immunocompetence in king penguins. Breast plumage colour appears to rely on the presence of pterin pigments, and the fact that pterins are implicated in immune function may underlie the honesty of this signal. Descriptors: Article Subject Terms Body conditions | Breast | Colour | Evolution | Hemagglutinins | Immunocompetence | Islands | Marine birds | Oceans | Ornamentation | Pigments | Plumage | Reproductive behaviour | Sexual selection | Article Taxonomic Terms Aptenodytes patagonicus | Article Geographic Terms PSE, Indian Ocean, Crozet Is., Possession I.
Background: According to the Green World Hypothesis of Hairston, Smith, and Slobodkin,
all plants are edible for some herbivores. Hence, the copious abundance of plant biomass,
typical for terrestrial ecosystems, depends on the collective regulatory action of predators on
the herbivore guild. According to the counterarguments of Polis and Strong, the defensive
traits of terrestrial plants attenuate terrestrial trophic cascades to species-specific trickles,
so elimination of predators might lead to increased abundance of inedible plants but will not
influence community-level plant biomass.
Question: Does the elimination of predators from a low arctic scrubland, with high-quality
forage plants and poorly edible evergreen ericoids, lead to a reduction of community-level plant
biomass or to an increased abundance of well-defended evergreen ericoids?
Methods: In 1991, we introduced grey-sided voles (Myodes rufocanus) to islands, initially
harbouring dense scrubland vegetation, and established permanent plots there. In 2000, we
transplanted vegetation blocks from a large three-trophic-level island with voles and predators,
to two-trophic-level islands with introduced voles but without resident predators, and also to
vole-free one-trophic-level islands, and back to the three-trophic-level island. Vole densities
were monitored by semi-annual live trapping. Vegetation was monitored by the point-frequency
Results: In the absence of predators, vole densities increased 3.7-fold and the communitylevel
plant biomass was decimated. The least palatable plant group, evergreen ericoids,
suffered especially heavily, whereas palatable herbaceous plants increased in abundance.
However, all three functional plant groups responded positively to the elimination of grey-sided
Conclusions: Our results corroborate the Green World Hypothesis, indicating that in the
absence of predators, plant defences do not prevent runaway consumption of the vegetation.The fate of plants in predator-free systems with browsing vertebrates depends primarily on the
accessibility of each plant during the limiting season. Evergreen ericoids then form the most
sensitive functional group.
Methods: In 2000, we constructed vole-proof exclosures on low arctic islands where
vegetation had, since 1991, been heavily impacted by grey-sided voles. In 2000 and 2003,
we surveyed the vegetation of the exclosures, of unfenced plots on the same islands, and
of control plots on a vole-free island. We used the point-frequency method for vegetation
Results: In the exclosures, the biomasses of most plant species increased, by and large, at the
same pace. The two woody species, which increased most rapidly, were the maximally palatable
bilberry (Vaccinium myrtillus) and the phenolics-laden, maximally unpalatable northern
crowberry (Empetrum nigrum ssp. hermaprhoditum). The recovery rates of these species
Conclusions: The high concentrations of phenolics typical for evergreen arctic dwarf shrubs
do not carry any obvious cost in the form of reduced capacity for compensatory growth. The
principle of trade-offs does not help to explain the variation in plant palatability.
Patterns of geographic parthenogenesis can provide insight into the ecological implications of the transition from sexual to parthenogenetic reproduction. We analysed quantitatively the environmental niches occupied by sexual and parthenogenetic geckos of the Heteronotia binoei complex in the Australian and zone. This complex consists of two independently derived maternal lineages of hybrid parthenogens, which, in turn, include two different triploid races that resulted from reciprocal backcrossing with the parental sexual taxa. The sexual progenitors are still extant and occupy very distinct environmental niches. The triploid parthenogenetic races are biased in their environmental niche towards those of the sexual races for which their genomes are biased and this dosage effect is apparent in both maternal lineages. Thus triploidy may have benefited the parthenogens through partial recovery of the parental niches. Although the parthenogens have a broader geographic distribution than their sexual progenitors, their environmental niche is narrower and biased towards one of the sexual races. In keeping with general patterns of geographic parthenogenesis. parthenogenetic H. binoei occupy a harsher environment than the sexual forms. occurring in regions of persistently low rainfall. Bioclimatic modelling suggests patterns of rainfall are important in limiting the distribution of sexual and parthenogenetic taxa. and extrapolation from the current bioclimatic profiles indicates potential for further eastward range expansion by the parthenogens.
The Australian and zone harbours a surprising number of parthenogenetic organisms. including the well known case of the grasshopper Warramaba virgo. Less well known is the case of the stick insects of the Sipyloidea complex, which. despite its presence in the literature for over 15 years. has gone entirely unnoticed by workers in the field. We draw attention to the remarkable similarities between the evolution of parthenogenesis in Warramaba and Sipyloidea and analyse the geographic distributions of parthenogenetic and sexual forms with respect to six Climatic variables. We provide evidence that a combination of Climatic and vegetative barriers are responsible for the current distribution patterns in these taxa. Comparisons are also made with patterns of geographic parthenogenesis in lizards of the Heteronotia binoei complex. In general. there has been a strong tendency for parthenogenesis to originate via hybridization in the western part of the and zone with subsequent eastward spread throughout mulga woodlands and mallee shrublands where rainfall is both low and aseasonal. We propose that the hybridization events leading to parthenogenesis in these diverse taxa were driven by a common biogeographic process - that is, by range shifts associated with changes in aridity during the late Pleistocene.
Questions: What was the distribution of fossil mammal taxa in the Miocene German Molasse Basin? Were there changes in community structure during the terrestrial development of the Molasse Basin? Were community dynamics similar in the Molasse Basin to those in the rest of Europe? Data: We gathered the available Miocene large mammal herbivore occurrences from the southern German Molasse Basin (museum data mainly from Munich (Germany), with additional data from museums in Stuttgart (Germany) and Vienna (Austria)). We used public data from NOW (Neogene of the Old World database, http://www.helsinki.fi/science/now) for comparison and as the source of ecological data for the species. Methods: We combined ecological data from the NOW database with distributions of herbivorous mammals within the Molasse Basin. We plotted the occurrences of taxa on a base map, and used the associated body size and dietary categories to plot these data on the map. We investigated the differences in the structure of communities in different time periods. We compared different time periods and differences among areas. We also compared the Molasse Basin and NOW data. Conclusions: The evolution of large-mammal communities in the Molasse Basin occurred in two phases: build up and decline. The build-up phase was characterized especially by a high abundance of small-sized browsers and mixed feeders. The diversity was especially high during the built-up phase, indicating a highly differentiated wetland habitat. The decline phase saw a very different community structure with fewer mixed feeders and with larger sized mammals dominating. The difference between these phases was largely the consequence of regional extinctions of species and genera. The Molasse Basin community dynamics also differ from those of the rest of Europe (NOW data).
The ability to create shelters that provide protection from the environment is widespread among animals. However, in spite of the central role roosts play in the life of bats (Chiroptera), only a few species have developed the ability to make their own refuges, one of them being the Neotropical Lophostoma silvicolum. This bat creates and inhabits cavities in active arboreal nests of the termite Nasutitermes corniger. We measured temperature in cavities inside active and dead termite nests, and in tree holes occupied by closely related bats, to determine whether energetic benefits compensate for the cost of excavating the hard nests. The inside temperatures of active termite nests were very stable and 2.1–2.8 °C warmer than those of the other two potential roost types. The observed temperature difference is estimated to allow euthermic
L. silvicolum to save about 5% of their daily energy expenditure when roosting in active
termite nests instead of dead nests or tree holes. Suitable roosting conditions result from the presence of termites and are independent of nest architecture. Our results indicate that the benefits of higher temperatures may be one of the driving forces promoting the evolution of active roost making in bats.
There is growing interest in the potential for population divergence (and hence speciation) to be
driven by co-evolutionary arms races due to conflicts of interest between the sexes over matings
and investment in offspring. It has been suggested that the signature of sexually antagonistic
co-evolution may be revealed in crosses between populations through females showing
the weakest response to males from their own population compared with males from other
populations. The rationale behind this prediction is that females will not have been able to
evolve counter-adaptations to manipulative signals from males with which they have not
co-evolved. Recent theoretical treatments suggest that this prediction is not strictly exclusive
to the sexual conflict theory, but it remains the case that population crosses can provide
insights into the evolution of mate choice within populations. We describe crosses between six
populations of the red flour beetle Tribolium castaneum. Although successful matings are no
more or less likely between populations compared to within populations, females do increase
their oviposition rate in response to males from other populations, relative to males from their
own population. Our results are therefore consistent with the proposition that sexual conflict
has driven population divergence in this species. However, we argue that the available evidence
is more supportive of the hypothesis that increased female investment in response to males from
other populations is a side-effect of inbreeding avoidance within populations.
There have been many attempts to document links between reproductive allocation and factors such as adult body size and demography. This paper suggests that among closely related taxa, two dimensionless numbers, each a benefit–cost ratio summarizing reproductive timing, allocation and demography, are invariants and thus are useful to classify life histories. The two numbers are E/α and C·E, where E is average adult life span, α is age-at-first-reproduction and C is average mass (per adult) devoted to reproduction per unit of time, divided by the average adult body mass (m); C is usually called ‘reproductive effort’. Since E−1 is the average adult mortality rate, C/E−1 is the reproductive effort (benefit) per unit death (cost). Similarly, E/α is the amount of time for reproduction (E) divided by the time cost to get there (α). Combining these two numbers with the relative size (I) of an offspring (I/m) yields a new classification scheme for life histories; this is contrasted with other classification schemes (e.g. r and K).
Questions: Under what climatic conditions is long leaf longevity, or evergreen-ness, favoured? Under what physiological conditions of leaves is long leaf longevity, or evergreen-ness, favoured? Why is evergreen-ness favoured in both tropical and frigid regions? What is the difference in biological meaning of evergreen-ness between tropical and frigid regions? Mathematical method: Optimization with two variables, expansion and shedding times of leaves. The objective function for optimality is the amount of assimilating product per unit time of an individual leaf. We obtained the optimal expansion and shedding times of leaves by numerical calculation. Key assumptions: (1) Air temperature varies seasonally with average temperature and the amplitude (climatic condition). (2) The key parameters of a leaf are construction cost, photosynthetic rate, and ageing rate (physiological condition). (3) A leaf adopts the optimal strategies of expansion and shedding times both under various climatic conditions and physiological conditions. Predictions: (1) There are two climatic conditions in which evergreen-ness is optimal. The first is where average temperature is over 30°C and the amplitude is very small, as in the tropics. The other is in cold regions, such as a frigid area. (2) Low maximum photosynthetic rate and high construction cost are likely to select for evergreen leaves.
Flower orientation is an important character influencing plant fitness. Zygomorphic flowers
are known to orient vertically. We conducted field experiments in which we changed the
flower angle of zygomorphic Commelina communis to determine how flower orientation affects pollinator behaviour. We confirmed that Commelina flowers oriented vertically like other zygomorphic flowers. Then, we artificially prepared control, upward- and downward-oriented flowers and exposed them to natural pollinators (syrphid flies and bumblebees). We found that the frequency of approach by syrphid flies and bumblebees was not influenced by flower angle, but there were fewer landings on downward-oriented flowers than on control and upwardoriented flowers. Moreover, the upward flower orientation increased illegitimate landings (landing on the flower without touching the stigmas or mating-related anthers) compared with controls. Thus, vertical flower orientation in zygomorphic flowers serves to control pollinator landings. Our findings suggest that deviations from vertical orientation may reduce fitness in C. communis by reducing the efficiency of insect-mediated pollen transfer.
In this paper, we use the theory of adaptive dynamics to highlight the differences in evolution- ary behaviour when contrasting formulations of the carrying capacity are used. We use two predator-prey systems, one with a fixed carrying capacity and one in which the carrying capacity is an emergent property compounded of an intrinsic growth rate and a susceptibility to crowding. We consider prey evolution in both systems and link the evolving parameters by a trade-off which requires that prey with higher per capita growth experience a greater risk of predation. We find that the two approaches for representing the carrying capacity can lead to markedly different evolutionary behaviour. In particular, the possibility of exhibiting evolu- tionary branching requires an emergent carrying capacity. This is significant, since evolutionary branching is regarded as a possible mechanism by which sympatric speciation may occur.
Question: How is variation in offspring size (between broods) related to brood size? Hypotheses: Variance in offspring size (between broods) should decrease with increasing brood size as predicted by Charnov and colleagues' (Charnov and Downhower, 1995; Charnov et al., 1995) small brood invariant. The range in resources put towards reproduction (for mothers producing a certain brood size) should be invariant over brood size (Downhower and Charnov, 1998). We also test assumptions underlying these predictions. Data studied: We use previously collected data on six parasitoid wasp species. Conclusions: As predicted, variance in offspring size among broods decreased with increasing brood size. However, this decrease did not follow closely the quantitative predictions of Charnov and colleagues (Charnov and Downhower, 1995; Charnov et al., 1995). We found some support for the prediction that the range in resources invested in reproduction is invariant over brood size. The assumption that mean offspring size is constant over brood size was violated in three of six species. The assumpt [KEYWORDS: brood size ; litter size ; parasitoid wasps ; resource allocation ; trade-off OPTIMAL OFFSPRING SIZE ; CLUTCH-SIZE ; SEX-CHANGE ; BODY-SIZE ; HYMENOPTERA ; EVOLUTION ; FITNESS ; FIELD ; INVARIANTS ; EULOPHIDAE]
Ecological models of behaviour are typically based on the assumption that decisions can be evaluated with a single resource currency. Here we present models that predict the tactics of consumers collecting two nutritionally distinct resources: fuel that is used for activity and food used for growth (F4G). Both models assume that foragers seek to maximize F4G gain subject to collecting enough fuel for activity. Our first model determines the optimal tempos of foraging for each resource. While foraging for fuel, consumers use and collect the same resource and optimal behaviour is identical to the predictions of a single resource model. However, because consumers use fuel to acquire F4G, they are predicted to work harder to acquire F4G when fuel is more available. Our second model examines how consumers should allocate their time among foraging for fuel, foraging for F4G and defence of F4G sources. Optimal investment in defence increases when fuel is available (because expenditures can be quickly recovered) and when F4G is scarce (because fewer opportunities exist for obtaining new sources of F4G). Our results suggest that behaviours will appear wasteful when foraging environments are fuel-rich and overly frugal when F4G is common but fuel is scarce.
Questions: What are the relative contributions of kin selection and individual selection to the evolution of dispersal rates in fragmented landscapes? How do environmental parameters influence the relative contributions of both evolutionary forces? Features of the model: Individual-based simulation model of a metapopulation. Logistic local growth dynamics and density-dependent dispersal. An optional shuffling algorithm allows the continuous destruction of any genetic structure in the metapopulation. Ranges of key variables: Depending on dispersal mortality (0.05-0.4) and the strength of environmental fluctuations, mean dispersal probability varied between 0.05 and 0.5. Conclusions: For local population sizes of 100 individuals, kin selection alone could account for dispersal probabilities of up to 0.1. It may result in a ten-fold increase of optimal dispersal rates compared with those predicted on the basis of individual selection alone. Such a substantial contribution of kin selection to dispersal is restricted to cases where the overall dispersal probabilities are small (textless 0.1). In the latter case, as much as 30% of the total fitness of dispersing individuals could arise from the increased reproduction of kin left in the natal patch.
This paper was published as Evolutionary Ecology Research, 2009, 11, 949-963. It is available from http://www.evolutionary-ecology.com/v1.html. Metadata only entry Embargoed until September 2010. Full text of this item does not appear in the LRA. Question: How can the evolution of turn-taking be explained in species without language? Features of model: Using a genetic algorithm incorporating mutation and crossover, we studied noisy decision making in three repeated two-player games in which we predicted on theoretical grounds that cooperative turn-taking would evolve and three games in which we expected synchronized cooperation to evolve. Ranges of key variables: We set population size to 20, number of rounds to be played by each pair in each generation to 200, and number of evolutionary generations to 2000, and we repeated each simulation 10 times to check the stability of the results. Results: Cooperative turn-taking and (unexpectedly) a form of double turn-taking evolved in the alternation games, and joint cooperation evolved in the synchronization games. We propose a mechanism to explain how cooperative turn-taking can evolve mechanically, even without communication or insight, as it did in our simulations.