Showing papers in "The American Naturalist in 1994"
TL;DR: It is shown that "nonbiological" gradients in species richness arise inevitably from the assumption of a random latitudinal (or elevational) association between the size and placement of species' ranges, and that a spurious Rapoport effect can be caused by sampling bias alone.
Abstract: G. C. Stevens showed that an equatorward increase in species richness is often paralleled by a decrease in the mean latitudinal range of species-a pattern he called Rapoport's rule. He reported a similar pattern for elevational gradients and suggested that both latitudinal and elevational species richness gradients may be a result of the Rapoport effect. Using null models that assume no environmental gradients, we show that "nonbiological" gradients in species richness arise inevitably from the assumption of a random latitudinal (or elevational) association between the size and placement of species' ranges. These models predict a peak in species richness at tropical latitudes and, at intermediate elevations (regardless of latitude), common patterns in empirical data. Using simulated sampling from a parametric distribution of ranges that incorporates a richness gradient but no Rapoport effect, we then show that a spurious Rapoport effect can be caused by sampling bias alone, even when total sampling effort...
603Â citations
TL;DR: This work develops a general model in which predation is inflicted evenhandedly on two prey species competing for a single resource and shows that the R* and P* rules hold: the winning prey both depresses resources to the lowest level and sustains the higher predator density.
Abstract: Because mechanistic models of interspecific interactions are often complex, one should deliberately seek simple unifying principles that transcend system-specific details. Earlier work on resource ...
583Â citations
TL;DR: It is shown that competition is frequently a diversifying force that creates differences between species and differences within species when other closely related species are absent, and the lake environment can be viewed as a set of non-Hutchinsonian or "environmental" niches that exist apart from the species that occupy them.
Abstract: Biologists concerned with the evolutionary consequences of competition seem unaware of a large literature on freshwater fishes that bears directly on the debates over ecological character displacem...
574Â citations
TL;DR: Tree phenologies from eight disparate tropical forests are consistent with the hypothesis that leaf and flower production have been selected to coincide with seasonal peaks of irradiance.
Abstract: Tree phenologies from eight disparate tropical forests are consistent with the hypothesis that leaf and flower production have been selected to coincide with seasonal peaks of irradiance.
514Â citations
TL;DR: It is argued that sexual selection gradients are the key to understanding how the intensity of sexual selection is affected by mate provisioning, parental investment, and sex ratio.
Abstract: Following principles used by A. J. Bateman, we identify the relationship between fecundity and mating success as the central feature in the operation of mating systems. Using selection theory from the field of quantitative genetics, we define the sexual selection gradient as the average slope of the relationship between fecundity and mating success and show how it can be estimated from data. We argue that sexual selection gradients are the key to understanding how the intensity of sexual selection is affected by mate provisioning, parental investment, and sex ratio.
427Â citations
TL;DR: In this paper, a general model that relaxes the assumptions that resources and organisms are sufficiently mixed that all organisms experience the same resource concentration and the organisms themselves regulate the resource concentration of their shared environment is developed.
Abstract: Classical resource competition theory can be generalized to apply to a variety of specific resource types and specific supply media (e.g., soil, water, or air). We develop a general model that relaxes the assumptions that (1) resources and organisms are sufficiently mixed that all organisms experience the same resource concentration and (2) the organisms themselves regulate the resource concentration of their shared environment. These assumptions are shown to apply to a limited subset of conditions in which the resource input rate is low and the resource transport rate in the environment is high. Under such conditions, the coexistence criteria of our general model converge with those of classical resource competition models. Such conditions may be met in some aquatic environments, but under other conditions, in which resource transport rates may be low or input fluxes high, the general model makes predictions that differ radically from those of the classical models. Specifically, our model predicts that, ...
425Â citations
TL;DR: It is shown that distribution patterns of New England salt-marsh plants are strongly influenced by facilitative associations among neighboring plants, a simple by-product of neighbors buffering one another from potentially limiting physical stresses and thus only occur in physically harsh habitats.
Abstract: The contribution of positive interactions such as facilitations and mutualisms to the structure and organization of natural communities has received little recent attention. Here we show that distribution patterns of New England salt-marsh plants are strongly influenced by facilitative associations among neighboring plants. Positive interactions among marsh plants appear to be the simple by-product of neighbors buffering one another from potentially limiting physical stresses and thus only occur in physically harsh habitats. Positive associations such as these are likely common but unappreciated forces in harsh environments that have been largely overlooked by contemporary ecologists because of their preoccupation with competitive phenomena.
421Â citations
TL;DR: It is argued that the error in this assumption that birds see color patterns as humans do may well be a major reason that support for various evolutionary hypotheses involving color is an area of controversy, and suggests methods for overcoming the shortcomings of existing studies.
Abstract: Many long-standing evolutionary hypotheses make predictions about trends in color patterns. Examples of these include crypsis, mimicry and warning coloration, fruit coloration, flower coloration, the handicap principle of honest advertisement, Fisher's runaway process, the parasite theory of sexual selection, and sensory drive theories of signaling. The majority of tests of these hypotheses, particularly with regard to sexual selection, have been conducted on objects that birds perceive visually, with human vision used to assess color. This assumes that birds see color patterns as humans do, an assumption that is seriously flawed. First, birds see very well parts of the spectrum that humans cannot. Second, birds have at least four dimensions to their color vision, compared to only three in humans. Third, birds have a complex system of oil droplets in their retinas, which may alter the number of hues they perceive. Thus, an object will not appear to have the same hues for a human and a bird, and maybe not ...
408Â citations
TL;DR: The authors decompose the complex relationship between species richness and area into its components: grain, extent, and number of samples, and find that species richness patterns were neither self-similar nor hierarchical.
Abstract: The complex relationship between species richness and area can be simplified by decomposing spatial scale into its components: grain, extent, and number of samples. We designed a 256 x 256-m study grid in the Oosting Natural Area in the Duke Forest, Orange County, North Carolina, such that the effects of these components can be disentangled. We found that grain, extent, and the number of samples all influenced the species-area relationship, although the effects of grain were dominant. We also found that species richness patterns were neither self-similar nor hierarchical. The degree to which diversity occurs in "hot spots" increases as a function of both grain and extent, but diversity hot spots tend to persist across a wide range of grains.
373Â citations
TL;DR: It is suggested that temporally fluctuating selection can indeed contribute significantly to the maintenance of genetic variation when the effects of overlapping generations and age-specific or stage-specific selection are considered, and it is found that an evolutionarily stable population must have positive genetic variance maintained by selection.
Abstract: Population genetics theory suggests that temporally fluctuating selection on pheno- types can act to maintain genetic variance only under very restrictive conditions. However, this conclusion is based on models with discrete nonoverlapping generations. We propose here that temporally fluctuating selection can indeed contribute significantly to the maintenance of genetic variation when the effects of overlapping generations and age-specific or stage-specific selection are considered. We develop a simple model for a population with overlapping generations, experiencing stabilizing selection with a temporally fluctuating optimum, and subject to repeated invasions by mutants with alternative phenotypes. We find that an evolutionarily stable popula- tion must have positive genetic variance maintained by selection so long as the product (variance of fluctuations) times (amount of generation overlap) times (selection intensity) is sufficiently high. This result applies to haploid, diploid, single-locus, or multilocus inheritance, and it does not depend on any form of heterozygote advantage to maintain genetic variance. However, it depends on the map between genotype and phenotype being constrained. If a single genotype can produce an arbitrary distribution of phenotypes, then genetic variance is not maintained by selection.
367Â citations
TL;DR: The results for the analytically tractable models appear to explain the cause and pattern of coexistence in the simulation model of a complex forest-simulation model of competition among individuals for light and a nutrient.
Abstract: We study two kinds of mechanistic and spatial models of plant competition in heterogeneous environments. First, we study lottery models in which the outcome of competition among juveniles for vacant space is determined by one of several underlying mechanistic submodels. Environmental heterogeneity that affects the outcome of competition is present in all of these models and may be spatial, temporal, biotic in origin, abiotic in origin, or any mixture of these. The principal result is that the models predict coexistence if the plant species are sufficiently dissimilar-if they sufficiently partition the spatial, temporal, biotic, or abiotic environmental heterogeneity. Second, we study a complex forest-simulation model of competition among individuals for light and a nutrient. Whereas the nonspatial version of this model predicts that a single species will outcompete all others, the spatial version predicts that coexistence is possible. We show that the results for the analytically tractable models appear t...
TL;DR: At least four factors that influence the relative fitness of generalists and specialists have been identified and activity selection has been modeled in two ways.
Abstract: Species are often faced with an evolutionary trade-off between performing a few activities well (specialists) and many activities poorly (generalists). This trade-off is a central concern for evolutionary ecologists interested in the coexistence of species and the coexistence of genotypes within species. At least four factors that influence the relative fitness of generalists and specialists have been identified (table 1). The first factor is the shape of the fitness et. Following Levins (1962, 1968), figure 1 shows fitness associated with two activities, such as resources that require different techniques to capture or habitats that require different behaviors to exploit successfully. Each genotype is represented by a point, and the set of genotypes that exist in the population is represented by a line. Both fitness ets in figure 1have a negative slope, which indicates that fitness associated with one activity can be increased only at the expense of fitness associated with the other activity. Generalists are only slightly inferior to the specialists in the convex fitness et (A) and are greatly inferior to the specialists in the concave fitness et (B). A variety of models conclude that the fitness et must be concave for the advantages of specialization to outweigh the costs (see, e.g., Levins 1962, 1968; MacArthur and Levins 1964, 1967; MacArthur and Pianka 1966; Pulliam 1974; Rosenzweig 1974; Lawlor and Maynard Smith 1976; Wilson and Turelli 1986). It is important to emphasize that fitness ets do not always have a negative slope. Some activities can be performed well without compromising other activities, which leads to a jack of all trades that is also a master of all. Several authors have properly cautioned that trade-offs between activities must be demonstrated and not assumed (e.g., Bernays and Graham 1988; Futuyma nd Moreno 1988). Nevertheless, few would doubt the basic existence of trade-offs or their importance in the evolution of specialization. The second factor affecting fitness is activity selection. The foregoing models assume that each genotype performs both activities so that fitness is a weighted average of the xand y-coordinate values. If animals can choose to perform one activity and avoid the other, however, specialists can be favored even when the fitness et is convex because the cost of specializing-inferior performance at the other activity-is seldom experienced (see, e.g., Imanishi 1938, 1941, 1949; Kani 1944; Svardson 1949; MacArthur and Pianka 1966; Charnov 1976). Activity selection has been modeled in two ways. First, we can assume that a
TL;DR: A dynamic model for adaptive seasonal decline in clutch size is constructed and it is expected that such conflicts between the advantages of early and late transitions are common to life-history decisions.
Abstract: A seasonal decline in clutch size is typical of bird populations. This phenomenon may result from a conflict between the advantages of early breeding (greater offspring value) and the advantages of delay (greater accumulated condition and hence clutch size). We construct a dynamic model for adaptive seasonal decline in clutch size on the basis of these premises. The model requires a small number of well-supported assumptions; it is formulated and analyzed in both analytical and graphical forms. We outline some novel predictions and suggest tests of our conclusions. Initial comparisons of our predictions with results available from wild birds in the field are favorable. We briefly extend the model to consider the evolution of multiple clutches within a season and the effects of between-season costs of reproduction. Although this work focuses on avian clutch size, the analysis provides a general framework for studies of conditiondependent transitions in life histories. Such transitions between stages (e.g.,...
TL;DR: Counter to the current fashion, which downplays the importance of inbreeding in stochastic environments, it is concluded that, while inbreeding depression is not necessarily the primary cause of extinction, it can be critical.
Abstract: Although traditional population fragmentation theory and management has been strongly oriented toward concerns arising from inbreeding depression, recent papers suggest that small populations will be eliminated by demographic and/or environmental events before inbreeding becomes a problem. We explore the interaction between these factors by developing a stochastic, discrete time Leslie model that incorporates inbreeding depression. We model small population dynamics with three realistic demographic schedules: low growth rate "ungulates," medium growth rate "felids," and high growth rate "rodents," examining the impact of survival and fertility depression commensurate with inbreeding effects reported in the literature. Focusing on the first few generations after habitat fragmentation and isolation, we find that (a) high growth rate populations are affected only by strong inbreeding depression, but low growth rate populations are extremely vulnerable to even minor inbreeding depression; (b) vulnerability to...
TL;DR: Analysis of the properties of the mathematical model of the within-host population dynamics of an invading microparasite and the immune response are interpreted as support for the proposition that the immune system may be responsible for the evolution and maintenance of the virulence of those microparsites that it controls and clears.
Abstract: We examine effects of the vertebrate immune system on the evolution and maintenance of virulence of microparasites (viruses, bacteria, or unicellular eukaryotes) We employ a mathematical model of the within-host population dynamics of an invading microparasite and the immune response In this model, the virulence of the parasite is proportional to its rate of replication within the host We show that if the rate of transmission of parasites from infected hosts is proportional to their within-host density, then the total transmission reaches a maximum for parasites with intermediate rather than low or high virulence When hosts can only be infected with a single parasite strain, then parasites having intermediate virulence not only release the most transmissible forms from individual infected hosts but also drive other parasite strains to extinction in the population at large We interpret the results of our analysis of the properties of this model as support for the proposition that the immune system may
TL;DR: A nonparametric method to estimate the form of multivariate selection on a suite of quantitative traits and emphasizes the ubiquity of "correlational" selection and illustrates how traits jointly determine fitness.
Abstract: We present a nonparametric method to estimate the form of multivariate selection on a suite of quantitative traits. Its advantages are threefold. First, the procedure is flexible and does not force estimates of the surface to conform to a specific mathematical shape. The need for a flexible method is illustrated by an example using quadratic regression. Second, estimates of multidimensional surfaces can be visualized in two or three dimensions. This simplification is accomplished by making cross sections of the surface in the few most interesting directions. Finally, the method is designed to handle survival and other nonnormal fitness components. We apply the procedure to two data sets. In song sparrows, the survival surface is approximated by a ridge favoring an allometric relation between body mass and wing length. Survival in human infants rises steeply with increasing birth mass and maternal gestation period to a broad flat dome. Our results emphasize the ubiquity of "correlational" selection and ill...
TL;DR: It is suggested that the immunosuppression by sexual hormones is indeed in the animal's interest and therefore adaptive and how signals based on sex hormones could be evolutionarily stable is suggested.
Abstract: Recently, Folstad and Karter (1992) suggested a new mechanistic model of parasite-driven sexual selection (Hamilton and Zuk 1982). Their idea was based on the finding that sex hormones, such as testosterone, have a dual effect: on the one hand, they induce development of secondary sexual traits, and, on the other hand, they suppress the immune system. Folstad and Karter concluded that a major cost of sexual ornamentation may be immunosuppression and the associated pathogenic effects of parasite populations that are consequently less well controlled by hosts. They further suggested that \"testosterone profiles and/or responsiveness, and consequently both immunosuppression and the expression of male secondary sexual characters, are self-regulated in response to parasite burden\" (p. 605). Thus, secondary sexual traits should have the potential to reliably reveal current parasite burden. By presenting this model, Folstad and Karter (1992) drew attention to the potential of including both immunology and endocrinology in evolutionary hypotheses. However, their interpretation of immunosuppression by sex hormones as a nonadaptive phenomenon begs the question of how signals based on sex hormones could be evolutionarily stable. They envision one scenario, but within their signaling system a mutant eliminating one of the connections among the immune system, sex hormones, and sexual ornamentation could possibly succeed as a cheater. The connection between sexual hormones and the immune system could be prevented by omitting the specific hormone receptors (see, e.g., Grossman and Roselle 1986) in tissues of the immune system. Additionally, the connection between hormones and sexual ornaments could be eliminated in a mutant using other ways to induce its sexual traits (e.g., via the nervous system or with any substances that need not negatively affect the immune system). The reliability of the signal could still persist through the energetic and metabolic requirements of the traits (Zahavi 1977; Grafen 1990a, 1990b; Iwasa et al. 1991). As an alternative causal explanation of the seemingly paradoxical properties of sexual hormones, we suggest that the immunosuppression by sexual hormones is indeed in the animal's interest and therefore adaptive. Hormones are chemical messengers that regulate the activity of certain tissues in the body (Hadley 1988). Their immunosuppressive ffects might be one of the consequences of an adaptive resource reallocation (Wedekind 1992). That is, the development of the sexual ornamentation requires energy and metabolites whose availability may be limited.
TL;DR: Under many circumstances, female animals, or the female function of a hermaphroditic plant or animal, can experience sexual selection.
Abstract: A. J. Bateman's approach to animal mating systems is generalized to provide a framework for studying sexual selection in plants and hermaphroditic animals. The key feature of this generalization is the realization that the relationship between fecundity and mating success can take a variety of forms beyond those observed in Bateman's data. Thus, under many circumstances, female animals, or the female function of a hermaphroditic plant or animal, can experience sexual selection. Path diagrams are used to show that sexual selection is a component of fecundity selection. Finally, statistical procedures are discussed for measuring sexual selection in plant and animal populations with separate or combined sexes.
TL;DR: This review integrates recent advances in these fields around the central theme of kin recognition, showing how behavior, disease, and molecular genetics can be integrated in a context of evolution and natural selection.
Abstract: The major histocompatibility complex (MHC) has been theorized to play a critical role in kin recognition in two contexts, mating and cooperation. Recent work has greatly strengthened this view. The MHC is uniquely important for understanding the evolution of kin recognition because so much is known about its genetics, cell biology, and molecular evolutionary history. Our review integrates recent advances in these fields around the central theme of kin recognition. The chemical cues involved in mammals are controversial and appear to involve microorganisms in some cases. The genes that code MHC glycoproteins, although highly diverse within many species, are similar across generic boundaries in primates and rodents. The potential mechanisms of selection for the incredible allelic diversity at MHC loci are as controversial as ever, but they now also include behavioral mechanisms of disassortative mating and inbreeding avoidance. These studies show how behavior, disease, and molecular genetics can be integrat...
TL;DR: It is concluded that the maximum intake rate of mammalian herbivores will scale closely with the scaling of daily energy requirements, and this conclusion was corroborated by 39 published observations of the maximum I.
Abstract: The rate of food intake exerts an important influence on many aspects of herbivore ecology, including diet and habitat choices, social organization, and predator avoidance. When food is spatially concentrated, short-term dry matter intake rate (I, g/min) is determined largely by morphology of the mouth and mechanics of food consumption. Morphology (tooth size and jaw musculature) and mechanics (cropping and chewing processes) are hypothesized to scale with body mass (M) for mammalian herbivores. By using a simple model of processes regulating short-term I, we developed and tested hypotheses on the scaling of these parameters in 12 species of mammalian herbivores whose masses ranged from 0.05 kg to 547 kg. Specifically, this model predicts that I is controlled by the size of bite taken, by the time required to crop a bite, and by the rate at which food in the mouth can be processed. Maximum bite size scaled with M0 72, whereas cropping time did not scale with body mass and averaged 0.015 min/bite across sp...
TL;DR: The independently derived genotypes are somewhat more variable in these life-history traits than in their relative fitnesses, which indicates that they acquired different genetic adaptations to the seasonal environment.
Abstract: Twelve populations of the bacterium Escherichia coli were propagated for 2,000 generations in a seasonal environment, which consisted of alternating periods of feast and famine. The mean fitness of the derived genotypes increased by -35% relative to their common ancestor, based on competition experiments in the same environment. The bacteria could have adapted, in principle, by decreasing their lag prior to growth upon transfer to fresh medium (L), increasing their maximum growth rate (Vm), reducing the concentration of resource required to support growth at half the maximum rate (Ks), and reducing their death rate after the limiting resource was exhausted (D). We estimated these parameters for the ancestor and then calculated the opportunity for selection on each parameter. The inferred selection gradients for Vm and L were much steeper than for Ks and D. The derived genotypes showed significant improvement in Vm and L but not in Ks or D. Also, the numerical yield in pure culture of the derived genotypes was significantly lower than the yield of the common ancestor, but the average cell size was much larger. The independently derived genotypes are somewhat more variable in these life- history traits than in their relative fitnesses, which indicates that they acquired different genetic adaptations to the seasonal environment. Nonetheless, the evolutionary changes in life-history traits exhibit substantial parallelism among the replicate populations.
TL;DR: The thesis of adaptationism is clarified, how the structure of optimality models relates to that thesis is shown, and how the thesis ofAdaptationism is testable are described.
Abstract: The use of optimality models in the investigation of adaptation remains controversial. Critics charge that advocates of the optimality approach assume that the traits they analyze are optimal. Advocates of the approach deny this but admit to assuming that the traits have adaptive explanations. This controversy is part of the ongoing debate about adaptationism. We believe that this controversy remains unresolved in part because of ambiguity in the definition of adaptationism. In this article, we clarify the thesis of adaptationism, show how the structure of optimality models relates to that thesis, and describe how the thesis of adaptationism is testable. In addition, we describe the types of analyses that are essential to a test of an optimality model if the optimality of the trait is to be assessed and if assessments of the success of specific models are to contribute to a test of adaptationism. These analyses allow one to distinguish between the hypothesis that natural selection has had some influence o...
TL;DR: Hypotheses to explain the maintenance of male ejaculate delivery patterns that are consistent with sperm competition and bet-hedging theory are examined, as are potential selection pressures responsible for sperm-size evolution.
Abstract: The gametic strategy of males comprises the amount of energy invested per sperm, the total amount invested in sperm production, and the pattern of sperm allocation among successive reproductive bouts. All of these variables were measured for each of the four species constituting the nannoptera species group of the Drosophilidae. Extreme interspecific variation was identified for all variables and enigmatic male reproductive strategies, including submaximal insemination of females, partitioning of ejaculate among successive mates, and production of few large sperm, were observed. Variation among species in female remating behavior was found to occur concomitantly with male remating behavior, probably because of female fertility demands. Relationships among testes size, sperm size, sperm numbers, and mating systems in these fruit flies are examined. These relationships are not consistent with patterns identified in studies of vertebrate taxa and suggest fundamental differences between vertebrates and invert...
TL;DR: A modified version of the polygyny threshold model that takes female aggression into account is presented and shows that secondary females generally have a reduced reproductive success compared with simultaneous, monogamous females.
Abstract: In many polygynous birds the reproductive success of females is strongly dependent on male parental care, and females mated with the same male will compete for a limited amount of male assistance. Mated females may benefit from trying to prevent or delay the settlement of other females. Hence, female aggression may affect male mating success and thus play a role in the evolution of avian mating systems. We present a modified version of the polygyny thresh- old model that takes female aggression into account. A review on female aggression, male allocation of parental care, and delayed breeding of secondary females supports the model. An alternative model also predicts delayed breeding of secondary females when they benefit from reducing the breeding overlap with the respective primary females. However, recent studies suggest that secondary females may benefit from breeding as early as possible relative to the primary female. Our review also shows that secondary females generally have a reduced repro- ductive success compared with simultaneous, monogamous females. This difference may arise from a restricted mate search by females and does not necessarily contradict the polygyny threshold model.
TL;DR: It is suggested that there is selection at the MHC involved with maternal-fetal interactions and nonrandom mating, and can be considered another classic example of an adaptive polymorphism, one that may be used as an evolutionary genetic paradigm.
Abstract: The major histocompatibility complex (MHC) is becoming one of the most thoroughly investigated regions in vertebrate genomes. This region was first studied because of its importance in tissue transplantation and the immune system in humans. Now there is detailed genetic knowledge on many of the over 80 genes in the MHC, both about the extent and pattern of genetic variation in this region and about the molecular structure and functions of the molecules coded by its genes. For example, the level of amino acid heterozygosity is often extremely high, reaching over 60% at some amino acid sites with primary immune function (i.e., those involved with presenting peptides to the T-cell receptor). The frequency distribution of alleles at many of the loci in this region is quite even, and, in some populations, there is an observed deficiency of homozygotes. These and other observations point to the importance of some type of balancing selection operating in this region. It is generally assumed that this selection i...
TL;DR: The results provide the first quantitative evidence that allometry for SSD may evolve in response to sexual selection favoring large males, even in taxa in which females are the larger sex.
Abstract: Sexual size dimorphism (SSD) typically increases with body size (hyperallometry) in taxa in which males are the larger sex and decreases with body size (hypoallometry) in taxa in which females are larger. We demonstrate the commonality of these trends, both of which indicate greater evolutionary divergence in male size than in female size and strong covariation between the sexes. We postulate that both of these components of allometry evolve in response to sexual selection on male size coupled with genetic correlations for size between males and females, and we argue that this hypothesis can be generalized to taxa in which females are the larger sex. For such taxa, we predict hypoallometry for SSD, sexual selection on male size, and a correlation between the intensity of sexual selection and male size. An analysis of total length in 31 populations of the water strider, Aquarius remigis, demonstrates significant hypoallometry for SSD. Comparisons of mating and single males within 12 populations reveal sign...
TL;DR: In general, increases in adult longevity select against seed dormancy, but exceptions to this pattern are discussed and the idea that established plant traits are uncoupled from those of the regenerative phase, as assumed by J. P. Grime's competition-stress-ruderal model, is considered critically.
Abstract: The effects of adult longevity, the timing of reproduction, and population age/stage structure on the evolution of seed dormancy are explored in both constant and variable environment models. In the constant environment models complete germination is the evolutionarily stable strategy (ESS) regardless of adult longevity. Incorporating a cost of reproduction on subsequent survival does not alter this result. In contrast, in a variable environment changes in adult longevity can exert a strong selection pressure against seed dormancy. Incorporating a cost of reproduction for iteroparous species reduces adult longevity, which selects for more seed dormancy. The magnitude of the change in ESS germination probability depends on several factors, including which life-history stage is variable (e.g., fecundity, seedling survival), whether seeds can detect favorable sites for establishment, and the age/stage structure of the population. In general, increases in adult longevity select against seed dormancy, but exce...
TL;DR: It is shown that many different evolutionary mechanisms can, in principle, account for any one interspecific pattern, and it is argued that these confounded mechanisms can only be unraveled if patterns of selection or genetic variation and covariation are directly measured in many species within a clade.
Abstract: It has been suggested recently that new quantitative methods for analyzing comparative data permit the identification of evolutionary processes. Specifically, it has been proposed that new comparative methods can distinguish the direct effects of natural selection on the distribution of a trait within a clade from the effects of drift, indirect selection, genotype-by-environment interaction, and uncontrolled environmental variation. Such methods can supposedly unravel the relative importance of these factors by the phylogenetic analysis of traits, performance attributes, and habitats. We argue that they cannot. We show that many different evolutionary mechanisms can, in principle, account for any one interspecific pattern, and we illustrate our case using examples from the comparative literature. We argue that these confounded mechanisms can only be unraveled if patterns of selection or genetic variation and covariation are directly measured in many species within a clade. Even though comparative methods ...
TL;DR: This work presents a path model that contradicts Bateman's principle by asserting that floral attractiveness characters might well affect fitness more deterministically through female than through male function and envisioning an episodic selection scenario that has the same outcome as Batemen's principle but is based specifically on the ecology and mechanics of pollination.
Abstract: For over a decade, Bateman's principle has been used to argue that the showy petals and sweet nectar of flowers are evolutionarily more male than female-that they are adaptations principally for promoting the export of pollen rather than the setting of seed. Here we present alternative views. (1) We question whether the assumptions of Bateman's principle have been generally upheld for angiosperms. (2) We present a path model that contradicts Bateman's principle by asserting that floral attractiveness characters might well affect fitness more deter- ministically through female than through male function. (3) We envision an episodic selection scenario that has the same outcome as Bateman's principle but is based specifically on the ecology and mechanics of pollination. In the end, we recognize that selection on the displays and rewards of flowers is probably often gender biased (one way or the other), but we warn against reflexive invocation of Bateman's principle, which is neither the only nor the best way to think about the problem.
TL;DR: Analysis of a simulation model of an environment composed of discrete patches that persist for a fixed period and a generalized life-history pattern shows that a dominant brachyptery allele can readily spread in a monomorphically macropterous population and that at equilibrium genetic variability is maintained.
Abstract: Wing dimorphism occurs commonly among many species of insects. This dimorphism is hypothesized to be maintained by a trade-off between the costs of being macropterous (winged, flight capable) and the long-term benefits of migration in a heterogeneous environment. In this article this hypothesis is investigated with a simulation model consisting of an environment composed of discrete patches that persist for a fixed period and a generalized life-history pattern. The analysis shows that a dominant brachyptery (rudimentary wings, flight incapable) allele can readily spread in a monomorphically macropterous population and that at equilibrium genetic variability is maintained. The invasion of the brachyptery allele depresses population size but enables the population to spread into regions in which the monomorphically macropterous population cannot persist. Further analysis of the model shows that patch persistence time (T), the cost of being macropterous (c), the proportion of migrants (m), and the probabilit...