Why do parasites harm their hosts? The general understanding is that if the transmission rate and virulence of a parasite are linked, then the parasite must harm its host to maximize its transmission. The exact nature of such trade-offs remains largely unclear, but for vertebrate hosts it probably involves interactions between a microparasite and the host immune system. Previous results have suggested that in a homogeneous host population in the absence of super- or coinfection, within-host dynamics lead to selection of the parasite with an intermediate growth rate that is just being controlled by the immune system before it kills the host (Antia et al. 1994). In this paper, we examine how this result changes when heterogeneity is introduced to the host population. We incorporate the simplest form of heterogeneity—random heterogeneity in the parameters describing the size of the initial parasite inoculum, the immune response of the host, and the lethal density at which the parasite kills the host. We find that the general conclusion of the previous model holds: parasites evolve some intermediate growth rate. However, in contrast with the generally accepted view, we find that virulence (measured by the case mortality or the rate of parasite-induced host mortality) increases with heterogeneity. Finally, we link the within-host and between-host dynamics of parasites. We show how the parameters for epidemiological spread of the disease can be estimated from the within-host dynamics, and in doing so examine the way in which trade-offs between these epidemiological parameters arise as a consequence of the interaction of the parasite and the immune response of the host.

Corresponding Editor: H. Ochman

Complexity analysis is capable of highlighting those gross evolutionary changes in gene promoter regions (loosely termed “promoter shuffling”) that are undetectable by conventional DNA sequence alignment. Complexity analysis was therefore used here to identify the modular components (blocks) of the orthologous β-globin gene promoter sequences of 22 vertebrate species, from zebrafish to humans. Considerable variation between the β-globin gene promoters was apparent in terms of block presence/absence, copy number, and relative location. Some sequence blocks appear to be ubiquitous, whereas others are restricted to a specific taxon. Block similarities were also evident between the promoters of the paralogous human β-like globin genes. It may be inferred that a wide variety of different mutational mechanisms have operated upon the β-globin gene promoter over evolutionary time. Because these include gross changes such as deletion, duplication, amplification, elongation, contraction, and fusion, as well as the steady accumulation of single base-pair substitutions, it is clear that some redefinition of the term “promoter shuffling” is required. This notwithstanding, and as previously described for the vertebrate growth hormone gene promoter, the modular structure of the β-globin promoter region and those of its paralogous counterparts have continually been rearranged into new combinations through the alteration, or shuffling, of preexisting blocks. Some of these changes may have had no influence on promoter function, but others could have altered either the level of gene expression or the responsiveness of the promoter to external stimuli. The comparative study of vertebrate β-globin gene promoter regions described here confirms the generality of the phenomenon of sequence block shuffling and thus supports the view that it could have played an important role in the evolution of differential gene expression.

Corresponding Editor: M. Riley

Calochortus and the family Liliaceae s.s. have often been considered each other's closest relatives, based partly on their shared possession of bulbs, visually showy flowers, winged wind-dispersed seeds, and narrow parallel-veined leaves. We present a well-supported molecular phylogeny for these groups and their close relatives in the core Liliales, based on sequence variation in the chloroplast-encoded rbcL and ndhF genes. This analysis identifies Liliaceae s.s. as monophyletic, including one clade (((Lilium, Fritillaris, Nomocharis), Cardiocrinum), Notholirion) that appears to have diversified in the Himalayas roughly 12 million years ago and another ((Erythronium, Tulipa), (Gagea, Lloydia)) that arose in East Asia at about the same time. Medeola and Clintonia are sister to Liliaceae s.s. and bear rhizomes, inconspicuous flowers, fleshy animal-dispersed fruits, and broad reticulate-veined leaves. Calochortus is sister to Tricyrtis; both Tricyrtis and the neighboring clade of Prosartes-Streptopus-Scoliopus share several of the traits seen in Medeola-Clintonia. The core Liliales thus provide compelling examples of both concerted convergence and phylogenetic niche conservatism. Invasion of open, seasonal habitats was accompanied by the independent evolution of bulbs, showy flowers, wind-dispersed seeds, and narrow parallel-veined leaves in Calochortus and Liliaceae s.s. Conversely, persistence in shady habitats was accompanied by the retention of rhizomes, inconspicuous flowers, animal-dispersed seeds, and broad reticulate-veined leaves in their sister groups. We advance arguments for the context-specific adaptive value of each of these traits, as well as evidence of parallel trends in other groups. Concerted convergence—convergence in several different traits, favored by the same shared set of ecological conditions, in two or more lineages—is an important evolutionary process that can mislead evolutionary analyses based solely on phenotypic variation.

Corresponding Editor: D. Baum

A highly variable mitochondrial DNA (mtDNA) restriction fragment length polymorphism (RFLP) locus is used to assess the population structure of mitochondrial genomes in the gynodioecious plant Silene vulgaris at two spatial scales. Thirteen mtDNA haplotypes were identified within 250 individuals from 18 populations in a 20-km diameter region of western Virginia. The population structure of these mtDNA haplotypes was estimated as θ_ST = 0.574 (± 0.066 SE) and, surprisingly, genetic differentiation among populations was negatively correlated with geographic distance (Mantel r = −0.246, P < 0.002). Additionally, mtDNA haplotypes were spatially clumped at the scale of meters within one population. Gender in S. vulgaris is determined by an interaction between autosomal male fertility restorers and cytoplasmic male sterility (CMS) factors, and seed fitness is affected by an interaction between gender and population sex ratio; thus, selection acting on gender could influence the distribution of mtDNA RFLP haplotypes. The sex ratio (females:hermaphrodites) varied among mtDNA haplotypes across the entire metapopulation, possibly because the haplotypes were in linkage disequilibrium with different CMS factors. The gender associated with some of the most common haplotypes varied among populations, suggesting that there is also population structure in male fertility restorer genes. In comparison with reports of mtDNA variation from other published studies, we found that S. vulgaris exhibits a large number of mtDNA haplotypes relative to that observed in other species.

Corresponding Editor: M. Morgan

The role of partial self-incompatibility in plant breeding system evolution has received little attention. Here, we examine the genetic basis of modifiers conferring self-fertility in the creeping bellflower, Campanula rapunculoides L. (Campanulaceae), a partially self-incompatible herb. A survey of 35 individuals from two natural populations indicates that 45% of them are strongly self-incompatible, 40% intermediately self-incompatible, and 15% weakly self-incompatible and that some plants show a strong breakdown in self-incompatibility over floral age. We generated 101 F₁ families by random crossing among 31 parental plants and estimated the heritability of self-fertility in day 1 and day 4 female-phase flowers, the genetic correlation between day 1 and day 4 self-fertility, and the coefficient of additive genetic variance of self-fertility. We use linear regression and data from additional crosses to examine whether there are significant maternal effects in the expression of self-fertility. We use Fain's test to determine if a major gene influences self-fertility and, finding no evidence, use data from additional crosses on an F₂ generation to estimate the mean number and dominance of genes conferring self-fertility. These analyses indicate that the heritability (h²) of self-fertility is 0.24 in day 1 female-phase flowers and 0.44 in day 4 flowers, self-fertility is primarily additive but shows some recessive effects, and self-fertility is estimated to be controlled by four genetic factors. In addition, we have evidence that there may be maternal effects for self-fertility, especially for weakly self-incompatible plants. The significance of these results in the context of mating system evolution is discussed.

Corresponding Editor: M. Morgan

We analyzed the genetic basis of morphological differences between two wild species of teosinte (Zea diploperennis and Z. mays ssp. parviglumis), which are relatives of maize. These two species differ in a number of taxonomically important traits including the structure of the tassel (male inflorescence), which is the focus of this report. To investigate the genetic inheritance of six tassel traits, quantitative trait locus (QTL) mapping with 95 RFLP markers was employed on a population of 425 F₂ plants. Each trait was analyzed by interval mapping (IM) and composite interval mapping (CIM) to identify and characterize the QTL controlling the differences in tassel morphology. We detected two to eight QTL for each trait. In total, 30 QTL with IM and 33 QTL with CIM were found for tassel morphology. QTL for several of the traits mapped near each other, suggesting pleiotropy and/or linkage of QTL. The QTL showed small to moderate magnitudes of effect. No QTL of exceptionally large effect were found as seen under domestication and in the case of some other natural species. Thus, the model involving major QTL of large effect seems not to apply to the traits and species analyzed. A mixture of QTL with positive and negative allelic effects was found for most tassel traits and may suggest a history of periodic changes in the direction of selection during the divergence of Z. diploperennis and Z. mays ssp. parviglumis or fixation of QTL alleles by random genetic drift rather than selection.

Corresponding Editor: J. Conner

Studies that have tested and failed to support the hypothesis that escalated species (e.g., those with predation-resistant adaptations) are more susceptible to elimination during mass extinctions have concentrated on the distribution and degree of morphological defenses in molluscan species. This morphological approach to determining level of escalation in bivalves may be oversimplified because it does not account for metabolic rate, which is an important measure of escalation that is less readily accessible for fossils. Shell growth rates in living bivalves are positively correlated with metabolic rate and thus are potential indicators of level of escalation. To evaluate this approach, we used oxygen isotopes to reconstruct shell growth rates for two bivalve species (Macrocallista marylandica and Glossus markoei) from Miocene-aged sediments of Maryland. Although both species are classified as non-escalated based on morphology, the isotopic data indicate that M. marylandica was a faster-growing species with a higher metabolic rate and G. markoei was a slower-growing species with a lower metabolic rate. Based on these results, we predict that some morphologically non-escalated species in previous tests of extinction selectivity should be reclassified as escalated because of their fast shell growth rates (i.e., high metabolic rates). Studies that evaluate the level of escalation of a fauna should take into account the energetic physiology of taxa to avoid misleading results.

Corresponding Editor: D. Geary

Assortative mating (prezygotic isolation) and reduced hybrid fitness (postzygotic isolation) are typically invoked to explain the stability of hybrid zones. In the tension zone model, these factors work in opposition to migration, which promotes genetic homogeneity. Many marine animals migrate over long distances through a planktonic larval stage. Therefore, strong reproductive isolation is needed to maintain stable marine hybrid zones. However, surprisingly little is known about mating preferences and hybrid fitness in marine organisms. Smooth-shelled mussels (Mytilus spp.) form a well-known species complex, with hybridization over extensive areas such as the contact zone of M. edulis and M. galloprovincialis around European Atlantic coasts. This paper reports direct experimental evidence of assortative fertilization, hybrid larval inviability, and early heterosis for growth rate in M. edulis and M. galloprovincialis. Four crosses between pure M. edulis and M. galloprovincialis were analyzed with a new polymerase-chain-reaction-based diagnostic marker. Gamete competition between taxa was allowed in two out of the four crosses. Genotype frequencies observed at an early stage (36 h after fertilization) unambiguously revealed assortative fertilization when gamete competition was allowed. A significant reduction in hybrid viability was subsequently observed during the larval stage. At the same stage an antagonistic effect, heterosis, was observed on growth rate. However, even if heterosis is observed in the F₁, it is expected to vanish in subsequent hybrid generations. Although specialization for different habitats and asynchronous spawning have been mentioned as factors contributing to the maintenance of the blue mussel hybrid zone in Europe, we argue that assortative fertilization and reduced hybrid fitness are important factors that also contribute to the stabilization of this zone. These results emphasize that multiple factors may act concomitantly in a barrier to gene flow, especially in complex life cycles. Furthermore, they show that assortative mating through gamete preference, as already demonstrated for sea urchins, may play a role in speciation processes taking place in the sea.

Corresponding Editor: R. Burton

One of the hypotheses of growing interest in studies of responses to thermal environments suggests that trade-offs and other trait associations may be altered by temperature. Here, the commonly observed positive association between body size and longevity was examined at two adult test temperatures, 14°C and 25°C, in cold-stress-selected lines (S) and their controls (C) in 25°C-reared Drosophila melanogaster. Thorax length (TL) and developmental time (DT) were also scored in 25°C-reared individuals before and after one generation of truncation selection on longevity. The topography of the selection surface that relates longevity to thorax and wing size was temperature dependent and differed both between lines and between sexes. Longevity increased monotonically with body size (TL) in C and S females at 25°C but, surprisingly, longevity decreased with body size in S individuals at 14°C. Body size did not diverge between S and C lines and showed no response to longevity selection. However, DT increased by 25°C-longevity selection in C individuals and decreased by 14°C-longevity selection in S individuals. These results suggest that trait associations (including the commonly observed trade-off between body size and DT) can greatly depend on temperature, as a shift in the sign of the correlation is possible at low temperature. Genotype × temperature interaction is an important source of variation in the relationship between soma size and longevity.

Corresponding Editor: C. López-Fanjul

Body size is one of the most important quantitative traits under evolutionary scrutiny. Sexual size dimorphism (SSD) in a given species is expected to result if opposing selection forces equilibrate differently in both sexes. We document variation in the intensity of sexual and fecundity selection, male and female body size, and thus SSD among 31 and 27 populations of the two dung fly species, Scathophaga stercoraria and Sepsis cynipsea, across Switzerland. Whereas in S. cynipsea females are larger, the SSD is reversed in S. stercoraria. We comprehensively evaluated Fairbairn and Preziosi's (1994) general, three-tiered scenario, hypothesizing that sexual selection for large male size is the major driving force of SSD allometry within these two species. Sexual selection intensity on male size in the yellow dung fly, S. stercoraria, was overall positive, greater, and more variable among populations than fecundity selection on females. Also, sexual selection intensity in a given population correlated positively with mean male body size of that population for both the field-caught fathers and their laboratory-reared sons, indicating a response to selection. In S. cynipsea, sexual selection intensity on males was lower overall and significantly positive, about equal in magnitude, but more variable than fecundity selection on females. However, there was no correlation between the intensity of sexual selection and mean male body size among populations. In both species, the laboratory-reared offspring indicate genetic differentiation among populations in body size. Despite fulfillment of all key prerequisites, at least in S. stercoraria, we did not find hypoallometry for SSD (Rensch's rule, i.e., greater evolutionary divergence in male size than female size) for the field-caught parents or the laboratory-reared offspring: Female size was isometric to male size in both species. We conclude that S. cynipsea does not fit some major requirements of Fairbairn and Preziosi's (1994) scenario, whereas for S. stercoraria we found partial support for it. Failure to support Rensch's rule within the latter species may be due to phylogenetic or other constraints, power limitations, erroneous estimates of sexual selection, insufficient genetic isolation of populations, or sex differences in viability selection against large size.

Corresponding Editor: J. Merilä

Ecological speciation occurs when reproductive isolation evolves ultimately as a result of divergent natural selection between populations inhabiting different environments or exploiting alternative resources. I tested a prediction of the ecological model concerning the fitness of hybrids between two young, sympatric species of threespine sticklebacks (Benthics and Limnetics). The two species are ecologically and morphologically divergent: the Benthic is adapted to feeding on invertebrates in the littoral zone of the lake whereas the Limnetic is adapted to feeding on zooplankton in the open water. The growth rate of two types of hybrids, the Benthic backcross and the Limnetic backcross, as well as both parent species, was evaluated in enclosures in both parental habitats in the lake. The use of backcrosses is ideal because a comparison of their growth rates in the two habitats estimates an ecologically dependent component of their fitness while controlling for any intrinsic genetic incompatibilities that may exist between the Benthic and Limnetic genomes. The backcross results revealed a striking pattern of ecological dependence: in the littoral zone, Benthic backcrosses grew at approximately twice the rate of Limnetic backcrosses, while in the open water, Limnetic backcrosses grew at approximately twice the rate of Benthic backcrosses. Such a reversal of relative fitness of the two cross-types in the two environments provides strong evidence that divergent natural selection has played a central role in the evolution of postmating isolation between Benthics and Limnetics. Although the rank order of growth rates of all cross-types in the littoral zone was Benthic > Benthic backcross > Limnetic backcross > Limnetic, neither backcross differed significantly from the parent from which it was mainly derived. Implications of this result are discussed in terms of ecological speciation and possible introgressive hybridization between the species. Results in the open water were less clear and were not fully consistent with the ecological model of speciation, mainly as a result of the low growth rate of Limnetics. However, analysis of the diet of the fish in the open water suggests that these enclosures may not have been fully successful at replicating the food regimes characteristic of this habitat.

Corresponding Editor: G. Wallis

The evolution of sociality in insects holds a central place in evolutionary theory. By examining the phylogenetic patterns of solitary and social behavior and how they correlate with ecological variables, we may identify factors important in the evolution of sociality. In this study, we investigated historical and biogeographical patterns of sociality in a socially polymorphic bee species (one that demonstrates both social and solitary nesting behavior). This unique system allows for a more powerful examination of evolutionary transitions in sociality than interspecific studies of obligately social and solitary species. We conducted a phylogenetic analysis among populations of the halictine bee Halictus rubicundus and then identified relationships among mitochondrial DNA sequence data, sociality, environmental conditions at the nesting site, and geographic location of populations of this species. Within North America, populations of H. rubicundus expressing social and solitary behavior belong to different genetic lineages. Sociality is also correlated with at least one environmental variable used in this study. Taken together, the results support the predictions for genetic control of sociality, but they are still consistent with social behavior at some level being determined by the environmental conditions at the nesting site.

Corresponding Editor: K. Ross

Seasonal polyphenism, in which different forms of a species are produced at different times of the year, is a common form of phenotypic plasticity among insects. Here I show that the production of dark fifth-instar caterpillars of the eastern black swallowtail butterfly, Papilio polyxenes, is a seasonal polyphenism, with larvae reared on autumnal conditions being significantly darker than larvae reared on midsummer conditions. Both rearing photoperiod and temperature were found to have individual and synergistic effects on larval darkness. Genetic analysis of variation among full-sibling families reared on combinations of two different temperatures and photoperiods is consistent with the hypothesis that variation in darkness is heritable. In addition, the genetic correlation in larval darkness across midsummer and autumnal environments is not different from zero, suggesting that differential gene expression is responsible for the increase in larval darkness in the autumn. The relatively dark autumnal form was found to have a higher body temperature in sunlight than did the lighter midsummer form, and small differences in temperature were found to increase larval growth rate. These results suggest that this genetically based seasonal polyphenism in larval color has evolved in part to increase larval growth rates in the autumn.

Corresponding Editor: T. Kawecki

Ectothermic organisms, such as insects and reptiles, rely on external heat sources to control body temperature and possess physiological and behavioral traits that are temperature dependent. It has therefore been hypothesised that differences in body temperature resulting from phenotypic properties, such as color pattern, may translate into selection against thermally inferior phenotypes. We tested for costs and benefits of pale versus dark coloration by comparing the behaviors (i.e., basking duration and bouts) of pygmy grasshopper (Tetrix undulata) individuals exposed to experimental situations imposing a trade-off between temperature regulation and feeding. We used pairs consisting of two full-siblings of the same sex that represented different (genetically coded) color morphs but had shared identical conditions from the time of fertilization. Our results revealed significant differences in behavioral thermoregulation between dark and pale individuals in females, but not in males. Pale females spent more time feeding than dark females, regardless of whether feeding was associated with a risk of either hypothermia or overheating. In contrast, only minor differences in behavior (if any) were evident between individuals that belonged to the same color morph but had been painted black or gray to increase and decrease their heating rates. This suggests that the behavioral differences between individuals belonging to different color morphs are genetically determined, rather than simply reflecting a response to different heating rates. To test for effects of acclimation on behaviors, we used pairs of individuals that had been reared from hatchlings to adults under controlled conditions in either low or high temperature. The thermal regime experienced during rearing had little effect on behaviors during the experiments reported above, but significantly influenced the body temperatures selected in a laboratory thermal gradient. In females (but not in males) preferred body temperature also varied among individuals born to mothers belonging to different color morphs, suggesting that a genetic correlation exists between color pattern and temperature preferences. Collectively, these findings, at least in females, are consistent with the hypothesis of multiple-trait coevolution and suggest that the different color morphs represent alternative evolutionary strategies.

Corresponding Editor: T. Mousseau

The hypothesis that predator-induced defenses in anuran larvae are maintained by divergent selection across multiple predation environments has not been fully supported by empirical results. One reason may be that traits that respond slowly to environmental variation experience a fitness cost not incorporated in the standard adaptive model, due to a time lag between detecting the state of the environment and expressing the phenotypic response. I measured the rate at which behavior and morphology of Rana temporaria tadpoles change when confronted with a switch in the predation environment at two points in development. Hatchling tadpoles that had been exposed during the egg stage to Aeshna dragonfly larvae were not phenotypically different from those exposed as eggs to predator-free conditions, and both responded similarly to post-hatching predator treatments. When 25-day-old tadpoles from treatments with and without dragonflies were subjected to a switch in the environment, their activity budgets reversed completely within 24–36 h, and their body and tail shape began changing significantly within 4 days. The behavioral response was conservative: Tadpoles switched from high-risk to predator-free treatments were slower to adjust their activity. The study confirmed that behavioral traits are relatively labile and exhibit strong plasticity, but it did not reveal such a pattern at the level of individual traits: Morphological traits that developed slowly did not show the least plasticity. Thus, I found that differences in lability of traits were useful for predicting the magnitude of plasticity only for fundamentally different kinds of characters.

Corresponding Editor: T. Mousseau

There has been recent interest in the role genetic incompatibility may play in mate or sperm choice. One source of incompatibility may be too similar or disparate genomes. An isolated population of the ornate dragon lizard, Ctenophorus ornatus, was followed over a breeding season and parentage assigned to the offspring using microsatellites. It was found that the adults in the population had an eight per cent chance of mating with a relative. I examined whether C. ornatus mate (or fertilize their eggs) with respect to genetic similarity. There was no difference in a female's relatedness to the male in whose territory she resided and her average relatedness to all other males. Neither was there a difference in the relatedness of the male that sired a female's offspring and the female's average relatedness to all other males. There was no evidence of a cost to mating with a more genetically similar mate, because offspring survival was not influenced by degree of inbreeding or outbreeding. However, females that were more outbred had fewer offspring survive. In this small population there are two possible explanations for the decreased fitness associated with outbreeding. First, the species may have an evolutionary history of inbreeding and thus may be susceptible to outbreeding depression. Second, as fitter individuals produce more offspring, these offspring have an increased probability of breeding with relatives, leading to an indirect relationship between fitness and outbreeding.

Corresponding Editor: H. L. Gibbs

We studied the genetic diversity of a coral reef fish species to investigate the origin of the differentiation. A total of 727 Acanthurus triostegus collected from 15 locations throughout the Pacific were analyzed for 20 polymorphic loci. The genetic structure showed limited internal disequilibrium within each population; 3.7% of the loci showed significant Hardy-Weinberg disequilibrium, mostly associated with Adh*, and we subsequently removed this locus from further analysis of geographic pattern. The genetic structure of A. triostegus throughout the tropical Pacific Ocean revealed a strong geographic pattern. Overall, there was significant population differentiation (multilocus F_ST = 0.199), which was geographically structured according to bootstraps of neighbor-joining analysis on Nei's unbiased genetic distances and AMOVA analysis. The genetic structure revealed five geographic groups in the Pacific Ocean: western Pacific (Guam, Philippines, Palau, and Great Barrier Reef); central Pacific (Solomons, New Caledonia, and Fiji); and three groups made up of the eastern populations, namely Hawaiian Archipelago (north), Marquesas (equatorial), and southern French Polynesia (south) that incorporates Clipperton Island located in the northeastern Pacific. In addition, heterozygosity values were found to be geographically structured with higher values grouped within Polynesian and Clipperton populations, which exhibited lower population size. Finally, the genetic differentiation (F_ST) was significantly correlated with geographic distance when populations from the Hawaiian and Marquesas archipelagos were separated from all the other locations. These results show that patterns of differentiation vary within the same species according to the spatial scale, with one group probably issued from vicariance, whereas the other followed a pattern of isolation by distance. The geographic pattern for A. triostegus emphasizes the diversity of the evolutionary processes that lead to the present genetic structure with some being more influential in certain areas or according to a particular spatial scale.

Corresponding Editor: L. Bernatchez

Life-history theory suggests that the variation in the seasonal timing of reproduction within populations may be explained on the basis of individual optimization. Optimal breeding times would vary between individuals as a result of trade-offs between fitness components. The existence of such trade-offs has seldom been tested empirically. We experimentally investigated the consequences of altered timing of current reproduction for future reproductive output in the European coot (Fulica atra). First clutches of different laying date were cross-fostered between nests, and parents thereby experienced a delay or an advance in the hatching date. The probability and success of a second brood, adult survival until and reproduction in the next season were then compared to the natural variation among control pairs. Among control pairs the probability of a second brood declined with the progress of season. Delayed pairs were less likely and advanced pairs were more likely to produce a second brood. These changes were quantitatively as predicted from the natural seasonal decline. The number of eggs in the second clutch was positively related to egg number in the first clutch and negatively related to laying date. Compared to the natural variation, delayed females had more and advanced females had fewer eggs in their second clutch. The size of the second brood declined with season, but there was no significant effect of delay or advance. Local adult survival was higher following a delay and reduced following an advance. The effect of the experiment on adult survival was independent of sex. Laying date and clutch size of females breeding in the next year were not affected by treatment. The study demonstrates the existence of a trade-off between increased probability of a second brood and decreased parental survival for early breeding. Timing-dependent effects of current reproduction on future reproductive output may thus play an important role in the evolution of the seasonal timing of reproduction.

Corresponding Editor: B. Crespi

Elaborate plumage and complex songs of male birds are two of the best-known examples of sexually selected traits, yet the interaction between these traits is poorly understood. Theory suggests that among a suite of potential displays, animals will emphasize traits that are most conspicuous, least costly, or best signal condition and reduce display of other traits. Here we examined the relationship between song and plumage elaborations in cardueline finches, songbirds that are highly variable in plumage displays and songs, but that share a similar mating system. We statistically controlled for body mass, habitat characteristics, and phylogenetic relationships and found that across species song complexity was strongly negatively related to elaboration of plumage ornamentation. When plumage coloration was partitioned into carotenoid-based and melanin-based components, song complexity was negatively related to elaboration of male carotenoid-based coloration but unrelated to elaboration of melanin-based coloration. These observations support the idea that, for condition-dependent traits, animal species trade off trait expression in response to changes in the costs or the information content of these traits. We discuss several alternative explanations for the observed pattern.

Corresponding Editor: T. Smith

The race for reaching mates by the time they are receptive, or sexual selection by scramble competition, has received little attention. We argue that smaller males are favored in species in which the male must climb to reach females located in high habitat patches. This new explanation we term the “gravity hypothesis” of sexual size dimorphism (SSD). We show that a simple biomechanical model of animal movement predicts that: (1) selection should favor a comparatively smaller size in the searching sex when searching involves climbing; and (2) this effect should be stronger in larger species than in smaller species. In reaching high habitats, smaller, faster searchers will be favored either through sexual selection by scramble competition and/or by escaping predation easier by running faster on vertical surfaces. Different spider species are found at a wide range of heights. We compiled a dataset of spider taxa and arranged their habitats according to four height categories, ranked from soil surface to trees. We show that, after controlling for phylogeny, both predictions of the gravity hypothesis of SSD are met. Thus, it appears that the constraint imposed by gravity on climbing males is a selective factor in determining male dwarfism.

Corresponding Editor: T. Kawecki

No Abstract Available.

Corresponding Editor: D. Futuyma