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PubMed2004

Reticulate sympatric speciation in Cameroonian crater lake cichlids.

BACKGROUND: Traditionally the rapid origin of megadiverse species flocks of extremely closely related species is explained by the combinatory action of three factors: Disruptive natural selection, disruptive sexual selection and partial isolation by distance. However, recent empirical data and theoretical advances suggest that the diversity of complex species assemblages is based at least partially on the hybridization of numerous ancestral allopatric lineages that formed hybrids upon invasion of new environments. That reticulate speciation within species flocks may occur under sympatric conditions after the primary formation of species has been proposed but not been tested critically. RESULTS: We reconstructed the phylogeny of a complex cichlid species flock confined to the tiny Cameroonian crater lake Barombi Mbo using both mitochondrial and nuclear (AFLP) data. The nuclear phylogeny confirms previous findings which suggested the monophyly and sympatric origin of the flock. However, discordant intra-flock phylogenies reconstructed from mitochondrial and nuclear data suggest strongly that secondary hybridization among lineages that primarily diverged under sympatric conditions had occurred. Using canonical phylogenetic ordination and tree-based tests we infer that hybridization of two ancient lineages resulted in the formation of a new and ecologically highly distinct species, Pungu maclareni. CONCLUSIONS: Our findings show that sympatric hybrid speciation is able to contribute significantly to the evolution of complex species assemblages even without the prior formation of hybrids derived from allopatrically differentiated lineages.

Journal Article↗

PubMed2003

The turn of the sword: length increases male swimming costs in swordtails.

Sexual selection via female mate choice can result in the evolution of elaborate male traits that incur substantial costs for males. Despite increased interest in how female mating preferences contribute to the evolution of male traits, few studies have directly quantified the locomotor costs of such traits. A sexually selected trait that could affect movement costs is the sword exhibited by male swordtail fishes: while longer swords may increase male mating success, they could negatively affect the hydrodynamic aspects of swimming activities. Here, we examine the energetic costs of the sword in Xiphophorus montezumae by experimentally manipulating sword length and measuring male aerobic metabolism during two types of activity, routine swimming and courtship swimming. Direct measurements of oxygen consumption indicate that males with longer swords expend more energy than males with shortened swords during both types of swimming. In addition, the sword increases the cost of male courtship. Thus, while sexual selection via female choice favours long swords, males with longer swords experience higher metabolic costs during swimming, suggesting that sexual and natural selection have opposing effects on sword evolution. This study demonstrates a hydrodynamic cost of a sexually selected trait. In addition, this study discriminates between the cost of a sexually selected trait used in courtship and other courtship costs.

Animals↗

PubMed2006

Monogamy in the maternally mouthbrooding Lake Tanganyika cichlid fish Tropheus moorii.

Supported by evidence for assortative mating and polygynandry, sexual selection through mate choice was suggested as the main force driving the evolution of colour diversity of haplochromine cichlids in Lakes Malawi and Victoria. The phylogenetically closely related tribe Tropheini of Lake Tanganyika includes the genus Tropheus, which comprises over 100 colour variants currently classified into six morphologically similar, polyphyletic species. To assess the potential for sexual selection in this sexually monochromatic maternal mouthbrooder, we used microsatellite-based paternity inference to investigate the mating system of Tropheus moorii. In contrast to haplochromines in Lake Malawi, multiple paternity is rare or even absent in broods of T. moorii. Eighteen of the 19 analysed families were consistent with genetic monogamy, while either a mutation or more than one sire explained the genotype of one offspring in another brood. We discuss the differences in breeding behaviour between T. moorii and the Lake Malawi haplochromines, and evaluate additional factors or alternatives to sexual selection as promoters of colour diversification. A preliminary survey of other Tropheini species suggested that multiple paternity is infrequent in the entire tribe.

Alleles↗

PubMed2006

Sensory drive in cichlid speciation.

The role of selection in speciation is a central yet poorly understood problem in evolutionary biology. The rapid radiations of extremely colorful cichlid fish in African lakes have fueled the hypothesis that sexual selection can drive species divergence without geographical isolation. Here we present experimental evidence for a mechanism by which sexual selection becomes divergent: in two sibling species from Lake Victoria, female mating preferences for red and blue male nuptial coloration coincide with their context-independent sensitivities to red and blue light, which in turn correspond to a difference in ambient light in the natural habitat of the species. These results suggest that natural selection on visual performance, favoring different visual properties in different spectral environments, may lead to divergent sexual selection on male nuptial coloration. This interplay of ecological and sexual selection along a light gradient may provide a mechanism of rapid speciation through divergent sensory drive.

Adaptation, Biological↗

PubMed2001

Sexual conflict and the evolution of female mate choice and male social dominance.

Conflicts between the sexes over control of reproduction are thought to lead to a cost of sexual selection through the evolution of male traits that manipulate female reproductive physiology and behaviour, and female traits that resist this manipulation. Although studies have begun to document negative fitness effects of sexual conflict, studies showing the expected association between sexual conflict and the specific behavioural mechanisms of sexual selection are lacking. Here we experimentally manipulated the opportunity for sexual conflict in the cockroach. Nauphoeta cinerea and showed that, for this species, odour cues in the social environment influence the behavioural strategies and fitness of males and females during sexual selection. Females provided with the opportunity for discriminating between males but not necessarily mating with preferred males produced fewer male offspring than females mated at random. The number of female offspring produced was not affected, nor was the viability of the offspring. Experimental modification of the composition of the males' pheromone showed that the fecundity effects were caused by exposure to the pheromone component that makes males attractive to females but also makes males less likely to be dominant. Female mate choice therefore carries a demographic cost but functions to avoid male manipulation and aggression. Male-male competition appears to function to circumvent mate choice rather than directly manipulating females, as the mate choice can be cryptic. The dynamic struggle between the sexes for control of mating opportunities and outcomes in N. cinerea therefore reveals a unique role for sexual conflict in the evolution of the behavioural components of sexual selection.

Animals↗

PubMed2006

Sex chromosomes and male ornaments: a comparative evaluation in ray-finned fishes.

Theory predicts that the mechanism of genetic sex determination can substantially influence the evolution of sexually selected traits. For example, female heterogamety (ZZ/ZW) can favour the evolution of extreme male traits under Fisher's runaway model of sexual selection. We empirically test whether the genetic system of sex determination has played a role in the evolution of exaggerated male ornaments in actinopterygiian fishes, a clade in which both female-heterogametic and male-heterogametic systems of sex determination have evolved multiple times. Using comparative methods both uncorrected and corrected for phylogenetic non-independence, we detected no significant correlation between sex-chromosome systems and sexually selected traits in males. Results suggest that sex-determination mechanism is at best a relatively minor factor affecting the outcomes of sexual selection in ray-finned fishes.

Animals↗

PubMed1989

Ecological causes for the evolution of sexual dimorphism: a review of the evidence.

Can sexual dimorphism evolve because of ecological differences between the sexes? Although several examples of this phenomenon are well known from studies on birds, the idea has often been dismissed as lacking general applicability. This dismissal does not stem from contradictory data so much as from the difficulties inherent in testing the hypothesis, and its apparent lack of parsimony, in comparison to the alternative explanation of sexual selection. The only unequivocal evidence for the evolution of sexual dimorphism through intersexual niche partitioning would be disproportionate dimorphism in trophic structures (e.g., mouthparts). This criterion offers a minimum estimate of the importance of ecological causes for dimorphism, because it may fail to identify most cases. A review of published literature reveals examples of sexually dimorphic trophic structures in most animal phyla. Many of these examples seem to be attributable to sexual selection, but others reflect adaptations for niche divergence between the sexes. For example, dwarf non-feeding males without functional mouthparts have evolved independently in many taxa. In other cases, males and females differ in trophic structures apparently because of differences in diets. Such divergence may often reflect specific nutritional requirements for reproduction in females, or extreme (sexually selected?) differences between males and females in habitats or body sizes. Ecological competition between the sexes may be responsible for intersexual niche divergence in some cases, but the independent evolution of foraging specializations by each sex may be of more general importance. If ecological causation for dimorphism can be demonstrated in so many cases, despite the inadequacies of the available criteria, the degree of sexual size dimorphism in many other animal species may well also have been influenced by ecological factors. Hence, it may be premature to dismiss this hypothesis, despite the difficulty of testing it.

Animals↗

PubMed2004

No evidence that polyandry benefits females in Drosophila melanogaster.

Understanding the evolution of polyandry (mating with multiple males) is a major issue in the study of animal breeding systems. We examined the adaptive significance of polyandry in Drosophila melanogaster, a species with well-documented costs of mating in which males generally cannot force copulations. We found no direct fitness advantages of polyandry. Females that mated with multiple males had no greater mean fitness and no different variance in fitness than females that mated repeatedly with the same male. Subcomponents of reproductive success, including fecundity, egg hatch rate, larval viability, and larval development time, also did not differ between polyandrous and monogamous females. Polyandry had no affect on progeny sex ratios, suggesting that polyandry does not function against costly sex-ratio distorters. We also found no evidence that polyandry functions to favor the paternity of males successful in precopulatory sexual selection. Experimentally controlled opportunities for precopulatory sexual selection had no effect on postcopulatory sperm precedence. Although these results were generally negative, they are supported with substantial statistical power and they help narrow the list of evolutionary explanations for polyandry in an important model species.

Analysis of Variance↗

PubMed1999

Predation enhances complexity in the evolution of electric fish signals.

Theories of sexual selection assume that predation is a restrictive, simplifying force in the evolution of animal display characters and many empirical studies have shown that predation opposes excessive elaboration of sexually selected traits. In an unexpected turnaround, I show here that predation pressure on neotropical, weakly electric fish (order Gymnotiformes) seems to have selected for greater signal complexity, by favouring characters that have enabled further signal elaboration by sexual selection. Most gymnotiform fish demonstrate adaptations that lower detectability of their electrolocation/communication signals by key predators. A second wave phase added to the ancestral monophasic signal shifts the emitted spectrum above the most sensitive frequencies of electroreceptive predators. By using playback trials with the predatory electric eel (Electrophorus electricus), I show that these biphasic signals are less detectable than the primitive monophasic signals. But sexually mature males of many species in the family Hypopomidae extend the duration of the second phase of their electric signal pulses and further amplify this sexual dimorphism nightly during the peak hours of reproduction. Thus a signal element that evolved for crypsis has itself been modified by sexual selection.

Animals↗

PubMed2004

Condition dependence of sexual ornament size and variation in the stalk-eyed fly Cyrtodiopsis dalmanni (Diptera: Diopsidae).

We used the stalk-eyed fly Cyrtodiopsis dalmanni to examine predictions made by condition-dependent handicap models of sexual selection. Condition was experimentally varied by manipulation of larval food availability. Cyrtodiopsis dalmanni is a highly dimorphic species exhibiting strong sexual selection, and the male sexual ornament (exaggerated eyespan) showed strong condition-dependent expression relative to the homologous trait in females and nonsexual traits. Male eyespan also showed a great increase in standardized variance under stress, unlike nonsexual traits. The inflated variance of the male ornament was primarily attributable to condition-dependent (but body-size-independent) increase in variance. Thus, evaluation of male eyespan allows females to gain additional information about male condition over and above that given by body size. These findings accord well with condition-dependent handicap models of sexual selection.

Analysis of Variance↗

PubMed1997

Intrasexual competition and body weight dimorphism in anthropoid primates.

Body weight dimorphism in anthropoid primates has been thought to be a consequence of sexual selection resulting from male-male competition for access to mates. However, while monogamous anthropoids show low degrees of weight dimorphism, as predicted by the sexual selection hypothesis, polygynous anthropoids show high variation in weight dimorphism that is not associated with measures of mating system or sex ratio. This observation has led many to debate the role of other factors such as dietary constraints, predation pressure, substrate constraints, allometric effects, and phylogeny in the evolution of anthropoid weight dimorphism. Here, we re-evaluate variation in adult body weight dimorphism in anthropoids, testing the sexual selection hypothesis using categorical estimates of the degree of male-male intrasexual competition ("competition levels"). We also test the hypotheses that interspecific variation in body weight dimorphism is associated with female body weight and categorical estimates of diet, substrate use, and phylogeny. Weight dimorphism is strongly associated with competition levels, corroborating the sexual selection hypothesis. Weight dimorphism is positively correlated with increasing female body weight, but evidence suggests that the correlation reflects an interaction between overall size and behavior. Arboreal species are, on average, less dimorphic than terrestrial species, while more frugivorous species tend to be more dimorphic than folivorous or insectivorous species. Several alternative hypotheses can explain these latter results. Weight dimorphism is correlated with taxonomy, but so too are competition levels. We suggest that most taxonomic correlations of weight dimorphism represent "phylogenetic niche conservatism"; however, colobines show consistently low degrees of weight dimorphism for reasons that are not clear.

Animals↗

PubMed2003

Life history and the male mutation bias.

If DNA replication is a major cause of mutation, then those life-history characters, which are expected to affect the number of male germline cell divisions, should also affect the male to female mutation bias (alpha(m)). We tested this hypothesis by comparing several clades of bird species, which show variation both in suitable life-history characters (generation time as measured by age at first breeding and sexual selection as measured by frequency of extrapair paternity) and in alpha(m), which was estimated by comparing Z-linked and W-linked substitution rates in gametologous introns. Alpha(m) differences between clades were found to positively covary with both generation time and sexual selection, as expected if DNA replication causes mutation. The effects of extrapair paternity frequency on alpha(m) suggests that increased levels of sexual selection cause higher mutation rates, which offers an interesting solution to the paradox of the loss of genetic variance associated with strong directional sexual selection. We also used relative rate tests to examine whether the observed differences in alpha(m) between clades were due to differences in W-linked or Z-linked substitution rates. In one case, a significant difference in alpha(m) between two clades was shown to be due to W-linked rates and not Z-linked rates, a result that suggests that mutation rates are not determined by replication alone.

Age Factors↗

PubMed2003

Exploring complex fitness surfaces: multiple ornamentation and polymorphism in male guppies.

The sexual ornamentation used by male guppies to attract females comprises many components, each of which varies considerably among males. Although natural and sexual selection have been shown to contribute to divergence among populations in male sexual ornaments, the role of sexual selection in maintaining polymorphism within populations is less clear. We used both parametric quadratic regression and nonparametric projection pursuit regression techniques to reveal the major axes of non-linear sexual selection on male ornaments. We visualized the fitness surfaces defined by these axes using thin-plate splines to allow a direct comparison of the two methodologies. Identification of the major axes of selection and their visualization was critical in determining the form and strength of nonlinear selection. Both types of analysis revealed fitness surfaces comprising three peaks, suggesting that there is more than one way to make an attractive guppy. Disruptive selection may be an important process underlying the presence of multiple sexual ornaments and may contribute to the maintenance of the high levels of polymorphism in male sexual ornaments found in guppy populations.

Animals↗

PubMed2004

Deciphering the swordtail's tale: a molecular and evolutionary quest.

The power of sexual selection to influence the evolution of morphological traits was first proposed more than 130 years ago by Darwin. Though long a controversial idea, it has been documented in recent decades for a host of animal species. Yet few of the established sexually selected features have been explored at the level of their genetic or molecular foundations. In a recent report, Zauner et al.1 describe some of the molecular features associated with one of the best characterized of sexually selected traits, the male-specific tail "sword" seen in certain species of the fish genus Xiphophorus. Zauner et al. find that the msxC gene, a gene previously implicated in fin development from work in zebrafish, is dramatically and specifically upregulated in the development of the ventral caudal fin rays, which give rise to the sword, in males. The results provide the first molecular insight into the development of this sexually selected trait while prompting new questions about the structure of the entire genetic network that underlies this trait. To fully understand the molecular-genetic and evolutionary history of this network, however, it will be essential to determine whether sword-development is a basal or derived trait in Xiphophorus.

Animals↗

PubMed2004

Sexy sons: a dead end for cytoplasmic genes.

Critics of sexual conflict theory argue that females may gain a net reproductive benefit from mating with manipulative males because the direct costs that they suffer may be offset by the production of sexy, i.e. manipulative, sons. However, this exclusive focus on nuclear gene effects represents an incomplete view of female fitness. Females differ fundamentally from males in transmitting not only nuclear genes but also a wide range of cytoplasmic genetic elements (CGEs) that can have profound effects, from male killing to influencing development of the nervous system and cognitive ability. Maternal transmission of CGEs has two major implications for sexual selection. First, the evolution of male fitness traits, such as sperm competitive ability, may be constrained because response to selection on mitochondrial genomes can occur only through the female line. Second, CGEs bear the direct costs of male manipulation but gain no indirect benefits when females produce sexy sons. This should result in perpetual antagonistic coevolution between nuclear genes involved in male manipulation and CGEs that promote female resistance to male sexually selected traits. Explicit consideration of the consequences of selection acting on CGEs is therefore necessary for a better understanding of the relationship between sexual selection and sexual conflict.

Animals↗

PubMed2002

Female choice depends on size but not symmetry of dorsal eyespots in the butterfly Bicyclus anynana.

The eyespots on the ventral wings of Bicyclus anynana butterflies are exposed when at rest and interact with predators. Those on the dorsal surface are not exposed in this way, and may be involved in courtship and mate choice. In this study, we examined whether the size and fluctuating asymmetry (FA) of dorsal eyespots are reliable signals of male quality. High developmental stability is considered to result in low FA, and to be associated with high quality. Individuals of high quality are predicted to produce sexually selected traits that are large and symmetrical, at a relatively low cost. In this study, we manipulated eyespot development to uncouple eyespot size and FA in order to examine their independent roles in signalling to the female. Individual females in cages were given the choice between two or three males differing in eyespot traits. The results indicate that although size per se of the eyespots is used as a signal, FA and wing size are not. We discuss the use of FA in studies of sexual selection and aspects of sexual selection on dorsal eyespot size.

Animals↗

PubMed2000

Signal decoding and receiver evolution. An analysis using an artificial neural network.

We use a connectionist model, a recurrent artificial neural network, to investigate the evolution of species recognition in sympatric taxa. We addressed three questions: (1) Does the accuracy of artificial neural networks in discriminating between conspecifics and other sympatric heterospecifics depend on whether the networks were trained only to recognize conspecifics, as opposed to being trained to discriminate between conspecifics and sympatric heterospecifics? (2) Do artificial neural networks weight most heavily those signal features that differ most between conspecifics and sympatric heterospecifics, or those features that vary less within conspecifics? (3) Does selection for species recognition generate sexual selection? We find that: (1) Neural networks trained only on self recognition do not classify species as accurately as networks trained to discriminate between conspecifics and heterospecifics. (2) Neural networks weight signal features in a manner suggesting that the total sound environment as opposed to the relative variation of signals within the species is more important in the evolution of recognition mechanisms. (3) Selection for species recognition generates substantial variation in the relative attractiveness of signals within the species and thus can result in sexual selection.

Biological Evolution↗

PubMed2003

Mate choice and toxicity in two species of leaf beetles with different types of chemical defense.

Evidence for the use of defensive compounds for sexual purposes is scarce, even though sexual selection might have some importance for the evolution of defensive traits. This study investigates the effect of defense-related traits and body size on mating success in two sister species of leaf beetle differing in their type of chemical defense. Oreina gloriosa produces autogenous cardenolides, whereas O. cacaliae sequesters pyrrolizidine alkaloids from its food plant. Larger O. gloriosa males with more toxin or higher toxin concentration had a mating advantage, likely due to direct or indirect female choice. In the laboratory, particular pairings recurred repeatedly in this species, indicating mate fidelity. O. gloriosa females were also subject to sexual selection, possibly by male choice, because larger females and those with higher toxin concentration mated more readily and more often. In O. cacaliae, in contrast, sexual selection for toxicity and body size was not detected, or at best was much weaker. Because toxicity is heritable in O. gloriosa but environment-dependent in O. cacaliae, individuals of the former species could be choosing well-defended partners with "good genes." Our study suggests that sexual selection may contribute to the maintenance of heritable defensive traits.

Adaptation, Physiological↗