Sexual selection and speciation: the comparative evidence revisited

The spectacular diversity in sexually selected traits in the animal kingdom has inspired the hypothesis that sexual selection can promote species divergence. In recent years, several studies have attempted to test this idea by correlating species richness with estimates of sexual selection across phylogenies. These studies have yielded mixed results and it remains unclear whether the comparative evidence can be taken as generally supportive. Here, we conduct a meta‐analysis of the comparative evidence and find a small but significant positive overall correlation between sexual selection and speciation rate. However, we also find that effect size estimates are influenced by methodological choices. Analyses that included deeper phylogenetic nodes yielded weaker correlations, and different proxies for sexual selection showed different relationships with species richness. We discuss the biological and methodological implications of these findings. We argue that progress requires more representative sampling and justification of chosen proxies for sexual selection and speciation rate, as well as more mechanistic approaches.     

Conflicting intersexual mate choices maintain interspecific sexual interactions

Reproductive interference, interspecific sexual interactions that affect reproductive success, is found in various taxa and has been considered as a fundamental source of reproductive character displacement (RCD). Once RCD has occurred, persistent interspecific sexual interactions between species pairs are expected to diminish. However, reproductive interference has been reported from some species pairs that sympatrically coexist. Thus, the question arises, can reproductive interference persist even after RCD? We modeled the evolutionary dynamics of signal traits and mate recognition that determine whether interspecific sexual interactions occur. Our models incorporate male decision making based on the recognition of signal traits, whereas most previous models incorporate only female decision making in mate selection. Our models predict the following: (1) even when male decision making is incorporated, males remain promiscuous; (2) nevertheless, the frequency of interspecific mating is maintained at a low level after trait divergence; (3) the rarity of interspecific mating is due to strict female mate recognition and the consequent refusal of interspecific courtship by females; and (4) the frequency of interspecific mating becomes higher as the cost to females of refusing interspecific courtship increases. These predictions are consistent with empirical observations that males of some species engage in infrequent heterospecific mating. Thus, our models predict that reproductive interference can persist even after RCD occurred.     

Ecology of some species of catfishSynodontis (Pisces: Mochocidae) in the Kpong Headpond in Ghana

Synopsis Some aspects of the ecology of the freshwater catfishSynodontis (Pisces: Mochocidae) in Kpong Headpond (Ghana) were studied. Five species were encountered, namelySynodontis schall, S. gambiensis, S. ocellifer, S. velifer andS. eupterus. S. ocellifer andS. velifer were not recorded in the area before impoundment and might have entered the headpond as eggs or young from the main Volta Lake. Length: weight relationships for the two most important species can be described by the equations: Log W = - 1.76 + 3.12log L (maleS. schall), Log W = -1.33 + 2.89log L (femaleS. schall) and Log W = - 0.39 + 2.07log L (S. gambiensis, sexes combined).Synodontis is omnivorous and generally browses on benthic deposits. Adults feed mostly on chironomids, plant material and aquatic insects. The genus displays sexual dimorphism by having a short urino-genital papilla in most adult males. ForS. schall, one season of major spawning activity occurs per year from mid to late September with first time spawners between 20.0–30.0 cm SL. TheSynodontis species studied lay between 2000 to 209000 eggs per spawning season.     

The distribution of plant mating systems: study bias against obligately outcrossing species

Early models of plant mating-system evolution argued that predominant outcrossing and selfing are alternative stable states. At least for animal-pollinated species, recent summaries of empirical studies have suggested the opposite-that outcrossing rates do not show the expected bimodal distribution. However, it is generally accepted that several potential biases can affect conclusions from surveys of published outcrossing rates. Here, we examine one potential bias and find that published studies of outcrossing rates contain far fewer obligate outcrossers than expected. We approximate the magnitude of this study bias and present the distribution of outcrossing rates after compensating for it. Because this study examines only one potential bias, and finds it to be large, conclusions regarding either the frequency of mixed mating or the shape of the distribution of outcrossing rates in nature are premature.     

Factors influencing the degree of sexual size dimorphism within and among calanoid copepod species

Populations of Diaptomus leptopus (Copepoda: Calanoida) and other calanoid copepods exhibit varying degrees of sexual size dimorphism. We examined whether intraspecific or interspecific variation in dimorphism could be explained by allometry, and we examined the relationship between adult size attained and development rate to determine any relationship between the two. We compared the degree of sexual size dimorphism in D. leptopus and in other calanoid copepods inhabiting temporary and permanent habitats. Allometry did not explain variation in sexual size dimorphism within or among populations or among species. Permanence of habitat affected the degree of dimorphism: dimorphism was greater within and among species inhabiting temporary environments. Non-significant differences in development rate were found among populations and significant differences were found between sexes of D. leptopus when reared under identical laboratory conditions: males developed more rapidly than females but there was no general relationship between development rate and adult size. Potential adaptive hypotheses to explain the differences between populations inhabiting temporary and permanent habitats are discussed.     

Postsettlement growth of two estuarine crab species, Chasmagnathus granulata and Cyrtograpsus angulatus (Crustacea, Decapoda, Grapsidae): laboratory and field investigations

The estuarine grapsid crabs Chasmagnathus granulata and Cyrtograpsus angulatus belong to the most typical and dominant inhabitants of brackish coastal lagoons in southeastern South America. In a combined laboratory and field investigation of juvenile growth, we measured the increase in body size in these species under controlled conditions as well as in field experiments (in Mar Chiquita lagoon, Argentina), seasonal changes in size frequency distribution of a natural population, and growth related changes in selected morphometric traits of male and female juveniles (relations between carapace width, carapace length, propodus height and length of the cheliped, and pleon width). At 24°C, Cy. angulatus grew faster than Ch. granulata; it reached the crab-9 instar (C9; 13 mm carapace width) after 92 days, while Ch. granulata required 107 days to reach the C8 instar (7.4 mm). At 12°C, growth ceased in both species. The pleon begins to show sexual differences in the C5 (Cy. angulatus) and C8 instar (Ch. granulata), respectively, while the chelae differentiate earlier in Ch. granulata than in Cy. angulatus (in C4 vs C6). In the field, growth was maximal in summer, and was generally faster than in laboratory cultures. However, there is great individual variability in size (about 25% even in the first crab instar) and in size increments at ecdysis, increasing throughout juvenile growth. Our data indicate that, in the field, small-scale and short-term variations in feeding conditions, temperature, and salinity account for an extremely high degree of variability in the absolute and relative rates of growth as well as in the time to sexual differentiation. Received in revised form: 20 September 2001 Electronic Publication     

Heterostyly inPrimula. 3. Pollen water economy: a factor in the intramorph-incompatibility response

Summary The pollens from pin (long-styled) and thrum (short-styled) flowers ofPrimula differ markedly in water economy. While pin pollen is little affected by atmospheric humidity, the germinability of thrum pollen in liquid medium is sensitive to the conditions experienced during the immediately preceding period, being depressed when the grains are partly desiccated, although capable of being restored with subsequent controlled rehydration. The tubes produced by incompletely rehydrated thrum pollen show abnormal growth forms, suggesting that the effect is on the membranes of the vegetative cell, a conclusion supported by observations on membrane state using the fluorochromatic reaction. Thrum pollen stripped of part of the material carried in the cavities of the exine by rinsing with a lipid solvent loses much of its capacity for the uptake of atmospheric water, indicating that, in this morph, some factor facilitating hydration is transferred from the sporophytic parent during pollen maturation.Implications of these differences in the physiology of the pin and thrum pollens for the functioning of the intramorph-incompatibility system are discussed.     

The Genomic Selfing Syndrome Accompanies the Evolutionary Breakdown of Heterostyly

The evolutionary transition from outcrossing to selfing can have important genomic consequences. Decreased effective population size and the reduced efficacy of selection are predicted to play an important role in the molecular evolution of the genomes of selfing species. We investigated evidence for molecular signatures of the genomic selfing syndrome using 66 species of Primula including distylous (outcrossing) and derived homostylous (selfing) taxa. We complemented our comparative analysis with a microevolutionary study of P. chungensis, which is polymorphic for mating system and consists of both distylous and homostylous populations. We generated chloroplast and nuclear genomic data sets for distylous, homostylous, and distylous–homostylous species and identified patterns of nonsynonymous to synonymous divergence (d_N/d_S) and polymorphism (π_N/π_S) in species or lineages with contrasting mating systems. Our analysis of coding sequence divergence and polymorphism detected strongly reduced genetic diversity and heterozygosity, decreased efficacy of purifying selection, purging of large-effect deleterious mutations, and lower rates of adaptive evolution in samples from homostylous compared with distylous populations, consistent with theoretical expectations of the genomic selfing syndrome. Our results demonstrate that self-fertilization is a major driver of molecular evolutionary processes with genomic signatures of selfing evident in both old and relatively young homostylous populations.     

基于生物学物种定义探讨物种形成理论与验证的研究进展

物种形成是进化生物学研究的一个永恒主题, 由于生物群体进化是连续和动态的, 物种界限变得难于界定。本文首先讨论了3种地理物种形成模式(同域、邻域及异域), 并分析了近期报道的研究证据。其次, 评述了合子后生殖隔离机制的分子遗传基础和应用群体基因组数据分析的证据, 包括BDMI模型(Bateson-Dobzhansky-Muller incompatibility)、QTLs (quantitative trait loci)、霍尔丹法则及大X染色体效应。最后, 探讨了交配系统作为合子前隔离机制之一与物种形成的关系, 认为近交或自交通过扩大种群遗传结构分化, 增强不同交配系统的种群间不对称基因渐渗, 或种群间无基因渐渗等途径, 促进新物种形成。已知植物交配系统的演化更倾向于从异交(或自交不亲和)向自交(或近交亲和)方式, 花性状和基因组的分化推动形成所谓的自交综合征, 研究交配系统驱动或强化物种形成模式对认识植物物种形成机制有重要意义。     

Multimale mating and absence of canine tooth dimorphism in woolly spider monkeys (Brachyteles arachnoides)

The relatively low degree of canine tooth dimorphism in Australopithecus afarensis has been used as “primary evidence” to support the concept of a mating system of monogamous pair-bonding and male provisioning. A recent field study of woolly spider monkeys shows that these large primates, which lack canine tooth (and body size) dimorphism, are characterized by apolygynous mating system. Male parental care of infants is absent in this species. These data support the view that a lack of canine tooth dimorphism in an anthropoid species does not necessarily imply either a monogamous, pair-bonded mating system or male parental care.     

Sexual selection and peripatric speciation: the Kaneshiro model revisited

The Kaneshiro model proposes a role for sexual selection in peripatric speciation. During population bottlenecks, derived males lose attractive traits and become discriminated against by ancestral females, whereas derived females are selected to be less choosy. This permits novel mate choice cues to evolve in derived populations. In a quantitative analysis of laboratory experiments, we show that bottlenecked males have indeed become less attractive, but females have not lost their ancestral preferences. Contrary to the model, mating asymmetries were not limited to bottlenecks, but regularly occurred between derived and ancestral populations. The simplest explanation for the observed mating asymmetries is loss of genetic variation and inbreeding in the derived populations. The Kaneshiro process is unlikely to isolate small daughter populations from their ancestor but with slight modifications it may become a strong candidate for speciation in allopatry manifested as isolation between daughter populations in secondary contact.     

Speedy speciation in a bacterial microcosm: new species can arise as frequently as adaptations within a species

Microbiologists are challenged to explain the origins of enormous numbers of bacterial species worldwide. Contributing to this extreme diversity may be a simpler process of speciation in bacteria than in animals and plants, requiring neither sexual nor geographical isolation between nascent species. Here, we propose and test a novel hypothesis for the extreme diversity of bacterial species—that splitting of one population into multiple ecologically distinct populations (cladogenesis) may be as frequent as adaptive improvements within a single population''s lineage (anagenesis). We employed a set of experimental microcosms to address the relative rates of adaptive cladogenesis and anagenesis among the descendants of a Bacillus subtilis clone, in the absence of competing species. Analysis of the evolutionary trajectories of genetic markers indicated that in at least 7 of 10 replicate microcosm communities, the original population founded one or more new, ecologically distinct populations (ecotypes) before a single anagenetic event occurred within the original population. We were able to support this inference by identifying putative ecotypes formed in these communities through differences in genetic marker association, colony morphology and microhabitat association; we then confirmed the ecological distinctness of these putative ecotypes in competition experiments. Adaptive mutations leading to new ecotypes appeared to be about as common as those improving fitness within an existing ecotype. These results suggest near parity of anagenesis and cladogenesis rates in natural populations that are depauperate of bacterial diversity.     

Sexual differences in auditory sensitivity: mismatch of hearing threshold and call frequency in a tettigoniid (orthoptera,tettigoniidae: Zaprochilinae)

Summary Sexual dimorphism of the ear of an undescribed species of zaprochiline tettigoniid is described. The internal trachea, dedicated to hearing in other tettigoniids, is unmodified in the male but fully developed in the female. The external auditory spiracle is also lost in the male. In contrast, there is no difference between the sexes in the number of sensilla within the hearing organ. The male is 10 dB less sensitive than the female. The characteristic frequency of the hearing organ at 35 kHz does not match the carrier frequency of the male's call at 51 kHz. As a result of this mismatch the female is remarkably insensitive to the male's call (threshold at 75 dB SPL), and the male is even less sensitive (thresholds80 dB SPL). In nature this provides a maximum hearing range of the male of less than 50 cm.     

Reversed sexual size dimorphism in microtines: Are females larger than males or are males smaller than females?

Summary We analysed sexual size dimorphism for 21 populations of microtine rodents. Female to male size ratio varied considerably among populations from females significantly larger than males (ratio=1.18) to males larger than females (ratio=0.78). In a multiple regression analysis female to male home range size ratio explained 94% of the total variation in body size dimorphism and was the only one of eight independent variables that was selected in a stepwise regression procedure. When females are the larger sex, males have home range sizes much larger than females. We suggest that the relationship between home range size ratio and body weight size dimorphism reflects different selection pressures on males and females in competition for resources and mates.     

Loading constraints,body size and mating preference in Gammarus species

Size relationships among pairing Gammarus were examined with reference to two hypotheses for sexual size dimorphism and assortative mating among aquatic Peracarida (Crustacea). The sizes of pairing and non-pairing animals were compared in different experimental conditions where the size of one or both sexes was controlled. The experimental results present a complex picture which suggests that both sexual selection and loading constraints are likely to play a role in determining mating decisions in these animals.Department of Adult & Continuing Education, University of Durham     

Sequence evolution of the sperm ligand zonadhesin correlates negatively with body weight dimorphism in primates

Sexual selection has repeatedly been shown to be the probable driving force behind the positive Darwinian evolution of genes affecting male reproductive success. Here we compare the sequence evolution of the sperm ligand zonadhesin with body mass dimorphism in primates. In contrast to previous related studies, the present approach takes into account not only catarrhine primates, but also platyrrhines and lemurs. In detail, we analyze the sequence evolution of concatenated zonadhesin fragments (555 bp) of four Lemuroidea, five Platyrrhini, and seven Catarrhini, using the rate ratio of nonsynonymous to synonymous substitutions (dn/ds=omega). Unexpectedly, subsequent regression analyzes between omega estimates for the terminal branches of a primate phylogeny and residual male body mass reveal that sequence evolution of zonadhesin decreases with increasing sexual dimorphism in body weight. Mapping published mating system classifications onto these results illustrates that unimale breeding species show a tendency for rather slow sequence evolution of zonadhesin and comparably pronounced sexual dimorphism in body weight. Female choice and sperm competition can be assumed to drive the evolution of zonadhesin. We speculate that the level of sperm competition is lower in more sexually dimorphic primates because males of these species monopolize access to fertile females more successfully. Thus, variation in sperm competition may be driving the observed negative correlation of sequence evolution and sexual dimorphism in body weight.     

Female mating preference for own males on species and population level in Chromaphyosemion killifishes (Cyprinodontiformes, Nothobranchiidae)

Chromaphyosemion killifishes are a very promising taxon for the examination of general principles and mechanisms underlying speciation. The polygamous mating system and high degree of sexual dimorphism in this group suggest that sexual selection and female mating preferences play a crucial role in the current radiation process. Because the emergence of mating preferences for individuals of the own population during allopatry is a necessary precondition for speciation by sexual selection, either via diverging mate recognition traits or reinforcement, we examined whether females of different nominal species (Chromaphyosemion bivittatum vs. Chromaphyosemion volcanum) or populations (C. bivittatum “Toko” vs. C. bivittatum “Ilor”) prefer their own males as mating partners. For this purpose, two different methods were used. First, we conducted simultaneous choice tests where females could choose between two different males, and second, we counted the eggs produced by females in consecutive matings with different males. Both approaches produced the same result: females of the populations under examination preferred to mate with their own males. Preference for own males was symmetrical in all tests. Males, on the other hand, do not discriminate between females, not even on the level of nominal species. Thus, our results are in accordance with the hypothesis that female choice is an important factor for speciation in Chromaphyosemion. Additionally, we present some preliminary data on post-zygotic isolation between the populations.     

A new sexually dimorphic mantidfly species of Allomantispa Liu et al., 2015 from China (Neuroptera: Mantispidae)

Allomantispa Liu et al., 2015 is the only extant genus of the mantidfly subfamily Drepanicinae from Eurasia, being endemic to the Oriental region. It currently consists of only two species from southeastern Tibet and northern Myanmar, respectively. Here we describe a new species of this rarely observed mantidfly genus, namely Allomantispa coniprocessa sp. nov. from Yunnan, China. Interestingly, the new species displays distinctive sexual dimorphic characters on the pronotum. The molecular identification reveals obvious genetic divergence between the allopatric individuals of the new species. Their morphological variations are illustrated and compared. In addition, some characters with taxonomic or phylogenetic significance present on the pronotum, the abdomen and the genitalia in Allomantispa are also discussed. Our finding updates the morphological and distributional information on Allomantispa and implies a more diverse fauna of this genus in the Oriental region than previously assumed.