Showing papers in "The American Naturalist in 1997"

Phylogenies and the comparative method: A general approach to incorporating phylogenetic information into the analysis of interspecific data

Abstract: This article considers the statistical issues relevant to the comparative method in evolutionary biology. A generalized linear model (GLM) is presented for the analysis of compar- ative data, which can be used to address questions regarding the relationship between traits or between traits and environments, the rate of phenotypic evolution, the degree of phylogenetic effect, and the ancestral state of a character. Our approach thus emphasizes the similarity among evolutionary questions asked in comparative studies. We then discuss ways of specifying the sources of error involved in a comparative study (e.g., measurement error, error due to evolution along a phylogeny, error due to misspecification of a phylogeny) and show how the impact of these sources of error can be taken into account in a comparative analysis. In contrast to most existing phylogenetic comparative methods, our procedure offers substantial flexibility in the choice of microevolutionary assumptions underlying the statistical analysis, allowing researchers to choose assumptions that are most appropriate for their particular set of data and evolutionary question. In developing the approach, we also propose novel ways of incorporating within- species variation and/or measurement error into phylogenetic analyses, of estimating ancestral states, and of considering both continuous (quantitative) and categorical (qualitative or "state") characters in the same analysis. In the last few years, there has been considerable debate regarding what can

Evolution of a Species' Range

Abstract: Gene flow from the center of a species' range can stymie adaptation at the periphery and prevent the range from expanding outward. We study this process using simple models that track both demography and the evolution of a quantitative trait in a population that is continuously distributed in space. Stabilizing selection acts on the trait and favors an optimum phenotype that changes linearly across the habitat. One of three outcomes is possible: the species will become extinct, expand to fill all of the available habitat, or be confined to a limited range in which it is sufficiently adapted to allow population growth. When the environment changes rapidly in space, increased migration inhibits local adaptation and so decreases the species' total population size. Gene flow can cause enough maladaptation that the peripheral half of a species' range acts as a demographic sink. The trait's genetic variance has little effect on species persistence or the size of the range when gene flow is sufficiently...

A theoretical framework for intraguild predation

Abstract: Many important issues in community ecology revolve around the interplay of competition and predation Species that compete may also be locked in predator-prey interactions, a mixture of competition and predation known as "intraguild predation" (IGP) There is growing evidence for the importance of IGP in many natural communities, yet little formal ecological theory addresses this particular blend of interactions In this article, we explore the consequences of incorporating IGP into standard models of exploitative competition and food chains (a general resource-consumer model, a Lotka-Volterra food chain model, and Schoener's exploitative competition model) Our theoretical analyses suggest a general criterion for coexistence in IGP systems: the intermediate species (the prey in intraguild predation) should be superior at exploitative competition for the shared resource, whereas the top species (the predator) should gain significantly from its consumption of the intermediate species Along gradients in en

The Roles of Harsh and Fluctuating Conditions in the Dynamics of Ecological Communities

Abstract: Harsh conditions (e.g., mortality and stress) reduce population growth rates directly; secondarily, they may reduce the intensity of interactions between organisms. Near‐exclusive focus on...

Sperm competition in fishes: the evolution of testis size and ejaculate characteristics.

Abstract: Fishes show one of the widest ranges of sperm competition intensity of any animal group. Here we present a comparative study whose aim is to investigate the effect of relative intensity of sperm competition on investment in spermatogenesis and the number and size of sperm produced. We find that both the gonadosomatic index (GSI5 (gonad weight/body weight) 3 100) and sperm numbers increase with intensity of sperm competition across species but that sperm length decreases. These new findings are consistent with a raffle-based mode of sperm competition in fishes. Most of these results (positive correlation of the GSI and sperm number with sperm competition intensity) concur with the predictions of current sperm competition the- ory. However, we also find that sperm longevity decreases with sperm length across species. Cur- rent models for continuous fertilization suggest that if length increases a sperm's speed but de- creases its longevity, sperm length should increase with sperm competition intensity, whereas models for instant fertilization suggest that sperm length should remain constant. The negative relationship found between sperm competition and sperm length therefore does not fit predictions of either model.

The relationship among area, elevation, and regional species richness in neotropical birds.

Abstract: The elevational gradient of species richness is often claimed to mirror the latitudinal gradient and has traditionally been explained by assuming a decrease in productivity with eleva- tion and more recently by Rapoport's rule. The influence of area on the pattern has rarely been considered. Analyses of all South American tropical land birds (more than one-fourth of the ex- tant bird species on Earth) are used to examine four species richness/elevation models: null model, Rapoport's rule, and monotonic or hump-shaped productivity/species richness relation- ships. To quantify the area effect, species-area curves were created for seven elevational zones. Not accounting for area, species richness declined monotonically with elevation, but area ac- counted for 67%-91% of the variation in species richness per zone. When area was factored out, a hump-shaped pattern emerged, with more species in the 500-1,000-m (P , .005) and 1,000- 1,500-m zones (P , .10) than in the 0-500-m zone. Rapoport's rule and the monotonic productivity/species richness relationship were thus not supported. Instead, elevational turnover rates and numbers of shared species between zones suggested that the hump-shaped pattern re- flects geometric constraints (as predicted by the null model) imposed by the narrow span of the gradient, and it is suggested that midelevational zones may represent sink habitats.

The Light: Nutrient Ratio in Lakes: The Balance of Energy and Materials Affects Ecosystem Structure and Process

Abstract: The amounts of solar energy and materials are two of the chief factors determining ecosystem structure and process. Here, we examine the relative balance of light and phosphorus in a set of freshwater pelagic ecosystems. We calculated a ratio of light: phosphorus by putting mixed‐layer mean light in the numerator and total P concentration in the denominator. This light: phosphorus ratio was a good predictor of the C:P ratio of particulate matter (seston), with a positive correlation demonstrated between these two ratios. We argue that the balance between light and nutrients controls “nutrient use efficiency” at the base of the food web in lakes. Thus, when light energy is high relative to nutrient availability, the base of the food web is carbon rich and phosphorus poor. In the opposite case, where light is relatively less available compared to nutrients, the base of the food web is relatively P rich. The significance of this relationship lies in the fact that the composition of sestonic material...

Developmental stability and fitness: a review.

Abstract: Developmental stability reflects the ability of individuals to undergo stable development of their phenotype under a range of environmental conditions. Developmental instability is measured in terms of fluctuating asymmetry or phenodeviance. A negative relationship between developmental instability and fitness has figured as a prominent untested assumption in the literature. A review of available information from the literature on the relationship between developmental instability and various fitness components such as growth, fecundity, and longevity suggests that there indeed is a general negative relationship. Symmetrical individuals do generally have faster growth, higher fecundity, and better survival than do more asymmetrical individuals. These differences appear partially to arise from lower competitive ability and higher risks of predation and parasitism of asymmetrical individuals compared with more symmetrical conspecifics. The relationship between developmental stability and fitness ma...

Secondary Metabolites Of Fleshy Vertebrate‐Dispersed Fruits: Adaptive Hypotheses And Implications For Seed Dispersal

Abstract: We discuss seven hypotheses to explain the adaptive significance of secondary me- tabolites in ripe fleshy fruits and their implications for seed dispersal. These hypotheses are the attraction/association, seed germination inhibition, attraction/repulsion, protein assimilation, gut retention time, directed toxicity, and defense trade-off hypotheses. We examine evidence that supports or refutes these hypotheses and suggest further tests of each. In addition, we summarize recent work with Solanum fruit pulp glycoalkaloids that bears directly on three of these hypothe- ses (directed toxicity, gut retention time, and defense trade-off ). We conclude that evidence ad- dressing many of these hypotheses is either observational or indirect, but most hypotheses find at least some level of support. Because many of the hypotheses are not mutually exclusive, we also conclude that synergistic interactions and multifunctionality in secondary metabolites may provide economical evolutionary solutions for plants facing disparate and temporally variable se- lective pressures that impinge on fruits and seeds.

How Does Immigration Influence Local Adaptation? A Reexamination of a Familiar Paradigm

Abstract: All species are distributed in space, but within limits. What determines these limits through evolutionary time is an important issue that is receiving increased attention (Hoffman and Blows 1994). For instance, paleobiologists have suggested that species' climatic range limits often reveal long-term stasis (Huntley et al. 1989), which presumably reflects stasis in basic ecological traits (e.g., physiological tolerance to climatic extremes). Stasis in species' borders provides a particularly compelling category of \"niche conservatism\" -the observation that phylogenetic lineages often seem to retain much the same ecological niche over substantial spans of evolutionary history (Holt and Gaines 1992; Ricklefs and Latham 1992; Holt 1996). There are three basic classes of explanations for niche conservatism, particularly at species borders: an absence of appropriate genetic variation (Bradshaw 1991; Bradshaw and McNeilly 1991); conservative selection, either within populations (Holt and Gaines 1992; Holt 1996) or among populations (Gomulkiewicz and Holt 1995), preventing evolutionary change in the niche; and gene flow swamping local selection (Mayr 1963). The third class of explanations may apply even if gene flow is not great enough to prevent local differentiation n response to spatially varying selection in much of the interior of a species' range (Endler 1977). Antonovics (1976) observed that peripheral populations may exist at densities considerably lower than more central populations. Rates of immigration that are low in absolute terms but high relative to the local population's abundance may suffice to swamp out local selec-

A comparative analysis of allometry for sexual size dimorphism: assessing rensch's rule

Abstract: Rensch's rule states that sexual size dimorphism (SSD) increases with body size (hyperallometry) in taxa in which males are the larger sex and decreases with body size (hypoallometry) in those in which females are larger. We use the independent contrasts method to assess the validity and generality of Rensch's rule within 21 independent animal taxa. Allometry is estimated as the slope of the major axis regression of contrasts for log(female size) versus contrasts for log(male size). Allometry consistent with Rensch's rule is significant in 33% of the taxa examined across a diverse range of invertebrate and vertebrate taxa. Significant allometry inconsistent with Rensch's rule occurs in only one taxon. Meta-analysis of these results reveals that Rensch's rule is general and highly significant. Only owls have allometry inconsistent with this trend. Rensch's rule is also shown to be associated with male-biased SSD, which is consistent with the hypothesis that sexual selection acting on male size drives the e...

Egg size plasticity in a seed beetle : an adaptive maternal effect

Abstract: In the seed beetle, Stator limbatus, the fitness consequences of egg size vary substantially among host plants. There is intense selection for laying large eggs when larvae will develop on seeds of Cercidium floridum (caused by high mortality penetrating the seed coat) but selection for laying small eggs when larvae will develop on seeds of Acacia greggii (caused by very low mortality penetrating the seed coat and an egg size/egg number trade-off). We test the hypothesis that host-associated variation in egg size within populations of S. limbatus represents an adaptive maternal effect in which females adjust egg size in response to host species. In laboratory experiments, S. limbatus females laid significantly larger and fewer eggs on C. floridum than on A. greggii. When switched between hosts, females readjusted egg size, producing progressively larger eggs on C. floridum and smaller eggs on A. greggii. When conditioned to lay either small eggs (on A. greggii) or large eggs (on C. floridum), and then for...

A "Good-Sperm" Model Can Explain the Evolution of Costly Multiple Mating by Females

Abstract: Multiple mating by females (or polyandry) is one of the most controversial subjects in sexual selection and mating systems theory (Smith 1984; Halliday and Arnold 1987; Birkhead and Moller 1992). In many animals, mass production of sperm is generally far cheaper than production of ova (but see Dewsbury 1982). Thus, multiple mating is clearly adaptive for males because the more females with which males mate, the more offspring they can sire (Trivers 1972). However, females can produce no more offspring than the number of their ova, and females in many species receive a lifetime supply of sperm from one mating or one mate (Thornhill and Alcock 1983). It is, therefore, difficult to explain the adaptive significance of polyandry from the viewpoint of female advantage. In many cases, females seem to obtain some direct benefit from males (e.g., nutrients in the seminal fluid, protection against predators, paternal care of offspring). However, in species in which no direct benefit can be detected, the females have been supposed to obtain some \"genetic benefit\" from additional mating. Since Walker's (1980) seminal article was published, two hypotheses have been advanced to explain the genetic benefit of polyandry: increased genetic diversity of offspring and acquisition of \"good genes.\" The first hypothesis i not promising. Meiosis and recombination can produce great genetic diversity in the sperm pool of one male (Williams 1975); for example, a male human (2n = 46) can produce 223 different types of spermatozoon without chromosomal crossover and translocation at the meiosis. Moreover, females never produce enough eggs to make full use of such sperm diversity from one mating. Therefore, many authors doubt whether multiple mating by females can actually increase genetic diversity of their offspring and, if any greater diversity is achieved, whether the diverse offspring are really beneficial to the females because an increase in genetic diversity (i.e., decrease in relatedness) within clutches may intensify sib competition (Sherman 1981; Westneat et al. 1990; Birkhead and Moller 1992). The good-gene hypothesis for polyandry assumes that when females encounter better males than the previous mates, they remate and fertilize ggs with the better male's sperm. Thus, this hypothesis regards the polyandry as a means of

An empirically based model for latitudinal gradient in vole population dynamics.

Abstract: Vole dynamics in northern Europe exhibit a well-defined geographical gradient, with oscillatory populations being confined to high latitudes. It has been proposed that oscillations in northern vole populations are driven by their interaction with specialist predators (weasels), while the more southern rodent populations are relatively stable because of regulation by generalist predators. We tested this generalist/specialist predation hypothesis by constructing an empirically based model for vole population dynamics, estimating its parameters, and makng predictions about the quantitative pattern of the latitudinal shift in vole dynamics. Our results indicated that the model accurately predicted the latitudinal shift in the amplitude and periodicity of population fluctuations. Moreover, the model predicted that vole dynamics should shift from stable to cha- otic as latitude is increased, a result in agreement with nonlinear time-series analysis of the data. The striking success of the model at predicting the shifts in amplitude and stability along the geographical gradient in northern Europe provides strong support for the key role of specialist and generalist predators in vole population dynamics.

Hybridization, Sexual Imprinting, and Mate Choice

Abstract: Hybridization of animal species is a special pattern of mating and hence a behavioral phenomenon, influenced by ecological, demographic, and individual factors. We examine demographic factors (sex ratio and mate availability) and characteristics of individuals (song and morphology) in an attempt to understand the occasional hybridization of three species of Darwin's finches (genus Geospiza) on the small Galapagos island of Daphne Major. We use field data from a 20-yr study to test five hypotheses. A relative scarcity of conspecific mates (the Hubbs principle) explains the hybridization of the rarer species (Geospiza fuliginosa and Geospiza scandens) but not the common species (Geospiza fortis). Female G. fortis pair with male Geospiza fuliginosa that sing songs similar to their own fathers' songs. Morphological trait values of their G. fuliginosa mates were also correlated with the trait values of their fathers. Geospiza fortis females pair with G. scandens that are morphologically similar to their mother...

Interspecific allometries are by-products of body size optimization

Abstract: Many physiological and life-history traits correlate with body weight in interspecific comparisons. To explain these allometries, we assume that the parameters of within-species functions describing the size dependence of production and mortality rates differ between species of the same taxon and that natural selection has optimized body size through optimal allocation of resources to growth and reproduction independently in each species. In a simple simulation model, we obtain good interspecific allometries for respiration, assimilation, production rates, age at maturity, and life expectancy. Some correlations, for example, those between age at maturity and life expectancy, remain significant after the effect of body size is removed. The slopes of the allometries depend not only on the average values of the parameters but also on their coefficients of variation. We show analytically how these slopes are determined for a simplifed model and how body size distributions are determined. In our model, intersp...

Group selection and assortative interactions

Abstract: Natural selection at all levels requires heritable phenotypic variation among units. At the group level, variation is often increased by reproduction coupled with limited dispersal, which forms the basis of kin selection and traditional group selection models. Assortative interactions are another possible mechanism for creating variation among groups that has received less attention. We present a series of models in which altruism is a continuously varying trait and individuals are free to choose their associates, based on information that is acquired through experience, observation, or cultural transmission. Assortative interactions can generate highly nonrandom variation among groups, favoring the evolution of altruism and other group-level adaptations among genealogically unrelated individuals. Altruism can evolve even when the initial phenotypic variation in altruism is not heritable, a form of genetic assimilation. The importance of assortative interactions depends in part on cognitive abilities that...

The effects of conspecific attraction and habitat quality on habitat selection in territorial birds (troglodytes aedon )

Abstract: When animals choose habitats and territories in which to live or reproduce, their preferences can be affected by two sets of factors: habitat quality and conspecifics. There is general agreement among theoreticians and empiricists that animals should prefer to settle in habitats with high ‘‘intrinsic quality,’’ for example, in areas that offer high resource densities, protection from predators and parasites, or other features that enhance growth, survivorship, or offspring production (Fretwell and Lucas 1970; Rosenzweig 1985, 1991; Ens et al. 1992; Schieck and Hannon 1993; Yosef and Grubb 1994; Sutherland 1996). There is much less agreement, however, in either the theoretical or the empirical literature with respect to the effects of conspecifics on habitat selection (review in Stamps 1994). One large and influential body of theory holds that, because conspecifics are competitors, individual fitness should monotonically decline as a function of conspecific density (Brown 1969; Fretwell and Lucas 1970; Fretwell 1972; Maynard Smith 1974; Rosenzweig 1985, 1991; Parker and Sutherland 1986; Halama and Dueser 1994; Morris 1995, 1996; Ovadia and Abramsky 1995; Sutherland 1996). Such models imply that the presence of previous settlers in a habitat should discourage newcomers from settling in that habitat. However, others have suggested that settlers might benefit by settling or living near conspecifics, in which case newcomers should be attracted to conspecifics while settling (review in Stamps 1988; see also following paragraph).

A Structural Equation Model of Plant Species Richness and Its Application to a Coastal Wetland

Abstract: Studies of plant species richness have often emphasized the role of either community biomass (as an indicator of density effects) or abiotic factors. In this article we present a general model that simultaneously examines the relative importance of abiotic and density effects. General and specific models were developed to examine the importance of abiotic conditions, disturbance, and community biomass on plant species richness. Models were evaluated using structural equation modeling based on data from 190 plots across a coastal marsh landscape. The accepted model was found to explain 45% of the observed variation in richness, 75% of biomass, and 65% of light penetration. Model results indicate that abiotic conditions have both direct effects on the species pool and indirect effects on richness mediated through effects on biomass and shading. Effects of disturbance were found to be indirect via biomass. Strong density effects on richness were indicated by the results, and canopy light penetration was foun...

Stability and Instability in Ungulate Populations: An Empirical Analysis

Abstract: While many populations of large mammals are stable from year to year, some show persistent oscillations associated with high mortality. This article investigates the causes of variation in population stability in ungulates by comparing the contrasting dynamics of two naturally regulated island populations: the Soay sheep population of Hirta (St. Kilda), where numbers fluctuate by 60%-70% every 3 or 4 yr as a result of overcompensatory winter mortality, and the red deer population of the North Block of Rum, where numbers have been stable for over a decade. We suggest that the contrasting dynamics of these two populations are caused by differences in fecundity and maturation rate. Red deer show relatively low fecundity and slow maturation, and increases in population density depress both fecundity and juvenile survival. In contrast, the relatively rapid development of Soay sheep allows them to evade densitydependent effects on reproduction and survival until the population exceeds winter carrying capacity b...

Host heterogeneity in susceptibility and disease dynamics: tests of a mathematical model.

Abstract: Most mathematical models of disease assume that transmission is linearly dependent on the densities of host and pathogen. Recent data for animal diseases, however, have cast doubt on this assumption, without assessing the usefulness of alternative models. In this article, we use a combination of laboratory dose-response experiments, field transmission experiments, and observations of naturally occurring populations to show that virus transmission in gypsy moths is a nonlinear function of virus density, apparently because of heterogeneity among individual gypsy moth larvae in their susceptibility to the virus. Dose-response experiments showed that larvae from a laboratory colony of gypsy moths are substantially less heterogeneous in their susceptibility to the virus than are larvae from feral populations, and field experiments showed that there is a more strongly nonlinear relationship between transmission and virus density for feral larvae than for lab larvae. This nonlinearity in transmission changes the dynamics of the virus in natural populations so that a model incorporating host heterogeneity in susceptibility to the virus gives a much better fit to data on virus dynamics from large-scale field plots than does a classical model that ignores host heterogeneity. Our results suggest that heterogeneity among individuals has important effects on the dynamics of disease in insects at several spatial and temporal scales and that heterogeneity in susceptibility may be of general importance in the ecology of disease.

High Parasite Load in House Finches (Carpodacus mexicanus) is Correlated with Reduced Expression of a Sexually Selected Trait

Abstract: Independent of age or geographic variation, males of many species of birds exhibit dramatic variation in the expression of elaborate secondary sexual characters. "Good genes" models of sexual selection propose that males with relatively low resistance to parasites suffer high parasite loads that inhibit their ability to express these characters fully. In turn, variation in such characters may reliably indicate male quality and may be used by females to choose males. This is the first study to monitor (via mark-recapture) the long-term effects of parasites on color and growth of plumage in individual birds. Specifically, we used house finches, Carpodacus mexicanus, a sexually dimorphic species in which females are known to prefer more brightly plumaged males for mating, to test the hypothesis that high parasite load in males is correlated with poor physiological condition and reduced development of male secondary sex characters. Our results clearly demonstrate that both ectoparasitic feather mite (Proctoph...

Omnivory as a stabilizing feature of natural communities.

Abstract: Omnivory—defined broadly as feeding on more than one trophic level—occupies a prominent position in discussions of food web architecture and dynamics, due in large part to an enduring conflict regarding omnivory's role in community dynamics. According to classical re- sults from mathematical food web theory, omnivory destabilizes ecological communities, whereas more recent conceptual syntheses suggest that omnivory should be a strongly stabilizing factor in food webs. Working with an arthropod assemblage at Mount Saint Helens, I experimen- tally addressed this controversy using a two-way factorial design that crossed a manipulation of the degree of omnivory with another ''disturbance'' manipulation that targeted a specific compo- nent of the assemblage. In this statistical design, significant interaction effects (i.e., how the com- munity impacts of the disturbance varied with the degree of omnivory) identified key stabilizing or destabilizing influences of omnivory. Overall, my experimental results indicated that increas- ing the degree of omnivory stabilized community dynamics, in keeping with recent conceptual syntheses.

Effect size in ecological experiments: the application of biological models in meta-analysis.

Abstract: Some of the most interesting and important questions in ecology require examination of the strength of different processes across environmental gradients and among organisms with different traits (Quinn and Dunham 1983; Tilman 1989; Cooper et al. 1990; Sarnelle 1992; Osenberg and Mittelbach 1996). Metaanalysis (see, e.g., Gurevitch et al. 1992; Gurevitch and Hedges 1993; Arnqvist and Wooster 1995; Curtis 1996) combines results from independent studies to examine patterns of effect across taxa or environments and, thus, may represent a powerful tool to test ecological theory. A meta-analysis requires that a common metric of effect size be extracted from each of the studies. Here, we focus on choosing a metric that best facilitates ecological inferences. We begin with a brief description of the standard definition of effect size in meta-analysis, as used in recent papers. We then discuss potential problems with this approach and suggest an alternative that is more explicitly tied to the dynamics of ecological systems. Using two examples drawn from predator-prey experiments, we then illustrate the limitations of the standard metric and the conceptual advantages of one ecologically based alternative. We conclude by discussing the link between metrics of effect size and ecological models.

Does sociality drive the evolution of communicative complexity? A comparative test with ground-dwelling sciurid alarm calls.

Abstract: While sociality has been hypothesized to drive the evolution of communicative complexity, the relationship remains to be formally tested. We derive a continuous measure of social complexity from demographic data and use this variable to explain variation in alarm repertoire size in ground‐dwelling sciurid rodents (marmots, Marmota spp.; prairie dogs, Cynomys spp.; and ground squirrels, Spermophilus spp.). About 40% of the variation in alarm call repertoire size was explained by social complexity in the raw data set. To determine the degree to which this relationship may have been influenced by historical relationships between species, we used five different phylogenetic hypotheses to calculate phylogenetically independent contrasts. Less variation was significantly explained in contrast‐based analyses, but a general positive relationship remained. Social complexity explained more variation in alarm call repertoire size in marmots, while sociality explained no variation in repertoire size in prair...

Analyzing Pollinator-Mediated Selection in a Plant Hybrid Zone: Hummingbird Visitation Patterns on Three Spatial Scales

Abstract: Clines across hybrid zones can be produced by several forms of natural selection. We illustrate an approach to studying pollinator-mediated selection in plant hybrid zones, using two species of Ipomopsis (Polemoniaceae) as a model system. We measured visitation to flowers in natural and experimental populations by two major types of pollinators, hummingbirds and hawkmoths, at up to three different spatial scales. Using measures of pollinator visitation, we calculated phenotypic selection gradients and characterized the form of selection in the hybrid zone. Hummingbirds overvisited Ipomopsis aggregata compared with Ipomopsis tenuituba and morphological hybrids at all spatial scales, especially the largest scale of kilometers. These responses may depend in part on the presence of other hummingbird-visited plants in the community. Hummingbird behavior produced directional selection favoring wide corolla tubes and intense red coloration. Hawkmoths, in contrast, overvisited plants with narrow corolla tubes. Wh...

Von Bertalanffy's Growth Equation Should Not Be Used to Model Age and Size at Maturity

Abstract: The study of life-history evolution has enjoyed considerable success in melding theoretical prediction with empirical observation (Roff 1992; Stearns 1992). Often relatively simple models yield remarkably accurate predictions. One area of particular interest has been the study of age at maturity (Kozlowski 1992; Bernardo 1993; references therein). Several models have been proposed to explain variation in age at maturity across species, and each usually employs a different set of assumptions regarding mortality rates, growth, fecundity, and the appropriate measure of fitness. Results are often sensitive to the choice of these components; therefore, itis important to know when simple formulations are adequate. Here we focus on one of these components: the growth function. In the first section, we present the general modeling framework common to most of the aforementioned studies and consider how to choose an appropriate growth equation. We suggest hat the growth trajectory should be specified by two separate equations: a prematurity equation in which essentially no surplus energy is devoted to reproduction a d a postmaturity equation in which all (determinate growth) or some (indeterminate growth) surplus energy is devoted to reproduction. Logical inconsistencies can arise in models of the evolution of age and size at maturity when such a specification is not made. We believe that the von Bertalanffy (VB) equation is misused in both indeterminate and determinate growth models of maturity. Under indeterminate growth, a separate specification of preand postmaturity growth curves is usually not employed. Under determinate growth, in which such a separate specification is made, the VB equation often fails to provide an appropriate description of prema-

Evidence for an evolutionary history of overcompensation in the grassland biennial Gentianella campestris (Gentianaceae).

Abstract: Swedish University of Agricultural Sciences, Department of Conservation Biology, Section of Conservation Botany, Box 7072, 750 07 Uppsala, Sweden; University of Oulu, Department of Biology, Linnanmaa, 90 570 Oulu, Finland, and University of Lund, Department for Theoretical Ecology, Ecology Building, 223 62 Lund, Sweden; University of Lund, Department for Theoretical Ecology, Ecology Building, 223 62 Lund, Sweden

Experimental studies of group selection: what do they tell us about group selection in nature?

Abstract: The study of group selection has developed along two autonomous lines. One approach, which we refer to as the adaptationist school, seeks to understand the evolution of existing traits by examining plausible mechanisms for their evolution and persistence. The other approach, which we refer to as the genetic school, seeks to examine how currently acting artificial or natural selection changes traits within populations and focuses on current evolutionary change. The levels of selection debate lies mainly within the adaptationist school, whereas the experimental studies of group selection lie within the genetic school. Because of the very different traditions and goals of these two schools, the experimental studies of group selection have not had a major impact on the group selection debate. We review the experimental results of the genetic school in the context of the group selection controversy and address the following questions: Under what conditions is group selection effective? What is the gen...

Habitat destruction, dispersal, and deterministic extinction in competitive communities

Abstract: An analytical model of competitive coexistence in spatial habitats, modified to address habitat destruction, predicts that the most abundant species can be among the first species driven extinct by habitat destruction, given that abundant species are the poorest dispersers and best competitors. This contrasts with the classical view of biased extinction of rare species. Here we explore the robustness of this prediction both analytically and in spatially explicit simulations of more realistic cases. The prediction proved surprisingly robust. The poorest dispersers, which in this model generally are the best competitors and may be the most abundant species, were among the first driven extinct by habitat destruction whether they were abundant or rare, had short or long range dispersal, or reproduced continuously or periodically; whether competitive displacement was immediate or gradual; whether habitat destruction was clumped, uniform, or random and whether destruction occurred at once or progressively; and ...