WHY DOES A TRAIT EVOLVE MULTIPLE TIMES WITHIN A CLADE? REPEATED EVOLUTION OF SNAKELINE BODY FORM IN SQUAMATE REPTILES

Abstract Why does a trait evolve repeatedly within a clade? When examining the evolution of a trait, evolutionary biologists typically focus on the selective advantages it may confer and the genetic and developmental mechanisms that allow it to vary. Although these factors may be necessary to explain why a trait evolves in a particular instance, they may not be sufficient to explain phylogenetic patterns of repeated evolution or conservatism. Instead, other factors may also be important, such as biogeography and competitive interactions. In squamate reptiles (lizards and snakes) a dramatic transition in body form has occurred repeatedly, from a fully limbed, lizardlike body form to a limbreduced, elongate, snakelike body form. We analyze this trait in a phylogenetic and biogeographic context to address why this transition occurred so frequently. We included 261 species for which morphometric data and molecular phylogenetic information were available. Among the included species, snakelike body form has evolved about 25 times. Most lineages of snakelike squamates belong to one of two ecomorphs, either short‐tailed burrowers or long‐tailed surface dwellers. The repeated origins of snakelike squamates appear to be associated with the in situ evolution of these two ecomorphs on different continental regions (including multiple origins of the burrowing morph within most continents), with very little dispersal of most limb‐reduced lineages between continental regions. Overall, the number of repeated origins of snakelike morphology seems to depend on large‐scale biogeographic patterns and community ecology, in addition to more traditional explanations (e.g., selection, development).     

DOES LARGE BODY SIZE IN MALES EVOLVE TO FACILITATE FORCIBLE INSEMINATION? A STUDY ON GARTER SNAKES

A trend for larger males to obtain a disproportionately high number of matings, as occurs in many animal populations, typically is attributed either to female choice or success in male-male rivalry; an alternative mechanism, that larger males are better able to coercively inseminate females, has received much less attention. For example, previous studies on garter snakes (Thamnophis sirtalis parietalis) at communal dens in Manitoba have shown that the mating benefit to larger body size in males is due to size-dependent advantages in male-male rivalry. However, this previous work ignored the possibility that larger males may obtain more matings because of male-female interactions. In staged trials within outdoor arenas, larger body size enhanced male mating success regardless of whether a rival male was present. The mechanism involved was coercion rather than female choice, because mating occurred most often (and soonest) in females that were least able to resist courtship-induced hypoxic stress. Males do physically displace rivals from optimal positions in the mating ball, and larger males are better able to resist such displacement. Nonetheless, larger body size enhances male mating success even in the absence of such male-male interactions. Thus, even in mating systems where males compete physically and where larger body size confers a significant advantage in male-male competition, the actual selective force for larger body size in males may relate to forcible insemination of unreceptive females. Experimental studies are needed to determine whether the same situation occurs in other organisms in which body-size advantages have been attributed to male-male rather than male-female interactions.     

DOES PHENOTYPIC PLASTICITY FOR ADULT SIZE VERSUS FOOD LEVEL IN DROSOPHILA MELANOGASTER EVOLVE IN RESPONSE TO ADAPTATION TO DIFFERENT REARING DENSITIES?

Recent studies with Drosophila have suggested that there is extensive genetic variability for phenotypic plasticity of body size versus food level. If true, we expect that the outcome of evolution at very different food levels should yield genotypes whose adult size show different patterns of phenotypic plasticity. We have tested this prediction with six independent populations of Drosophila melanogaster kept at extreme densities for 125 generations. We found that the phenotypic plasticity of body size versus food level is not affected by selection or the presence of competitors of a different genotype. However, we document increasing among population variation in phenotypic plasticity due to random genetic drift. Several reasons are explored to explain these results including the possibility that the use of highly inbred lines to make inferences about the evolution of genetically variable populations may be misleading.     

DOES DIZOCILPINE (MK-801) INHIBIT THE DEVELOPMENT OF MORPHINE-INDUCED BEHAVIOURAL SENSITIZATION IN RATS?

Intermittent morphine pretreatment (10 mg/kg/day for 14 days) induced long-lasting (one month post-treatment) sensitization to the locomotor effects of morphine and amphetamine in rats. Co-administration of the non-competitive NMDA-receptor antagonist dizocilpine (MK-801) (0.1 mg/kg) with morphine did not prevent the development of long-term behavioural sensitization. However, this dose of MK-801 did cause long-term sensitization to its own locomotor effects. Co-administration of 0.25 mg/kg MK-801 with morphine caused death in 60% of the animals. In the animals that survived MK-801 plus morphine pretreatment, neither short-term (3 days) nor long-term morphine-induced sensitization was observed. MK-801 alone (0.25 mg/kg/day for 14 days) induced short-term cross-sensitization to morphine. Thus, the development of long-term morphine-induced locomotor sensitization could only be prevented by a dose of MK-801 that yields a lethal combination with morphine. In addition, MK-801 induced sensitization to its own locomotor effects and cross-sensitization to morphine. These findings seriously question whether MK-801 can be used to study the development of morphine-induced behavioural sensitization. © 1997 Elsevier Science Inc.     

DOES DINOPHYSIS CAUDATA (DINOPHYCEAE) HAVE PERMANENT PLASTIDS?1

The marine photosynthetic dinoflagellates Dinophysis Ehrenb. species are obligate mixotrophs that require both light and the ciliate prey Myrionecta rubra (= Mesodinium rubrum) for long‐term survival. Despite rapid progress on the study of Dinophysis using laboratory cultures, however, whether it has its own permanent plastids or kleptoplastids (i.e., stolen plastids from its ciliate prey) is not fully resolved. Here, we addressed this issue using established cultures of D. caudata Saville‐Kent strain DC‐LOHABE01 and cross‐feeding/starvation experiments encompassing the prey M. rubra strain MR‐MAL01 cultures grown on two different cryptophytes (strains CR‐MAL01 and CR‐MAL11). To follow the fate of prey plastids, psbA gene as a tracer was amplified from individually isolated D. caudata cells, and the PCR products were digested with a restriction enzyme, SfaNI. The RFLP pattern of the PCR products digested by SfaNI revealed that D. caudata continued to keep CR‐MAL01–type plastids, while it lost CR‐MAL11–type plastids with increasing starvation time. Our results suggest that Dinophysis treats in different ways plastids taken up from different cryptophytes via its ciliate prey M. rubra. Alternatively, D. caudata may already have its own CR‐MAL01–type permanent plastid, with two types of plastids (CR‐MAL01 and CR‐MAL11) obtained from M. rubra being lost within 1 month. This result highlights the need to identify more accurately the origin of plastids in newly isolated photosynthetic Dinophysis species to resolve the issue of plastid permanence.     

ARE RATES OF DIVERSIFICATION IN SUBTERRANEAN SOUTH AMERICAN TUCO-TUCOS (GENUS CTENOMYS,RODENTIA: OCTODONTIDAE) UNUSUALLY HIGH?

Subterranean rodents have been used frequently as examples of explosive speciation in mammals. We tested for differential rates of diversification by using information from molecular phylogenies to focus primarily on tuco-tucos (Rodentia: Octodontidae), the most speciose lineage of subterranean rodents. Tuco-tucos were not significantly more diverse than their sister taxon (octodontines); however, a lineages-through-time analysis suggests an increase in diversification at the base of the tuco-tuco clade.     

DOES SHADE PROLONG JUVENILE DEVELOPMENT? A MORPHOLOGICAL ANALYSIS OF LEAF SHAPE CHANGES IN CUCURBIT A ARGYROSPERMA SUBSP. SORORIA (CUCURBITACEAE)

Several studies have concluded that shade extends the juvenile phase of plant development based on the prolonged production of juvenile-looking leaves along the shoot. Until now, the alternative hypothesis that leaves produced in shade converge in shape with more juvenile leaves through plastic responses of individual leaves has not been investigated. The literature has shown that differences in shape among leaves in a heteroblastic series are manifest very early in development, often at or near inception, whereas divergence in development between sun and shade leaves does not become apparent until considerably later. This study is the first to distinguish between these alternatives by comparing the developmental morphology of young leaves of the heteroblastic plant Cucurbita argyrosperma subsp. sororia. Differences in shapes of mature leaves along the shoot in sun and shade were quantified in terms of leaf area, perimeter, and shape using truss analysis. Developmental morphology from initiation through expansion was examined for representative transition and for later (adult) leaves using scanning electron microscopy and allometry. Determinants of shape established very early in development were the same for leaves at the same position grown in sun and shade. Differences in morphology between sun and shade leaves at the same position did not arise until these leaves reached lamina lengths greater than 1,000 μm. Thus, the less-lobed, more juvenile looking leaf produced at later positions in the shade arose through later developmental responses of individual leaves to shade, rather than through a prolonged phase of juvenile development.     

ABSENCE OF EVIDENCE IS NOT EVIDENCE OF ABSENCE: IS STEPHANODISCUS BINDERANUS (BACILLARIOPHYCEAE) AN EXOTIC SPECIES IN THE GREAT LAKES REGION?1

The eutrophic, freshwater diatom species Stephanodiscus binderanus (Kütz.) Willi Krieg. has long been considered a nuisance exotic alga introduced from Eurasia to the Great Lakes in North America in the early to mid‐20th century. However, our paleolimnological data from Lake Simcoe, Ontario, provide unequivocal evidence that this taxon has been present in the Great Lakes region since at least the late 17th century. Subfossil diatom valves were identified and enumerated at high resolution in ²¹⁰Pb‐dated sediment cores from four sites across the lake. The taxonomic identification of S. binderanus was confirmed using SEM. The historical presence of this species in Lake Simcoe indicates somewhat naturally productive conditions and also refutes the idea that S. binderanus is a nonindigenous species to North America. This study underscores the caution that should be applied to questions of diatom (and protistan) distributions in time and space. Clearly, the absence of evidence is not evidence of absence.     

INTERSEXUAL ARMS RACE? GENITAL COEVOLUTION IN NEPHILID SPIDERS (ARANEAE,NEPHILIDAE)

Genital morphology is informative phylogenetically and strongly selected sexually. We use a recent species-level phylogeny of nephilid spiders to synthesize phylogenetic patterns in nephilid genital evolution that document generalized conflict between male and female interests. Specifically, we test the intersexual coevolution hypothesis by defining gender-specific indices of genital complexity that summarize all relevant and phylogenetically informative traits. We then use independent contrasts to show that male and female genital complexity indices correlate significantly and positively across the phylogeny rather than among sympatric sister species, as predicted by reproductive character displacement. In effect, as females respond to selection for fecundity-driven fitness via giantism and polyandry (perhaps responding to male-biased effective sex ratios), male mechanisms evolve to monopolize females (male monogamy) via opportunistic mating, pre- and postcopulatory mate guarding, and/or plugging of female genitalia to exclude subsequent suitors. In males morphological symptoms of these phenomena range from self-mutilated genitalia to total castration. Although the results are compatible with both recently favored sexual selection hypotheses, sexually antagonistic coevolution, and cryptic female choice, the evidence of strong intersexual conflict and genitalic damage in both sexes is more easily explained as sexually antagonistic coevolution due to an evolutionary arms race.     

COMMENT ON GOHLI ET AL. (2013): “DOES PROMISCUITY EXPLAIN DIFFERENCES IN LEVELS OF GENETIC DIVERSITY ACROSS PASSERINE BIRDS?”

Gohli et al. (2013) report a positive relationship between genetic diversity and promiscuity across passerine birds, and suggest that female promiscuity acts as a form of balancing selection, maintaining differences in genetic variation across species. This is an interesting hypothesis, but the enormous variation in genetic diversity present within species is not taken into account in their analyses. This, combined with a small sample size at several levels, makes the relationship between genetic diversity and promiscuity very difficult to interpret. Demonstrating that species‐level differences in genetic diversity (if they occur at all) are affected by promiscuity would require a far more comprehensive study than is presently possible.     

COSPECIATION ANALYSIS OF AN OBLIGATE POLLINATION MUTUALISM: HAVEGLOCHIDION TREES (EUPHORBIACEAE) AND POLLINATING EPICEPHALA MOTHS(GRACILLARIIDAE) DIVERSIFIED IN PARALLEL?

Species-specific obligate pollination mutualism between Glochidion trees (Euphorbiaceae) and Epicephala moths (Gracillariidae) involves a large number of interacting species and resembles the classically known fig-fig wasp and yucca-yucca moth associations. To assess the extent of parallel cladogenesis in Glochidion-Epicephala association, we reconstruct phylogenetic relationships of 18 species of Glochidion using nuclear ribosomal DNA sequences (internal and external transcribed spacers) and those of the corresponding 18 Epicephala species using mitochondrial (the cytochrome oxidase subunit I gene) and nuclear DNA sequences (the arginine kinase and elongation factor-1alpha genes). Based on the obtained phylogenies, we determine whether Glochidion and Epicephala have undergone parallel diversification using several different methods for investigating the level of cospeciation between phylogenies. These tests indicate that there is generally a greater degree of correlation between Glochidion and Epicephala phylogenies than expected in a random association, but the results are sensitive to selection of different phylogenetic hypotheses and analytical methods for evaluating cospeciation. Perfect congruence between phylogenies is not found in this association, which likely resulted from host shift by the moths. The observed significant discrepancy between Glochidion and Epicephala phylogenies implies that the one-to-one specificity between the plants and moths has been maintained through a complex speciation process or that there is an underestimated diversity of association between Glochidion trees and Epicephala moths.     

IS SPECIALIZATION A DEAD END? THE PHYLOGENY OF HOST USE IN DENDROCTONUS BARK BEETLES (SCOLYTIDAE)

Ecological explanations for the prevalence of resource specialists are abundant, whereas phylogenetic evidence on their origins is scarce. In this paper, we provide a molecular phylogenetic study of the 19 specialist or generalist species in the bark beetle genus Dendroctonus, which collectively attack species in four different genera in the conifer family Pinaceae. Given substantial variation in diet breadth, we asked two general questions concerning the evolution of resource use in this group. How conservative is the evolution of host use in these insects? Does specialization tend to be derived (i.e., a “dead end”)? To answer these questions, we estimated the phylogeny of Dendroctonus using mitochondrial DNA sequences and mapped transitions in resource use on the resulting phylogeny estimate. The evolution of affiliations with Pinus and Picea hosts in Dendroctonus was conservative among beetle species (PTP test; P < 0.012), but there was no significant correspondence between the phylogeny of these beetles and the phylogeny among their Pinaceae hosts (among genera, P = 0.28; among Pinus species, P = 0.82). Degree of specialization, as measured in the proportion of hosts used, was bimodally distributed with “generalist” species utilizing < 60% of the congeneric hosts within their range and six specialist species utilizing < 40% of the available hosts. Among the generalists, we found a strong correlation between the number of hosts encountered and the number of hosts utilized (R = 0.97, P < 0.0001), whereas there was no significant correlation among the specialists (R = 0.27, P = 0.59). The evolution of specialization in Dendroctonus proved highly labile—specialists arose from generalists at least six separate times (without reversal) all in derived positions, and closer examination of some specialists revealed instances where they appear to have lost particular host species from their diet. However, evidence from the ecological literature also suggests that several Dendroctonus generalists may have increased their range of host genera within the Pinaceae.     

IS FLORAL SPECIALIZATION AN EVOLUTIONARY DEAD‐END? POLLINATION SYSTEM TRANSITIONS IN RUELLIA (ACANTHACEAE)

Pollination systems frequently reflect adaptations to particular groups of pollinators. Such systems are indicative of evolutionary specialization and have been important in angiosperm diversification. We studied the evolution of pollination systems in the large genus Ruellia. Phylogenetic analyses, morphological ordinations, ancestral state reconstructions, and a character mapping simulation were conducted to reveal key patterns in the direction and lability of floral characters associated with pollination. We found significant floral morphological differences among species that were generally associated with different groups of floral visitors. Floral evolution has been highly labile and also directional. Some specialized systems such as hawkmoth or bat pollination are likely evolutionary dead-ends. In contrast, specialized pollination by hummingbirds is clearly not a dead-end. We found evidence for multiple reverse transitions from presumed ancestral hummingbird pollination to more derived bee or insect pollination. These repeated origins of insect pollination from hummingbird-pollinated ancestors have not evolved without historical baggage. Flowers of insect-pollinated species derived from hummingbird-pollinated ancestors are morphologically more similar to hummingbird flowers than they are to other more distantly related insect-pollinated flowers. Finally, some pollinator switches were concomitant with changes in floral morphology that are associated with those pollinators. These observations are consistent with the hypothesis that some transitions have been adaptive in the evolution of Ruellia.     

DO REEFS DRIVE DIVERSIFICATION IN MARINE TELEOSTS? EVIDENCE FROM THE PUFFERFISH AND THEIR ALLIES (ORDER TETRAODONTIFORMES)

A major challenge in evolutionary biology lies in explaining patterns of high species numbers found in biodiversity hot spots. Tropical coral reefs underlie most marine hot spots and reef-associated fish faunas represent some of the most diverse assemblages of vertebrates on the planet. Although the standing diversity of modern reef fish clades is usually attributed to their ecological association with corals, untangling temporal patterns of codiversification has traditionally proved difficult. In addition, owing to uncertainty in higher-level relationships among acanthomorph fish, there have been few opportunities to test the assumption that reef-association itself leads to higher rates of diversification compared to other habitats. Here we use relaxed-clock methods in conjunction with statistical measures of species accumulation and phylogenetic comparative methods to clarify the temporal pattern of diversification in reef and nonreef-associated lineages of tetraodontiforms, a morphologically diverse order of teleost fish. We incorporate 11 fossil calibrations distributed across the tetraodontiform tree to infer divergence times and compare results from models of autocorrelated and uncorrelated evolutionary rates. All major tetraodontiform reef crown groups have significantly higher rates of diversification than the order as a whole. None of the nonreef-associated families show this pattern with the exception of the aracanid boxfish. Independent contrasts analysis also reveals a significantly positive relationship between diversification rate and proportion of reef-associated species within each family when aracanids are excluded. Reef association appears to have increased diversification rate within tetraodontiforms. We suggest that both intrinsic factors of reef habitat and extrinsic factors relating to the provincialization and regionalization of the marine biota during the Miocene (about 23-5 MY) played a role in shaping these patterns of diversity.     

HOW DO GEOLOGICAL SAMPLING BIASES AFFECT STUDIES OF MORPHOLOGICAL EVOLUTION IN DEEP TIME? A CASE STUDY OF PTEROSAUR (REPTILIA: ARCHOSAURIA) DISPARITY

A fundamental contribution of paleobiology to macroevolutionary theory has been the illumination of deep time patterns of diversification. However, recent work has suggested that taxonomic diversity counts taken from the fossil record may be strongly biased by uneven spatiotemporal sampling. Although morphological diversity (disparity) is also frequently used to examine evolutionary radiations, no empirical work has yet addressed how disparity might be affected by uneven fossil record sampling. Here, we use pterosaurs (Mesozoic flying reptiles) as an exemplar group to address this problem. We calculate multiple disparity metrics based upon a comprehensive anatomical dataset including a novel phylogenetic correction for missing data, statistically compare these metrics to four geological sampling proxies, and use multiple regression modeling to assess the importance of uneven sampling and exceptional fossil deposits (Lagerstätten). We find that range‐based disparity metrics are strongly affected by uneven fossil record sampling, and should therefore be interpreted cautiously. The robustness of variance‐based metrics to sample size and geological sampling suggests that they can be more confidently interpreted as reflecting true biological signals. In addition, our results highlight the problem of high levels of missing data for disparity analyses, indicating a pressing need for more theoretical and empirical work.     

Cell Wall β-(1,6)-Glucan of Saccharomyces cerevisiae: STRUCTURAL CHARACTERIZATION AND IN SITU SYNTHESIS*S?

Despite its essential role in the yeast cell wall, the exact composition of the β-(1,6)-glucan component is not well characterized. While solubilizing the cell wall alkali-insoluble fraction from a wild type strain of Saccharomyces cerevisiae using a recombinant β-(1,3)-glucanase followed by chromatographic characterization of the digest on an anion exchange column, we observed a soluble polymer that eluted at the end of the solvent gradient run. Further characterization indicated this soluble polymer to have a molecular mass of ∼38 kDa and could be hydrolyzed only by β-(1,6)-glucanase. Gas chromatographymass spectrometry and NMR (¹H and ¹³C) analyses confirmed it to be a β-(1,6)-glucan polymer with, on average, branching at every fifth residue with one or two β-(1,3)-linked glucose units in the side chain. This polymer peak was significantly reduced in the corresponding digests from mutants of the kre genes (kre9 and kre5) that are known to play a crucial role in the β-(1,6)-glucan biosynthesis. In the current study, we have developed a biochemical assay wherein incubation of UDP-[¹⁴C]glucose with permeabilized S. cerevisiae yeasts resulted in the synthesis of a polymer chemically identical to the branched β-(1,6)-glucan isolated from the cell wall. Using this assay, parameters essential for β-(1,6)-glucan synthetic activity were defined.The cell wall of Saccharomyces cerevisiae and other yeasts contains two types of β-glucans. In the former yeast, branched β-(1,3)-glucan accounts for ∼50–55%, whereas β-(1,6)-glucan represents 10–15% of the total yeast cell wall polysaccharides, each chain of the latter extending up to 140–350 glucose residues in length. The amount of 3,6-branched glucose residues varies with the yeast species: 7, 15, and 75% in S. cerevisiae, Candida albicans, and Schizosaccharomyces pombe, respectively (1). β-(1,6)-Glucan stabilizes the cell wall, since it plays a central role as a linker for specific cell wall components, including β-(1,3)-glucan, chitin, and mannoproteins (2, 3). However, the exact structure of the β-(1,6)-glucan and the mode of biosynthesis of this polymer are largely unknown. In S. pombe, immunodetection studies suggested that synthesis of this polymer backbone begins in the endoplasmic reticulum, with extension occurring in the Golgi (4) and final processing at the plasma membrane. In S. cerevisiae, Montijn and co-workers (5), by immunogold labeling, detected β-(1,6)-glucan at the plasma membrane, suggesting that the synthesis takes place largely at the cell surface.More than 20 genes, including the KRE gene family (14 members) and their homologues, SKN1 and KNH1, have been reported to be involved in β-(1,6)-glucan synthesis in S. cerevisiae, C. albicans, and Candida glabrata (6–10). Among all of these genes, the ones that seem to play the major synthetic role are KRE5 and KRE9, since their disruption caused significant reduction (100 and 80%, respectively, relative to wild type) in the cell wall β-(1,6)-glucan content (11–13).To date, the biochemical reaction responsible for the synthesis of β-(1,6)-glucan and the product synthesized remained unknown. Indeed, in most cases, when membrane preparations are incubated with UDP-glucose, only linear β-(1,3)-glucan polymers are produced, although some studies have reported the production of low amounts of β-(1,6)-glucans by membrane preparations (14–17). These data suggest that disruption of the fungal cell prevents or at least has a strong negative effect on β-(1,6)-glucan synthesis. The use of permeabilized cells, which allows substrates, such as nucleotide sugar precursors, to be readily transported across the plasma membrane, is an alternative method to study in situ cell wall enzyme activities (18–22). A number of methods have been developed to permeabilize the yeast cell wall (23), of which osmotic shock was successfully used to demonstrate β-(1,3)-glucan and chitin synthase activities (20, 24). Herein, we describe the biochemical activity responsible for β-(1,6)-glucan synthesis using permeabilized S. cerevisiae cells and UDP-[¹⁴C]glucose as a substrate. We also have analyzed the physicochemical parameters of this activity and chemically characterized the end product and its structural organization within the mature yeast cell wall.     

HAVE MALE AND FEMALE GENITALIA COEVOLVED? A PHYLOGENETIC ANALYSIS OF GENITALIC MORPHOLOGY AND SEXUAL SIZE DIMORPHISM IN WEB‐BUILDING SPIDERS (ARANEAE: ARANEOIDEA)

Sexual size dimorphism (SSD) can strongly influence the evolution of reproductive strategies and life history. If SSD is extreme, and other characters (e.g., genitalic size) also increase with size, then functional conflicts may arise between the sexes. Spiders offer an excellent opportunity to investigate this issue because of their wide range of SSD. By using modern phylogenetic methods with 16 species of orb-weaving spiders, we provide strong evidence for the "positive genitalic divergence" model, implying that sexual genitalic dimorphism (SGD) increases as SSD increases. This pattern is supported by an evolutionary mismatch between the absolute sizes of male and female genitalia across species. Indeed, our findings reveal a dramatic reversal from male genitalia that are up to 87x larger than female genitalia in size-monomorphic species to female genitalia that are up to 2.8x larger in extremely size-dimorphic species. We infer that divergence in SGD could limit SSD both in spiders, and potentially in other taxa as well. Further, male and female body size, as well as male and female genitalia size, are decoupled evolutionarily. Finally, we show a negative scaling (hypoallometry) of male and female genitalic morphology within sexes. Evolutionary forces specific to each sex, such as larger female size (increased fecundity) or smaller male size (enhanced mate-searching ability), may be balanced by stabilizing selection on relative genitalic size.