To date, theoretical or laboratory simulations of aposematic evolution in prey animals have focused narrowly on internally stored chemical defense as the source of unprofitability and ignore aposematic advertisement of physical defenses such as spines (and defensive hairs, claws, etc.). This has occurred even though aposematism in spiny animals has been recognized since the 19th century. In this paper we present the first detailed theoretical consideration of aposematism in spiny animals, focusing on questions of initial evolution, costs of display, and coevolution of displays with defenses. Using an individual-based evolutionary model, we found that spines (or similar physical defenses) can easily evolve without aposematism, but when spines do evolve, aposematic displays can also easily evolve if they help to make the prey animal distinctive and if they draw attention to the physical threat. When aposematic displays evolve, they cause reduced investment in costly spines, so that, in addition to signaling unprofitability, aposematic display may enhance the cost-effectiveness of antipredator defenses (one exception to this conclusion is if the display is itself as costly as the defense). For animals with stinging spines, combining physical and chemical defense, the evolution of aposematic display may lead to reduced investment in the toxin compared to the spine. This occurs because spines act as both secondary (repellent) defenses and as primary defenses (their own visible, honest advertisement), whereas internally stored toxins only (generally) act as repellent secondary defenses. We argue that conspicuous aposematism in spines functions as an attention-getting mechanism, whereas conspicuous aposematic display in purely toxic animals may be explained by signal reliability arguments. Finally, one (more) route by which aposematism may initially evolve is by spiny rather than purely chemically defended species, spreading to species with other forms of secondary defense as the signal becomes common.

Sharp and stable clinal variation is enigmatic when found in species with high gene flow. Classical population genetic models treat gene flow as a random homogenizing force countering local adaptation across habitat discontinuities. Under this view, dispersal over large spatial scales will lower the effectiveness of adaptation by natural selection at finer spatial scales. Thus, random gene flow will create a shallow phenotypic cline across an ecotone in response to a steep selection gradient. In sedentary marine species that disperse primarily as larvae, nonrandom dispersal patterns are expected due to coastal hydrodynamics. Surprisingly sharp phenotypic and genotypic clines have been documented in marine species with high gene flow. We are interested in the extent to which nonrandom dispersal could accentuate such clines. We model a linear species range in which populations have stable and uniform densities along a selection gradient; in contrast to random dispersal, convergent advection of larvae can amplify phenotypic differentiation if coupled with a semipermeable dispersal barrier in the convergence zone. The migration load caused by directional dispersal pushes the phenotypic mean away from the local trait optimum in downstream populations, that is, near the convergence zone. A dispersal barrier is possible as a result of colliding currents if the water and larvae are mostly displaced offshore, away from suitable settlement habitat. Disjunctions in a quantitative trait were enlarged in the convergence zone by faster current flows or a more complete dispersal barrier. With advection of larvae per generation one-third as far as the average dispersal distance by diffusion, convergence on a dispersal barrier with 40% permeability generated a trait disjunction across the convergence zone of two phenotypic standard deviations. Without directional dispersal, similar clines also developed across a habitat gap, where population density was low, or across dispersal barriers with less than 1% permeability. These findings suggest that the types of hydrographic phenomena often associated with marine transition zones can strongly affect the balance between gene flow and selection and generate surprisingly steep clines given the large-scale gene flow expected from larvae.

Simultaneous effects of host and parasite in determining quantitative traits of infection have long been neglected in theoretical and experimental investigations of host-parasite coevolution with the notable exception of gene-for-gene resistance studies. A cross-infection experiment, using five lines of the plant Arabidopsis thaliana and two strains of its oomycete pathogen Hyaloperonospora parasitica, revealed that three traits traditionally considered those of the parasite (number of infected leaves, transmission success, and time until 50% transmission), differed among specific combinations of host and parasite lines, being determined by the two protagonists of the infection. However, the two parasite strains did not differ significantly for most measured phenotypic traits of the infection. Globally, transmission increased with increasing virulence among the different host-parasite combinations, as assumed by most models of evolution of virulence. Surprisingly, however, there was no general relationship between parasite and host fitness, estimated respectively as transmission and seed production. Only one of the two strains showed the expected significant negative genetic correlation between these two variables. Our results thus highlight the importance of taking into account both host and parasite genetic variation because their interaction can lead to unexpected evolutionary outcomes.

Intratetrad mating, the fusion of gametes formed in a single meiosis, has unusual consequences for genetic diversity, especially in genome regions linked to mating type loci. Here we investigate the fate of modifier alleles that alter the rate of intratetrad mating, under models of heterozygote advantage and of genetic load resulting from recurrent mutation. In both cases, intratetrad mating is favored if the recombination rate between the selected locus and mating type is less than the frequency of lethal recessive alleles at that locus in the population. Positive feedback often accelerates the invasion of modifiers to the intratetrad mating rate. Recombination rate and intratetrad mating rate exert indirect selection on one another, resulting in a cascading decline in outcrossing, even in the absence of any cost of sex. However, under recurrent mutation, alleles for obligate intratetrad mating invade only very slowly, perhaps explaining why outcrossing can persist at low frequencies in a largely intratetrad mating population.

Homoploid hybrid speciation occurs through stabilization of a hybrid segregate (or segregates) isolated by premating and/or postmating barriers from parent taxa. Theory predicts that ecological and spatial isolation are of critical importance during homoploid hybrid speciation, and all confirmed homoploid hybrid species are ecologically isolated from their parents. Until recently, such species have been identified long after they originated, and consequently it has not been possible to determine the relative importance of spatial and ecological isolation during their origin. Here we present evidence for the recent origin (within the past 300 years) of a new homoploid hybrid species, Senecio squalidus (Asteraceae), in the British Isles, following long-distance dispersal of hybrid material from a hybrid zone between S. aethnensis and S. chrysanthemifolius on Mount Etna, Sicily, Italy. Historical records show that such hybrid material from Sicily was introduced to the Oxford Botanic Garden in Britain in the early part of the 18th century and that S. squalidus began to spread from there after approximately 90 years. A survey of randomly amplified polymorphic DNA/intersimple sequence repeats (RAPD/ISSR) marker variation demonstrated that S. squalidus is a diploid hybrid derivative of S. aethnensis and S. chrysanthemifolius that grow at high and low altitudes, respectively, on Mount Etna and that form a hybrid zone at intermediate altitudes. Senecio squalidus contained 11 of 13 RAPD/ISSR markers that were recorded at high frequency in S. chrysanthemifolius but were absent or occurred at low frequency in S. aethnensis, and 10 of 13 markers for which the reverse was true. Bayesian admixture analysis showed that all individuals of S. squalidus surveyed were of mixed ancestry with relatively high mean proportions of ancestry derived from both S. chrysanthemifolius and S. aethnensis (0.644 and 0.356, respectively). We argue that long-distance isolation of hybrid material from its parents on Mount Etna would have helped favor the origin and establishment of S. squalidus in the British Isles, regardless of whether the initial hybrid material introduced to Britain was preadapted to local conditions.

Steep clines in ecologically important traits may be caused by divergent natural selection. However, processes that do not necessarily invoke ongoing selection, such as secondary contact or restricted gene flow, can also cause patterns of phenotypic differentiation over short spatial scales. Distinguishing among all possible scenarios is difficult, but an attainable goal is to establish whether scenarios that imply selection need to be invoked. We compared the extent of morphological and genetic differentiation between geographically structured red and yellow floral races of Mimulus aurantiacus (bush monkeyflower; Phrymaceae). Flower color was assessed in a common garden as well as in the field to determine whether variation was genetic and to quantify the extent of geographical differentiation. Population genetic differentiation at marker loci was measured for both chloroplast and nuclear genomes, and the degree of population structure within and among the floral races was evaluated. Flower color shows both a strong genetic basis and a sharp geographic transition, with pure red-flowered populations in western San Diego County and pure yellow-flowered populations to the east. In the zone of contact, both pure and intermediate phenotypes occur. Patterns of genetic differentiation at marker loci are far less pronounced, as little of the variation is partitioned according to the differences in flower color. Phenotypic differentiation (Q_ST) between populations with different flower colors is much greater than neutral genetic differentiation (F_ST). When comparisons are made between populations of the same flower color, the opposite trend is evident. Limited neutral genetic structure between the floral races, combined with sharp differentiation in flower color, is consistent with the hypothesis that current or recent natural selection maintains the cline in flower color.

Costly resistance mechanisms have been cited as an explanation for the widespread occurrence of parasitic infections, yet few studies have examined these costs in detail. A malaria-mosquito model has been used to test this concept by making a comparison of the fitness of highly susceptible lines of mosquitoes with lines that are resistant to infection. Malaria infection is known to cause a decrease in fecundity and fertility of mosquitoes; resistant mosquitoes were thus predicted to be fitter than susceptible ones. Anopheles gambiae were selected for refractoriness/resistance or for increased susceptibility to infection by Plasmodium yoelii nigeriensis. Additional lines that acted as controls for inbreeding depression were raised in parallel but not exposed to selection pressure. Selections were made in triplicate so that founder effects could be detected. Resistance mechanisms that were selected included melanotic encapsulation of parasites within 24 h postinfection and the complete disappearance of parasites from the gut. Costs of immune surveillance were assessed after an uninfected feed, and costs of immune deployment were assessed after exposure to infection and to infection and additional stresses. Mosquito survivorship was unaffected by either resistance to infection or by an increased burden of infection when compared with low levels of infection. In most cases reproductive fitness was equally affected by refractoriness or by infection. Resistant mosquitoes did not gain a fitness advantage by eliminating the parasites. Costs were consistently associated with larval production and egg hatch rate but rarely attributed to changes in blood feeding and never to changes in mosquito size. No advantages appeared to be gained by the offspring of resistant mosquitoes. Furthermore, we were unable to select for refractoriness in groups of mosquitoes in which 100% or 50% of the population were exposed to infection every generation for 22 generations. Under these selection pressures, no population had become completely refractory and only one became more resistant. Variations in fitness relative to control lines in different groups were attributed to founder effects. Our conclusion from these findings is that refractoriness to malaria is as costly as tolerance of infection.

There is growing awareness of the importance of natural selection in driving genetic divergence and speciation, and several of the most apparent cases of this ecological speciation are provided by the existence of genetically distinct host forms in phytophagous insects. Such examples of host-associated differentiation (HAD) have become increasingly documented, and the implications of this phenomenon for the diversification of insects are becoming widely appreciated. However, instances of HAD remain rare relative to insect diversity and are sparsely distributed both ecologically and taxonomically. We sought to assess the frequency of HAD in a model herbivore community by examining genetic divergence in a variety of herbivores that feed on two closely related and broadly sympatric species of goldenrod (Solidago altissima and S. gigantea). Using mitochondrial DNA and allozyme data, in conjunction with previously published studies, we found that four of nine herbivores exhibited evidence of HAD, including possible host races or cryptic species. Using a range of reasonable substitution rate estimates for cytochrome oxidase I mitochondrial DNA, we found that HAD appears to have proceeded asynchronously across taxa. This pattern, along with the broadly sympatric distribution of host plants and the specialized life histories of the phytophagous insects, is consistent with sympatric divergence in some or all of these taxa. Although further behavioral and ecological study is needed, our survey of HAD in a community of herbivores indicates that ecological (perhaps sympatric) speciation may have been responsible for generating a significant fraction of the extant diversity of phytophagous insects.

Many studies of speciation rely critically on estimates of sexual isolation obtained in the laboratory. Here we examine the sensitivity of sexual isolation to alterations in experimental design and mating environment in two sister species of Drosophila, D. santomea and D. yakuba. We use a newly devised measure of mating frequencies that is able to disentangle sexual isolation from species differences in mating propensity. Variation in fly density, presence or absence of a quasi-natural environment, degree of starvation, and relative frequency of species had little or no effect on sexual isolation, but one factor did have a significant effect: the possibility of choice. Designs that allowed flies to choose between conspecific and heterospecific mates showed significantly more sexual isolation than other designs that did not allow choice. These experiments suggest that sexual isolation between these species (whose ranges overlap on the island of São Tomé) is due largely to discrimination against D. yakuba males by D. santomea females. This suggestion was confirmed by direct observations of mating behavior. Drosophila santomea males also court D. yakuba females less ardently than conspecific females, whereas neither males nor females of D. yakuba show strong mate discrimination. Thus, sexual isolation appears to be a result of evolutionary changes in the derived island endemic D. santomea. Surprisingly, as reported in a companion paper (Llopart et al. 2005), the genotypes of hybrids found in nature do not accord with expectations from these laboratory studies: all F₁ hybrids in nature come from matings between D. santomea females and D. yakuba males, matings that occur only rarely in the laboratory.

Despite the genetic tractability of many of Drosophila species, the genus has few examples of the “classic” type of hybrid zone, in which the ranges of two species overlap with a gradual transition from one species to another through an area where hybrids are produced. Here we describe a classic hybrid zone in Drosophila that involves two sister species, Drosophila yakuba and D. santomea, on the island of São Tomé. Our transect of this zone has yielded several surprising and anomalous findings. First, we detected the presence of an additional hybrid zone largely outside the range of both parental species. This phenomenon is, to our knowledge, unique among animals. Second, the genetic analysis using diagnostic molecular markers of the flies collected in this anomalous hybrid zone indicates that nearly all hybrid males are F₁s that carry the D. santomea X chromosome. This F₁ genotype is much more difficult to produce in the laboratory compared to the genotype from the reciprocal cross, showing that sexual isolation as seen in the laboratory is insufficient to explain the genotypes of hybrids found in the wild. Third, there is a puzzling absence of hybrid females. We suggest several tentative explanations for the anomalies associated with this hybrid zone, but for the present they remain a mystery.

In promiscuously mating species, there is strong selection on males to maximize their share of paternity through both defensive and offensive means. This has been most extensively examined using the Drosophila melanogaster model system. In these studies, sperm competition has been examined by mating a virgin female to two consecutive males and then determining the fertilization success of both the first male (defending, P₁) and the second male (offending, P₂). Recent evidence suggests that male defense may be influenced by female mating history (i.e., virgin versus nonvirgin). Here, by mating females to males with three different genotypes, we show that female mating history does not affect male defensive or offensive abilities in sperm competition. We also show that, although female lifetime fecundity was not correlated with the number of times that she mated, it was reduced by increased exposure to males. These data indicate that measures of P₁ and P₂ previously reported in D. melanogaster may be robust to the specific mating history of the females used in these studies.

The dipteran Drosophila melanogaster can express a form of reproductive quiescence or diapause when exposed to low temperature and shortened photoperiod. Among natural populations in the eastern United States, the frequency of lines that express reproductive diapause in the laboratory varies substantially and predictably with latitudinal origin. The goals of the present study were twofold: (1) to examine the impact of genetic variance for diapause expression on multiple traits associated with organismal fitness; and (2) to evaluate the potential for fitness trade-offs between diapause and nondiapause phenotypes that may result in the observed cline. Even prior to diapause entry or expression, inbred lines that express and do not express reproductive diapause in laboratory assays were constitutively distinct for life span, age-specific mortality rates, fecundity profiles, resistance to cold and starvation stress, lipid content, development time, and egg-to-adult viability. Furthermore, estimates of genetic correlations based on line means revealed significant differentiation for genetic variance/covariance matrices between diapause and nondiapause lines. The data indicate the potential for life-history trade-offs associated with variation for the diapause phenotype. The observed cline in diapause incidence in the eastern United States may be generated by these trade-offs and the associated spatial and/or temporal variation in relative fitness of these two phenotypes in natural populations.

Because workers in the eusocial Hymenoptera are more closely related to sisters than to brothers, theory predicts that natural selection should act on them to bias (change) sex allocation to favor reproductive females over males. However, selection should also act on queens to prevent worker bias. We use a simulation approach to analyze the coevolution of this conflict in colonies with single, once-mated queens. We assume that queens bias the primary (egg) sex ratio and workers bias the secondary (adult) sex ratio, both at some cost to colony productivity. Workers can bias either by eliminating males or by directly increasing female caste determination. Although variation among colonies in kin structure is absent, simulations often result in bimodal (split) colony sex ratios. This occurs because of the evolution of two alternative queen or two alternative worker biasing strategies, one that biases strongly and another that does not bias at all. Alternative strategies evolve because the mechanisms of biasing result in accelerating benefits per unit cost with increasing bias, resulting in greater fitness for strategies that bias more and bias less than the population equilibrium. Strategies biasing more gain from increased biasing efficiency whereas strategies biasing less gain from decreased biasing cost. Our study predicts that whether queens or workers evolve alternative strategies depends upon the mechanisms that workers use to bias the sex ratio, the relative cost of queen and worker biasing, and the rates at which queen and worker strategies evolve. Our study also predicts that population and colony level sex allocation, as well as colony productivity, will differ diagnostically according to whether queens or workers evolve alternative biasing strategies and according to what mechanism workers use to bias sex allocation.

Most studies of reinforcement have focused on the evolution of either female choice or male mating cues, following the long-held view in sexual selection theory that mating mistakes are typically more costly for females than for males. However, factors such as conspecific sperm precedence can buffer females against the cost of mating mistakes, suggesting that in some hybrid zones mating mistakes may be more costly for males than for females. Thus, the historical bias in reinforcement research may underestimate its frequency. In this study, we present evidence that reinforcement has driven the evolution of male choice in a hybrid zone between the highly promiscuous leaf beetles Chrysochus cobaltinus and C. auratus, the hybrids of which have extremely low fitness. In addition, there is evidence for male choice in these beetles and that male mating mistakes may be costly, due to reduced opportunities to mate with conspecific females. The present study combines laboratory and field methods to quantify the strength of sexual isolation, test the hypothesis of reproductive character displacement, and assess the link between relative abundance and the strength of selection against hybridization. We document that, while sexual isolation is weak, it is sufficient to produce positive assortative mating. In addition, reproductive character displacement was only detected in the relatively rare species. The strong postzygotic barriers in this system are sufficient to generate the bimodality that characterizes this hybrid zone, but the weak sexual isolation is not, calling into question whether strong prezygotic isolation is necessary for the maintenance of bimodality. Growing evidence that the cost of mating mistakes is sufficient to shape the evolution of male mate choice suggests that the reinforcement of male mate choice may prove to be a widespread occurrence.

The marine environment is characterized by few physical barriers, and pelagic fishes commonly show high migratory potential and low, albeit in some cases statistically significant, levels of genetic divergence in neutral genetic marker analyses. However, it is not clear whether low levels of differentiation reflect spatially separated populations experiencing gene flow or shallow population histories coupled with limited random genetic drift in large, demographically isolated populations undergoing independent evolutionary processes. Using information for nine microsatellite loci in a total of 1951 fish, we analyzed genetic differentiation among Atlantic herring from eleven spawning locations distributed along a longitudinal gradient from the North Sea to the Western Baltic. Overall genetic differentiation was low (θ = 0.008) but statistically significant. The area is characterized by a dramatic shift in hydrography from the highly saline and temperature stable North Sea to the brackish Baltic Sea, where temperatures show high annual variation. We used two different methods, a novel computational geometric approach and partial Mantel correlation analysis coupled with detailed environmental information from spawning locations to show that patterns of reproductive isolation covaried with salinity differences among spawning locations, independent of their geographical distance. We show that reproductive isolation can be maintained in marine fish populations exhibiting substantial mixing during larval and adult life stages. Analyses incorporating genetic, spatial, and environmental parameters indicated that isolating mechanisms are associated with the specific salinity conditions on spawning locations.

Behavioral changes have long been hypothesized to be an important driver of evolutionary diversification in animals, as they expose individuals to new environmental pressures and thus favor evolutionary divergence. There have been few empirical tests of this hypothesis, however, and the mechanisms linking behavioral changes and diversification processes remain controversial. We show here that Holarctic passerines with large brain size relative to body size, a character correlated with a high propensity for behavioral changes, generally have experienced more extensive subspecific diversification. This effect appears to be largely independent of other well-known mechanisms thought to promote diversification. As suggested by path analysis, relative brain size seems to affect diversification directly rather than indirectly through its presumed effect on range expansion, which is consistent with the original formulation of the behavioral drive hypothesis. Thus, the results support the long-held, intuitive hypothesis that behavioral changes facilitate evolutionary diversification.

Comparisons across multiple taxa can often clarify the histories of biogeographic regions. In particular, historic barriers to movement should have affected multiple species and, thus, result in a pattern of concordant intraspecific genetic divisions among species. A striking example of such comparative phylogeography is the recent observation that populations of many small mammals and reptiles living on the Baja California peninsula have a large genetic break between northern and southern peninsular populations. In the present study, I demonstrate that five species of near-shore fishes living on the Baja coastline of the Gulf of California share this genetic pattern. The simplest explanation for this concordant genetic division within both terrestrial and marine vertebrates is that the Baja Peninsula was fragmented by a Plio-Pleistocene marine seaway and that this seaway posed a substantial barrier to movement for near-shore fishes. For some fish species, the signal of this vicariance in mtDNA has been eroded by gene flow and is not evident with classic, equilibrium measures of population structure. Yet, significant divisions are apparent in coalescent analyses that jointly estimate divergence with gene flow. The genetic divisions within Gulf of California fishes also coincide with recognized biogeographic regions based on fish community composition and several environmental factors. It is likely that adaptation to regional environments and present-day oceanographic circulation limit gene exchange between biogeographic regions and help maintain evidence of past vicariance.

The tetrapod forelimb and hindlimb are serially homologous structures that share a broad range of developmental pathways responsible for their patterning and outgrowth. Covariation between limbs, which can introduce constraints on the production of variation, is related to the duplication of these developmental factors. Despite this constraint, there is remarkable diversity in limb morphology, with a variety of functional relationships between and within forelimb and hindlimb elements. Here we assess a hierarchical model of limb covariation structure based on shared developmental factors. We also test whether selection for morphologically divergent forelimbs or hindlimbs is associated with reduced covariation between limbs. Our sample includes primates, murines, a carnivoran, and a chiropteran that exhibit varying degrees of forelimb and hindlimb specialization, limb size divergence, and/or phylogenetic relatedness. We analyze the pattern and significance of between-limb morphological covariation with linear distance data collected using standard morphometric techniques and analyzed by matrix correlations, eigenanalysis, and partial correlations. Results support a common limb covariation structure across these taxa and reduced covariation between limbs in nonquadruped species. This result indicates that diversity in limb morphology has evolved without signficant modifications to a common covariation structure but that the higher degree of functional limb divergence in bats and, to some extent, gibbons is associated with weaker integration between limbs. This result supports the hypothesis that limb divergence, particularly selection for increased functional specialization, involves the reduction of developmental factors common to both limbs, thereby reducing covariation.

Comparative studies have increased greatly in number in recent years due to advances in statistical and phylogenetic methodologies. For these studies, a trade-off often exists between the number of species that can be included in any given study and the number of individuals examined per species. Here, we describe a simple simulation study examining the effect of intraspecific sample size on statistical error in comparative studies. We find that ignoring measurement error has no effect on type I error of nonphylogenetic analyses, but can lead to increased type I error under some circumstances when using independent contrasts. We suggest using ANOVA to evaluate the relative amounts of within- and between-species variation when considering a phylogenetic comparative study. If within-species variance is particularly large and intraspecific sample sizes small, then either larger sample sizes or comparative methods that account for measurement error are necessary.

In many species, individuals do not attain their full adult coloration until one or several years after reaching sexual maturity, and this signaling of juvenile status is thought to enable young individuals to avoid aggression from older, dominant conspecifics. We propose that hybridization may be one of several costs and benefits associated with such delayed maturation. We tested this idea in a hybrid zone of collared (Ficedula albicollis) and pied (F. hypoleuca) flycatchers on the Baltic islands of Öland and Gotland. One-year-old (subadult) male collared flycatchers differed from older birds in many plumage traits, and approached male pied flycatchers in phenotype. On both islands, subadult male collared flycatchers hybridized at a higher rate than adults. Mate-choice experiments in aviaries suggest that this difference is at least partly due to female pied flycatchers having a preference for subadults when constrained to choose a heterospecific mate. Because novel morphologies are often derived from changes in ontology, juvenile forms may resemble adults of closely related taxa. When such juveniles are reproductively mature, their phenotypic similarity to the adults of closely related species may increase their risk of hybridization.