Variation in Body Shape across Species and Populations in a Radiation of Diaptomid Copepods

Inter and intra-population variation in morphological traits, such as body size and shape, provides important insights into the ecological importance of individual natural populations. The radiation of Diaptomid species (~400 species) has apparently produced little morphological differentiation other than those in secondary sexual characteristics, suggesting sexual, rather than ecological, selection has driven speciation. This evolutionary history suggests that species, and conspecific populations, would be ecologically redundant but recent work found contrasting ecosystem effects among both species and populations. This study provides the first quantification of shape variation among species, populations, and/or sexes (beyond taxonomic illustrations and body size measurements) to gain insight into the ecological differentiation of Diaptomids. Here we quantify the shape of five Diaptomid species (family Diaptomidae) from four populations each, using morphometric landmarks on the prosome, urosome, and antennae. We partition morphological variation among species, populations, and sexes, and test for phenotype-by-environment correlations to reveal possible functional consequences of shape variation. We found that intraspecific variation was 18-35% as large as interspecific variation across all measured traits. Interspecific variation in body size and relative antennae length, the two traits showing significant sexual dimorphism, were correlated with lake size and geographic location suggesting some niche differentiation between species. Observed relationships between intraspecific morphological variation and the environment suggest that divergent selection in contrasting lakes might contribute to shape differences among local populations, but confirming this requires further analyses. Our results show that although Diaptomid species differ in their reproductive traits, they also differ in other morphological traits that might indicate ecological differences among species and populations.     

Sexual selection and the allometry of earwig forceps

Summary Positive intraspecific allometry, the tendency for large individuals to have relatively larger morphological traits, is thought to be more likely for secondary sexual traits than naturally selected traits. This is because secondary sexual traits are often used to signal individual quality and positive allometry should arise where the costs and/or benefits of signalling are size dependent. Here we examine the allometric relationships between forceps length, a sexually selected trait and elytra length, a naturally selected trait, in 42 species of earwig. Both forceps and elytra showed positive allometry. However, the degree of allometry was greater for forceps as predicted. If allometry arises due to sexual selection we would predict a greater degree of allometry in species with more exaggerated secondary sexual traits. Across species, the degree of forcep allometry did increase with forcep exaggeration. The relevance of positive allometry to reliable signalling is discussed.     

Fin-mutant female zebrafish (Danio rerio) exhibit differences in association preferences for male fin length

Females often choose to associate with males that have exaggerated traits. In fishes, this may reflect an overall preference for larger size in a potential mate. Female zebrafish (Danio rerio) prefer males with larger bodies but not longer fins. The availability of mutant and transgenic strains of zebrafish make this a unique model system in which to study the role of phenotypic variation in social and sexual behavior. We used mutant strains of zebrafish with truncated (short fin) and exaggerated (long fin) fins to further examine female preferences for fin length in dichotomous association tests. Wild type females showed no preferences between wild type males and short fin mutant males or between wild type males and long fin mutant males. short fin females also showed no preference for short fin males or wild type males while long fin females preferred to associate with long fin males over wild type males. These results suggest that the single gene long fin mutation that results in altered fin morphological may also be involved in a related female association preference.     

Intraspecific mating system evolution and its effect on complex male secondary sexual traits: Does male–male competition increase selection on size or shape?

Sexual selection is generally held responsible for the exceptional diversity in secondary sexual traits in animals. Mating system evolution is therefore expected to profoundly affect the covariation between secondary sexual traits and mating success. Whereas there is such evidence at the interspecific level, data within species remain scarce. We here investigate sexual selection acting on the exaggerated male fore femur and the male wing in the common and widespread dung flies Sepsis punctum and S. neocynipsea (Diptera: Sepsidae). Both species exhibit intraspecific differences in mating systems and variation in sexual size dimorphism (SSD) across continents that correlates with the extent of male–male competition. We predicted that populations subject to increased male–male competition will experience stronger directional selection on the sexually dimorphic male foreleg. Our results suggest that fore femur size, width and shape were indeed positively associated with mating success in populations with male‐biased SSD in both species, which was not evident in conspecific populations with female‐biased SSD. However, this was also the case for wing size and shape, a trait often assumed to be primarily under natural selection. After correcting for selection on overall body size by accounting for allometric scaling, we found little evidence for independent selection on any of these size or shape traits in legs or wings, irrespective of the mating system. Sexual dimorphism and (foreleg) trait exaggeration is therefore unlikely to be driven by direct precopulatory sexual selection, but more so by selection on overall size or possibly selection on allometric scaling.     

Within-population variation in the chemistry of life: the stoichiometry of sexual dimorphism in multiple dimensions

In sexual species, phenotypic divergence between males and females, or sexual dimorphism, is often the source of the most staggering examples of phenotypic variation in nature. Theory suggests that exaggerated sexual traits should drive sex-specific nutritional demands. Advances in spectrometry enable rapid quantification of the elements that make up individuals and traits, which can be used to assess patterns of intraspecific variation and the contribution of nutritionally-demanding sexual traits to these patterns. We measured dimorphism in the whole body stoichiometry of Hyalella amphipods and examined whether nutritional demands of exaggerated sexual traits differ from those of similar traits not under sexual selection. We found striking sexual dimorphism in multivariate whole body elemental composition (i.e., the ionome), including elements important for organismal growth and performance. In males, the exaggerated, sexually-selected claw-like appendage (posterior gnathopod) differed significantly in mass-specific stoichiometry from a similarly sized and serially homologous non-sexual trait (fifth pereopod), indicating that there are fundamental differences in the construction of sexual traits in relation to similar traits that are not under sexual selection. While sexually selected traits do differ from non-sexual traits in their ionomes, we found that possessing an exaggerated trait does not change organismal stoichiometry, indicating that trait exaggeration may not be directly driving ionomic sexual dimorphism. Finally, we found that larger traits are not comparatively larger resource sinks for any element, suggesting that the possession of larger traits is not a function of greater allocation of resources. Together, we discovered substantial sexual dimorphism at the lowest level of organization, chemical elements. Such information illuminates predictions about dimorphisms in foraging behavior, nutritional physiology, and sex-specific selection on the underlying loci. High throughput, multidimensional data on sexual divergence in stoichiometric composition is a powerful tool in understanding the evolutionary ecology of sexual dimorphisms.     

The cost of secondary sexual characters and the evolution of cost-reducing traits

Secondary sexual traits are characterized by their exaggerated expression relative to homologous nonsexual characters in other species. All models of sexual selection assume that sex traits are costly to produce and maintain, and individuals with reduced costs of production and maintenance of secondary sexual characters would be at a selective advantage. A number of morphological, physiological and behavioural traits may have evolved as a result of their cost-reducing properties: (1) body size, which does not change throughout life, that allows certain individuals to develop exaggerated sex traits, (2) cost-reducing traits, such as muscle size, that improve with practice and (3) actual cost-reducing traits, such as wing size in birds with song flight, which are produced in advance of or simultaneously with the sex trait. Cost-reducing traits may coevolve with secondary sexual characters and allow more extreme sexual signalling than would otherwise have been possible in their absence or in reduced versions.     

The cost of secondary sexual characters and the evolution of cost-reducing traits

Secondary sexual traits are characterized by their exaggerated expression relative to homologous nonsexual characters in other species. All models of sexual selection assume that sex traits are costly to produce and maintain, and individuals with reduced costs of production and maintenance of secondary sexual characters would be at a selective advantage. A number of morphological, physiological and behavioural traits may have evolved as a result of their cost-reducing properties: (1) body size, which does not change throughout life, that allows certain individuals to develop exaggerated sex traits, (2) cost-reducing traits, such as muscle size, that improve with practice and (3) actual cost-reducing traits, such as wing size in birds with song flight, which are produced in advance of or simultaneously with the sex trait. Cost-reducing traits may coevolve with secondary sexual characters and allow more extreme sexual signalling than would otherwise have been possible in their absence or in reduced versions.     

Lifestyle-Based Approaches Provide Insights into Body Size Variation Across Environmental Gradients in Anurans

Body size of organisms as a fitness-related phenotype has evolved in response to local conditions, often through the size-dependent thermoregulatory mechanisms. The direction and degree of this response should depend on animals’ lifestyle in terms of the preference for terrestrial or aquatic conditions, especially so for adult anurans that differ in lifestyle among species but all must maintain certain body temperatures for metabolism. It may be expected that anuran species frequently exposed to terrestrial environments characterized by fluctuant thermal conditions are more plastic in body size along thermal gradients than those highly relaying on aquatic environments where thermal conditions are relatively stable. We test this prediction using both interspecific and intraspecific data. With anurans in China as the model organisms, we show that across terrestrial species but not aquatic species, body size decreases with increasing ambient temperature. From the published literature worldwide, we summarized that more terrestrial versus fewer aquatic species follow the predicted ecogeographical size patterns. In addition, both interspecific and intraspecific data reveal that arboreal anurans do not exhibit the size cline, probably because relatively warm climates experienced by these species impose weak selective pressures on heat conservation or adaptation to tree-climbing constrains the variation in body size. Our finding highlights the importance of taking lifestyle into account when assessing macroevolutionary trends in body size for anurans in particular and ectothermic taxa in general.     

Tropical dung beetle morphological traits predict functional traits and show intraspecific differences across land uses

Functional traits and functional diversity measures are increasingly being used to examine land use effects on biodiversity and community assembly rules. Morphological traits are often used directly as functional traits. However, behavioral characteristics are more difficult to measure. Establishing methods to derive behavioral traits from morphological measurements is necessary to facilitate their inclusion in functional diversity analyses. We collected morphometric data from over 1,700 individuals of 12 species of dung beetle to establish whether morphological measurements can be used as predictors of behavioral traits. We also compared morphology among individuals collected from different land uses (primary forest, logged forest, and oil palm plantation) to identify whether intraspecific differences in morphology vary among land use types. We show that leg and eye measurements can be used to predict dung beetle nesting behavior and period of activity and we used this information to confirm the previously unresolved nesting behavior for Synapsis ritsemae. We found intraspecific differences in morphological traits across different land use types. Phenotypic plasticity was found for traits associated with dispersal (wing aspect ratio and wing loading) and reproductive capacity (abdomen size). The ability to predict behavioral functional traits from morphology is useful where the behavior of individuals cannot be directly observed, especially in tropical environments where the ecology of many species is poorly understood. In addition, we provide evidence that land use change can cause phenotypic plasticity in tropical dung beetle species. Our results reinforce recent calls for intraspecific variation in traits to receive more attention within community ecology.     

Components of phenotypic variation in avian ornamental and non-ornamental feathers

Phenotypic variation, measured as the coefficient of variation (CV), is usually larger in secondary sexual characters than in ordinary morphological traits. We tested if intraspecific differences in the CV between ornamental and non-ornamental feather traits in 67 evolutionary events of feather ornamentation in birds were due to differences in (1) the allometric pattern (slope of the regression line when regressing trait size on an indicator of body size), or (2) the dispersion of observations around the regression line. We found that only dispersion of observations around the regression line contributed significantly to total variation. A large dispersion of observations around the regression line for ornamental feathers is consistent with these characters showing condition-dependence, supporting indicator models of sexual selection more strongly than a pure Fisher process. Ornamental feathers in males demonstrated negative allometry when regressed on tarsus length, which is a measure of skeletal body size. This finding is consistent with ornamental feather traits being subject to directional selection due to female mate preferences, where large body size is less important than in male–male competition. This pattern of phenotypic variation for avian secondary sexual characters contrasts with patterns of variation for insect genitalia, supposedly subject to sexual selection, since the latter traits only differ from ordinary morphology traits in allometry coefficient. The prevailing regime of selection (directional or stabilizing) and the effects of environmental factors are proposed to account for these differences among traits.     

Patterns of fluctuating asymmetry in sexual ornaments predict female choice

Extravagant secondary sexual characters are assumed to have arisen and be maintained by sexual selection. While traits like horns, antlers and spurs can be ascribed to intrasexual competition, other traits such as extravagant feather ornaments, displays and pheromones have to be ascribed to mate choice. A number of studies have tested whether females exert selection on the size of male ornaments, but only some of these have recorded female preferences for the most extravagantly ornamented males. Here I demonstrate that female choice can be directly predicted from the relationship between the degree of fluctuating asymmetry and the size of a secondary sexual character. Fluctuating asymmetry is an epigenetic measure of the ability of individuals to cope with stress, and it occurs when an individual is unable to undergo identical development of an otherwise bilaterally symmetric trait on both sides of its body. There is a negative relationship between the degree of fluctuating asymmetry and the absolute size of an ornament in those bird species with a female preference for the largest male sex trait, while there is a flat or U-shaped relationship among species without a female preference. These results suggest that females prefer exaggerated secondary sexual characters if they reliably demonstrate the ability of males to cope with genetic and environmental stress. Some species may demonstrate a flat or U-shaped relationship between the degree of fluctuating asymmetry and the absolute size of an ornament because (i) the genetic variance in viability signalled by the secondary sex trait has been depleted; (ii) the secondary sex trait is not particularly costly and therefore does not demonstrate condition dependence; or because (iii) the sex traits can be considered arbitrary traits rather than characters reflecting good genes.     

Selection on sperm size in response to promiscuity and variation in female sperm storage organs

Sperm cells are exceptionally morphologically diverse across taxa. However, morphology can be quite uniform within species, particularly for species where females copulate with many males per reproductive bout. Strong sexual selection in these promiscuous species is widely hypothesized to reduce intraspecific sperm variation. Conversely, we hypothesize that intraspecific sperm size variation may be maintained by high among-female variation in the size of sperm storage organs, assuming that paternity success improves when sperm are compatible in size with the sperm storage organ. We use individual-based simulations and an analytical model to evaluate how selection on sperm size depends on promiscuity level and variation in sperm storage organ size (hereafter, female preference variation). Simulations of high promiscuity (10 mates per female) showed stabilizing selection on sperm when female preference variation was low, and disruptive selection when female preference variation was high, consistent with the analytical model results. With low promiscuity (2–3 mates per female), selection on sperm was stabilizing for all levels of female preference variation in the simulations, contrasting with the analytical model. Promiscuity level, or mate sampling, thus has a strong impact on the selection resulting from female preferences. Furthermore, when promiscuity is low, disruptive selection on male traits will occur under much more limited circumstances (i.e. only with higher among-female variation) than many previous models suggest. Variation in female sperm storage organs likely has strong implications for intraspecific sperm variation in highly promiscuous species, but likely does not explain differences in intraspecific sperm variation for less promiscuous taxa.     

Sexual dimorphism in a trophically polymorphic cichlid fish?

Sexual dimorphism in ecologically relevant traits is ubiquitous in animals. However, other types of intraspecific phenotypic divergence, such as trophic polymorphism, are less common. Because linkage to sex should often lead to balancing selection, understanding the association between sex and phenotypic divergence could help explain why particular species show high morphological variability. To determine if sexual dimorphism could be helping to maintain ecomorphological variation in a classic case of intraspecific trophic polymorphism, we examined the association between sex and morphological divergence in the cichlid Herichthys minckleyi. Although H. minckleyi with enlarged molariform teeth on their pharyngeal jaws have been reported to more commonly be male, we did not find an association between sex and pharyngeal morphotype. Sex was associated with divergence in body size (as measured through standard length). But, sex was not associated with any of the other trophic traits examined. However, pharyngeal morphotype did show an association with gut length, gape, and tooth number. Sexual dimorphism is not playing a central role in enhancing trophic diversity within H. minckleyi. J. Morphol. 276:1448–1454, 2015. © 2015 Wiley Periodicals, Inc.     

Multiple speciation events in an arthropod with divergent evolution in sexual morphology

Sexual selection can facilitate divergent evolution of traits related to mating and consequently promote speciation. Theoretically, independent operation of sexual selection in different populations can lead to divergence of sexual traits among populations and result in allopatric speciation. Here, we show that divergent evolution in sexual morphology affecting mating compatibility (body size and genital morphologies) and speciation have occurred in a lineage of millipedes, the Parafontaria tonominea species complex. In this millipede group, male and female body and genital sizes exhibit marked, correlated divergence among populations, and the diverged morphologies result in mechanical reproductive isolation between sympatric species. The morphological divergence occurred among populations independently and without any correlation with climatic variables, although matching between sexes has been maintained, suggesting that morphological divergence was not a by-product of climatic adaptation. The diverged populations underwent restricted dispersal and secondary contact without hybridization. The extent of morphological difference between sympatric species is variable, as is diversity among allopatric populations; consequently, the species complex appears to contain many species. This millipede case suggests that sexual selection does contribute to species richness via morphological diversification when a lineage of organisms consists of highly divided populations owing to limited dispersal.     

A Review for Life-history Traits Variation in Frogs Especially for Anurans in China

Environmental variation can promote differentiation in life-history traits in species of anurans. Increased environmental stress usually results in larger age at sexual maturity, older mean age, longer longevity, slower growth, larger body size, and a shift in reproductive allocation from offspring quantity to quality, and a stronger trade-off between offspring size and number. However, previous studies have suggested that there are inconsistent geographical variations in life-history traits among anuran species in China. Hence, we here review the intraspecific patterns and differences in life-history traits(i.e., egg size, clutch size, testes size, sperm length, age at sexual maturity, longevity, body size and sexual size dimorphism) among different populations within species along geographical gradients for anurans in China in recent years. We also provide future directions for studying difference in sperm performance between longer and shorter sperm within a species through transplant experiments and the relationships between metabolic rate and brain size and life-history.     

Contemporary sexual selection does not explain variation in male display traits among populations

Sexual selection is widely hypothesized to facilitate the evolution of reproductive isolation through divergence in sexual traits and sexual trait preferences among populations. However, direct evidence of divergent sexual selection causing intraspecific trait divergence remains limited. Using the wolf spider Schizocosa crassipes, we characterized patterns of female mate choice within and among geographic locations and related those patterns to geographic variation in male display traits to test whether divergent sexual selection caused by mate choice explains intraspecific trait variation. We found evidence of phenotypic selection on male behavior arising from female mate choice, but no evidence that selection varied among locations. Only those suites of morphological and behavioral traits that did not influence mate choice varied geographically. These results are inconsistent with ongoing divergent sexual selection underlying the observed intraspecific divergence in male display traits. These findings align with theory on the potentially restrictive conditions under which divergent sexual selection may persist, and suggest that long‐term studies capable of detecting periodic or transient divergent sexual selection will be critical to rigorously assess the relative importance of divergent sexual selection in intraspecific trait divergence.     

Allometry for sexual size dimorphism: testing two hypotheses for Rensch's rule in the water strider Aquarius remigis

Within any given clade, male size and female size typically covary, but male size often varies more than female size. This generates a pattern of allometry for sexual size dimorphism (SSD) known as Rensch's rule. I use allometry for SSD among populations of the water strider Aquarius remigis (Hemiptera, Gerridae) to test the hypothesis that Rensch's rule evolves in response to sexual selection on male secondary sexual traits and an alternative hypothesis that it is caused by greater phenotypic plasticity of body size in males. Comparisons of three populations reared under two temperature regimes are combined with an analysis of allometry for genital and somatic components of body size among 25 field populations. Contrary to the sexual-selection hypothesis, genital length, the target of sexual selection, shows the lowest allometric slope of all the assayed traits. Instead, the results support a novel interpretation of the differential-plasticity hypothesis: that the traits most closely associated with reproductive fitness (abdomen length in females and genital length in males) are "adaptively canalized." While this hypothesis is unlikely to explain Rensch's rule among species or higher clades, it may explain widespread patterns of intraspecific variation in SSD recently documented for many insect species.     

The role of mating preferences in shaping interspecific divergence in mating signals in vertebrates

Vertebrates represent one of the best-studied groups in terms of the role that mating preferences have played in the evolution of exaggerated secondary sexual characters and mating behaviours within species. Vertebrate species however, also exhibit enormous interspecific diversity in features of mating signals that has potentially led to reproductive isolation and speciation in many groups. The role that sexual selection has played in interspecific divergence in mating signals has been less fully explored. This review summarizes our current knowledge of how mating preferences within species have shaped interspecific divergence in mate recognition signals among the major vertebrate groups. Certain signal modalities appear to characterize mating signal diversification among different vertebrate taxa. Acoustic signals play an important role in mating decisions in anuran amphibians and birds. Here, different properties of the signal may convey information regarding individual, neighbor and species recognition. Mating preferences for particular features of the acoustic signal have led to interspecific divergence in calls and songs. Divergence in morphological traits such as colouration or ornamentation appears to be important in interspecific diversity in certain groups of fishes and birds. Pheromonal signals serve as the primary basis for species-specific mating cues in many salamander species, most mammals and even some fishes. The evolution of interspecific divergence in elaborate courtship displays may have played an important role in speciation of lizards, and particular groups of fishes, salamanders, birds and mammals. While much research has focused on the importance of mating preferences in shaping the evolution of these types of mating signals within species, the link between intraspecific preferences and interspecific divergence and speciation remains to be more fully tested. Future studies should focus on identifying how variation in mating preferences within a species shapes interspecific diversity in features of mating signals in order to better understand how sexual selection may have led to speciation in vertebrates.     

Does divergence in female mate choice affect male size distributions in two cave fish populations?

Sexual selection by female choice can maintain male traits that are counter selected by natural selection. Alteration of the potential for sexual selection can thus lead to shifts in the expression of male traits. We investigated female mate choice for large male body size in a fish (Poecilia mexicana) that, besides surface streams, also inhabits two caves. All four populations investigated, exhibited an ancestral visual preference for large males. However, only one of the cave populations also expressed this female preference in darkness. Hence, the lack of expression of female preference in darkness in the other cave population leads to relaxation of sexual selection for large male body size. While P. mexicana populations with size-specific female mate choice are characterized by a pronounced male size variation, the absence of female choice in one cave coincides with the absence of large bodied males in that population. Our results suggest that population differences in the potential for sexual selection may affect male trait variation.