Dissertations / Theses on the topic 'Sexual selection'

Saccharomyces yeasts are unique as a model system in evolutionary biology. They offer all the traditional benefits of fast generation times and easy maintenance found in other microbes such as Escherichia coli. In addition, Saccharomyces are diploid eukaryotes capable of asexual and sexual reproduction. In this thesis I develop Saccharomyces as a model organism for the study of sexual selection. I show that its mating pheromone is costly to produce and maintain, and that this cost is greater for lower quality individuals. This suggests that the pheromone may have evolved as a sexual signal under the Handicap Principle. I show that size can offer direct benefits during mating and that these are in fact selected for. I show that preferential mating also takes place to help clear deleterious mutations from a population. I also investigate mating barriers in yeast to better understand how yeast mating may take place in nature.

Sexual selection is generally accepted as being responsible for the rapid and divergent evolution of male genitalia and other primary reproductive characteristics in internal fertilisers, such as testes size and sperm length. Selection can act via three main processes: sperm competition, cryptic female choice and sexual conflict, however very few studies have directly addressed the patterns of selection, the degree of phenotypic and genotypic variability expected in genital morphology or the degree to which intromittent genitalia are dependent on male condition. The seedbug, Lygaeus equestris has greatly elongated intromittent genitalia, being almost as long the body. Here I determine whether this is a sexually selected trait and further assess the degree of genetic and phenotypic variability in the greatly elongated male intromittent organ in relation to other morphological components. Further to this, patterns of inheritance and allometry of such exaggerated genitalia were investigated, and of the degree of condition dependence of genital and general morphology was experimentally assessed by varying food availability during ontogeny. Finally, using experimental evolution, I manipulated the level of sexual selection by biasing adult sex-ratio (male-biased, equal-sex, female biased) and investigated potential correlated evolution of female reproductive morphology and fertile (eupyrene) and non-fertile (apyrene) sperm length and numbers in the Indian meal moth, Plodia interpunctella. The main findings indicated that genital length was sexually selected in L. equestris being negatively related to male fertilisation success and that there was great phenotypic variation in genitalia both across and within populations. Genital length was negatively allometric, in spite of being hugely elongated, and was significantly heritable with considerable evolvability. It was also evident that there was genetic variation in the condition dependence of genital length with a significant genotype-by-condition interaction and much reduced genetic variation in genital length in the poor food treatment. Male and female primary sexual traits of P. interpunctella were also shown to covary, but this pattern did not differ across treatments. Taken together, the results presented in this thesis do not support traditional hypotheses of genital evolution and instead suggests that male intromittent genital length of L. equestris is sexually selected in a similar way to secondary sexual characteristics. This is also true when examining primary sexual traits in P. interpunctella and further highlights the false dichotomy between primary and secondary sexual traits.

Over the last 100 years sexual selection has advanced into a vast field of theoretical and empirical research. While Darwin’s idea of female preference being an integral mechanism of sexual selection is no longer debated, our understanding of female preference is still very limited. For example, we know little about the genetic variation in female preference, and the costs of preference over and above the costs of mating with particular male phenotypes. Additionally, while costs of mate choice are well documented, the benefits of mate choice and their implications are still debated. For example, controversy exists over the inevitability of good gene benefits and their capability to promote adaptive sexual selection. Furthermore, the adaptiveness of sexual selection itself is debated. Our understanding of the traits involved in mate choice is also far from complete. Here I investigated aspects of sexual selection in Drosophila simulans, employing a range of behavioural approaches along with artificial selection and environmental manipulations. The findings presented here indicate that female preference can evolve when directly selected on, and that preference itself is not particularly costly. There was also no conclusive evidence for the good genes benefits of mate choice in D. simulans. These benefits are considered crucial in promoting the adaptiveness of sexual selection, and although we found sexual selection to be adaptive under some test conditions it was not adaptive in other conditions. Our investigations into traits involved in mate choice established sex-specific genetic variation in cuticular hydrocarbons and the genetic architecture of this trait was found to sex-specific evolution of cuticular hydrocarbons under natural and sexual selection. Additionally, we found that a secondary sexual character, the sex combs was positively allometric – just like most signalling and weapon traits, and there was no association between trait fluctuating asymmetry and trait size. These findings collectively indicate that sexual selection in D. simulans is consistent with classical models of this process.

In economic games the facial attributes of counterparts bias decisions to trust and decisions to enter play. We report research supporting hypotheses that trust and reciprocation decisions in trust games are biased by mechanisms of sexual selection. Hypotheses that trust game behaviour is modulated by inter-sexual competition were supported. 1) Attractive individuals elicit more cooperation. 2) Male participants display trust and reciprocation toward attractive female counterparts in excess of perceived trustworthiness (and this display is modulated by male self-reported physical dominance). 3) Female participants appear to respond to male trust as a signal of sexual interest and are therefore more likely to exploit the trust of attractive males. 4) In explicitly dating contexts females are more likely to prefer attractive males to pay for the meal. These results indicate that participants are biased by mate choice and mating display considerations while playing economic games in the lab. Hypotheses that trust game behaviour is modulated by intra-sexual competition for resources were also somewhat supported. 1) Male participants reporting an ability to win fights with same-sex peers are more exploitative of other males. 2) Cues to current circulating testosterone level in counterpart’s faces are less trusted but elicit more reciprocation. 3) The male sexually dimorphic trait facial width-to-height ratio (a trait which is related to both aggression and dominance) is related to an increased proportion of decisions to exploit others in the trust game while also being used by others as a cue to untrustworthiness. We conclude that trusting and trustworthy behaviour in both sexes is biased by mating market considerations predicted by intra- and inter-sexual selection.

Understanding sex-specific genetic variance for fitness is of fundamental importance to our understanding of evolution. This thesis presents the findings of empirical investigations into sex-specific genetic variance in fitness. The findings are discussed in terms of their implications for our understanding of the classic evolutionary paradoxes of what maintains genetic variance in fitness and what maintains sexual reproduction, as well as more specific implications regarding adaptation and population viability. Males and females reproduce and accrue fitness in fundamentally different ways, which inevitably comes at a detriment to the fitness of individuals of the opposite sex. This is known as sexual conflict, and because males and females use largely the same genome to develop, grow and reproduce, a genetic tug-of-war ensues. Alternative alleles at sexually antagonistic (SA) genes have opposing fitness effects in males and females. The consequence of this genetic tug-of-war is that alternative allelic variants at SA loci can be maintained in the population. Such SA genetic variation can therefore maintain genetic variance for fitness. Variance in fitness can also be maintained by a constant influx of mutations with weakly deleterious effects and weak selection against them, in what is referred to as mutation-selection balance. Because the average deleterious mutation will be detrimental to both sexes, this source of genetic variance in fitness will have predominantly sexually concordant (SC) effects. This thesis uses a wild-caught population of the seed beetle Callosobruchus maculatus to investigate these two mechanisms of maintaining genetic variance in fitness, as well as the consequences they bear on adaptation, population viability, and the maintenance of sexual reproduction. Results largely support much of the theoretical expectations for sexual conflict, sexual selection and maintenance of genetic variance in fitness, as well as stimulate new thoughts and hypotheses about the nature of SA genetic variation and its interaction with weakly deleterious partially recessive mutations.
Vår kunskap om könsspecifik selektion och genetisk variation för fitness är central för förståelsen av evolutionära processer. I den här avhandligen presenteras resultaten av empiriska undersökningar av just könsspecifik genetisk variation för fitness. Resultaten diskuteras med fokus på deras betydelse för de klassiska evolutionära paradoxerna angående vad som bibehåller genetisk variation i fitness och varför organismer som förökar sig sexuellt är så vanliga, men även mer specifika konsekvenser för en populations anpassningsförmåga och livskraftighet avhandlas. Evolutionen har ofta gynnat olika reproduktiva strategier hos hannar och honor, och dessa strategier kan medföra kostnader för det motsatta könet. Den könskonflikt som uppstår på grund av detta kan också inbegripa en genetisk dragkamp eftersom könen delar genetisk arvsmassa men gynnas av olika anpassningar. Konsekvensen är att alternativa varianter av gener gynnas hos honor och hanar, vilket resulterar i en form av balanserande selektion som kan bibehålla genetisk variation i en population. Genetisk variation i fitness kan även upprätthållas genom en jämvikt mellan ett konstant inflöde av genetisk variation via mutationer med svagt negativ effekt och svag selektion mot dessa mutationer.  Eftersom en negativ mutation normalt kommer vara skadlig för båda könen kommer den här typen av källa till genetisk variation i fitness ha liknande effekt hos könen.  I arbetet med denna avhandlig har jag använt en vilt infångad population av fröbaggaen Callosobruchus maculatus för att undersöka dessa två underliggande mekanismer bakom upprätthållandet av genetisk variation för fitness, samt vilka potentiella konsekvenser de kan ha för en populations anpassningsförmåga och för bibehållandet av sexuell reproduktion. Resultaten i denna avhandling stödjer i stort många av de antaganden som ligger till grund för teorin om könskonflikter, sexuell selektion och vad som upprätthåller genetisk variation för fitness. Resultaten ger också upphov till nya idéer och hypoteser angående  genetisk variation med könsspecifika effekter och dess interaktion med partiellt recessiva negativa mutationer.

The alternative abstract I uploaded should be used as the Swedish summary.

By performing a comparative analysis and using phylogenetic relationships of the Cervidaefamily this study aimed to address whether or not sexual selection may play a role in the extinctionof species by making species more vulnerable to extinction. The role of sexual selection in makingspecies more vulnerable to extinction is largely unexplored, and several factors such as ecologicaland life history traits may increase the risk of extinction.In all species of the family Cervidae (Gilbert et al. 2006, Geist 1998,Groves and Grubb2011,Meijaardand Groves2004,Price et al. 2005, Goss 1983) sexually selected characters plays amain role in determining species status and thus potentially their probability of extinction. In thisstudy the intensity of sexual selection (measured as sexual size dimorphism, antler size and matingsystem) and the rate of extinction (IUCN classification and anthropogenic effect) were counted asfactors to determine the role of sexual selection intensity in both species-rich and species-poorclades.By using the programme MESQUITE and phylogenetic trees, the results show an associationbetween species with larger body size and dimorphism, living in open habitats and having largerantler size expanded to more than three tines; such species are mostly non-territorial and formharems during the rutting season. The small species are territorial, live in closed habitats, aremonomorphic and have small antler size limited to two tines or less. Moreover species that aremore subjected to habitat degradation and anthropogenic effects tend to become smaller in size.Extinction risk for the species-rich clades with small sized, territorial and small antler sizedspecies is lower than for those consisting of species with larger antler size, larger body size, livingin open habitats and using harems as mating system.To sum up, the intensity of sexual selection in larger species in deer family put them in risk ofextinction; but on the other site, small species are more adapted to the environment by choosingdifferent strategy in mating system, and reducing antler and body size thus diminishing theextinction risk.

The work included in this thesis aims at exploring the environmental sensitivity of benefits and costs of sexual selection through a combined empirical and theoretical effort, to increase our understanding of the impact of environmental change on sexually reproducing populations.Can sexual selection promote adaptation to novel environments? Sexual selection for good genes should accelerate adaptation by granting higher reproductive success to individuals of high genetic quality. However, sexual conflict is a frequent outcome of sexual reproduction and may often be detrimental to population fitness. Experimental evolution has shown that the role of sexual selection in adaptation is variable, because of a complex balance between the detrimental and beneficial effects described above.The present thesis is investigating the role of sexual selection in adaptation by focusing on the sex-specific strength of selection and the intensity of intralocus sexual conflict (IaSC) in ancestral and novel environments. The sex-specific strength of selection is a valuable proxy for the benefits of sexual reproduction, since a male-bias in selection caused by sexual selection should allow efficient purging of deleterious alleles with little impact on female fecundity and cost to population fitness.This thesis investigates both sex-specific selection and IaSC across benign and novel environments in two species of seed beetles, Callosobruchus maculatus and Acanthoscelides obtectus, and includes a theoretical model of the effect of environmental change on of sexual selection. The empirical part of my results indicates that, generally, selection at the adult stage is male biased but that this male bias may be reduced under stress, pointing towards reduced benefits of sexual selection under rapid environmental change. Additional simulations suggest that the frequency dependent nature of sexual selection alone could explain this trend. No empirical support was found for the reduction of IaSC under stress.It is becoming crucial today to understand the impact of environmental change on natural populations. This thesis brings new material adding to our understanding of the role of sexual selection within that particular issue. The outcome of sexual selection is dependent on a variety of mechanisms, such as good genes processes and sexual conflict, which are very likely to be dependent on ecological factors and specificity of the system studied. For that reason, carefully controlled experiments on laboratory systems and mathematical modelling are necessary steps that should ultimately lead to the study of similar questions in natural systems.

Genotype-by-environment interactions (G x Es) describe genetic variation for phenotypic plasticity, such that the relative performance of genotypes varies across environments. These interactions have been studied in the context of natural selection for decades, but research interest in the evolutionary consequences of G x Es in sexual traits is more recent. Theory suggests that G x Es in sexual traits could be of fundamental importance to the operation of sexual selection across heterogeneous environments, but empirical research lags behind the theory. In this thesis, I review the current literature on the role of G x Es in sexual selection and identify areas for further research. Using cuticular hydrocarbons (CHCs) in the fruit fly Drosophila simulans as a model system for sexual selection, I examine G x Es in trait expression and quantify the effect of these G x Es in terms of sexual signal reliability and the coevolution of male and female sexual traits. To do so, I use a combination of quantitative genetics and laboratory environmental manipulations. First, I demonstrate that male CHC profile is subject to sexual selection through female mate choice and find some variation in patterns of mate choice across diets and temperatures (Chapter 3). Next, I identify G x Es in male and female CHC expression across diets and temperatures, although G x Es in male CHC profile across temperatures are weak (Chapter 4). I find that G x Es in male CHC expression can cause sexual signal unreliability, as predicted by theory, since male CHCs do not reliably signal heritable aspects of male attractiveness across diets and temperatures (Chapter 5). I also find G x Es in some aspects of female mate choice across temperatures (Chapter 6). In spite of the evidence for signal unreliability and variation in female mate choice across environments, I show that the overall outcome of mate choice is unaffected by G x Es, such that the same male genotypes are attractive across diets and temperatures (Chapters 5 and 6). From my results, it seems likely that females assess male attractiveness based on multiple male sexual signals, so that whilst male CHCs influence mate choice, CHC profile does not necessarily correlate well with overall male attractiveness. I discuss the implications of these results for the evolution of sexual traits and the genetic covariance between male and female sexual traits across environments. The research in this thesis highlights the importance of multivariate studies of sexual selection across environments for a more complete understanding of the evolution of sexual traits.

Fertilisation requires one sperm. The selection of the 'winning' sperm out of the millions inseminated into the female is not determined by a simple lottery, but rather a complex set of interactions between sperm and the female reproductive tract. These post-mating interactions, collectively termed post-copulatory sexual selection (PCSS), are influenced by many factors from biochemical to behavioural, making PCSS dynamic, complicated, and challenging to study. In some taxa, including birds, sperm take a surprising diversion in their long journey between copulation and conception. Females of many species can store sperm for extended periods of time in specialised storage sites within their reproductive tracts. Such storage can give females the opportunity to bias fertility outcomes through PCSS mechanisms. However, these processes occur deep within the female body, making them particularly difficult to document. A critical step towards understanding these mechanisms involves visualising them in real time, inside female tissue, under physiological conditions that are as close to natural as possible. Such visualisation is not trivial - especially in birds - due to the challenges of imaging through thick, muscular tissue. In this thesis, I demonstrate the optimisation of selective plane illumination microscopy (SPIM) and the development of imaging protocols, including sample preparation and cell labelling for this novel application. SPIM is a light sheet fluorescence microscopy technique, which allowed me to optical section live oviduct tissue to reveal the three-dimensional structure of sperm storage tubules (SSTs) in zebra finch [Taeniopygia guttata] females. This imaging technique, in combination with electron microscopy, also allowed me to make volumetric observations of zebra finch sperm mid-piece structure. These studies have helped us understand the adaptive significance of these structures, as well as the functioning of SSTs and the possible sperm-female interactions that occur inside them.

In this thesis I address several fundamental questions in sexual selection and mate choice theory, using the sandfly Lutzomyia longipalpis. I show that L. iongipalpis satisfies the criteria for a lek-breeding species. Males aggregated at sites near hosts which females visited to mate and feed, but a male's access to a resource did not appear to detennine his mating success. Females were free to reject unwanted males and male mating success was typically skewed. Field data suggest that the mating system may have evolved because of female preferences for larger leks, coupled with a hotspot-type mechanism: male distribution was correlated with resources availability, while females distribution was correlated with lek size. In the laboratory, females preferentially mated with middle-aged males. Within this age class, mating success was correlated with increased amounts of pheromone and increased investment in wing-fluttering. Across age classes, females appeared to gain a direct fitness payoff from their choice of mate through increased probability of fertilisation, but the presence of other mechanisms was not investigated. By testing simultaneously the predictions of current models using similar age males, I was able to assess their relative importance for the maintenance of female choice. I found weak evidence to support direct benefits: females that chose to mate with successful males survived longer post-oviposition than females with less successful mates. This did not translate into increased total longevity or increased fecundity. I found no evidence in favour of good-genes models: offspring of preferred males did not survive longer than offspring of less preferred males, nor were their daughters more fecund. Male attractiveness was, however, heritable: sons sired by preferred males achieved higher mating success than sons of less preferred fathers. These results suggest that a Fisherian mechanism is in part responsible for the maintenance of female mating preferences in L. iongipaipis.

Sexual conflict is a powerful evolutionary force, arising due to the differing reproductive interest of the sexes. As males and females contrast in what they require to increase their own relative fitness, this creates sexual conflict. This thesis investigates sexual conflict and sexual selection in the Indian meal moth Plodia interpunctella, a species known to be under strong intralocus sexual conflict for shared life-history traits and fitness. Through the use of experimental evolution, we altered the adult mating sex ratio to either male biased or female biased. This changes the selective forces between populations that may directly impact on intralocus sexual conflict. Male biased populations were found to have increased reproductive success for both sexes with corresponding changes in shared life-history traits compared to the female biased populations. Divergence between the two treatments is further supported by male biased populations showing increased protection from a natural viral pathogen. Post-copulatory sexual selection was examined in males through the sterile male method. Males evolving under a male biased sex ratio were subject to increased selection on traits involved in sperm competition from rival males’ ejaculates, yet sired the same total number of offspring as males evolving under a female biased sex ratio. Sexual selection is a non-mutually exclusive force acting in tandem with sexual conflict, potentially causing increased reproductive success and immunity protection in the male biased populations. Fecundity selection was examined in a stock population of P. interpunctella. Lifetime fecundity was increased without corresponding changes or trade-offs with body size or longevity, two life-history traits known to be important to increased reproductive success in females. Finally, the effects of dietary protein and carbohydrates were examined using the geometric framework. Sexually antagonistic selection over macronutrients was evident, with sexes showing preference for protein and carbohydrate in opposing regions of the nutrient landscape, indicating that intralocus conflict is unlikely to have been resolved in this moth species.

Sexual selection is a driving force in sexually reproducing organisms and strongly shapes their evolution. In the last three decades, sexual selection research has seen a rapid growth, and both theoretical and empirical work has clarified many components of pre- and postcopulatory sexual selection. Despite that, the coexistence of two basic observations still forms an unsolved evolutionary question: in natural populations genetic variation is found in almost all traits in the presence of strong natural and sexual selection. As selection should deplete variability those two observations are in direct conflict. This problem attracted the attention of many researchers, as it regards potentially most of the numerous traits describing an organism’s phenotype, or at least all the traits under some selection. During my PhD I explored part of this field of study, focusing on sexually selected male traits. Most of the efforts done to understand this evolutionary contradiction have been done in a precopulatory context, with particular attention to the prominent case of the so called ‘lek-paradox’. However, whenever females are sexually promiscuous, a directional selection for traits associated with sperm competition success is expected to arise. As ejaculate characteristics are expected, and actually known, to play a crucial role in determining the fitness outcome of males, selection acting on them should be strong and as a consequence their variability reduced. Yet, as for precopulatory traits, there are many experimental evidences that variability in postcopulatory traits is unexpectedly high. Many hypotheses have been formulated to explain the maintenance of genetic variability of sexually selected traits. During my PhD I tested some prediction of three main models applicable to both pre- and postcopulatory traits: first that selection constrains and non linear selection are acting on the set of traits defining the male phenotype. Second, I verified that resource trade-offs are present between pre- and postcopulatory traits, as proposed by Parker’s sperm competition theory. Third, I tested the fundamental assumption of the ‘genic capture hypothesis’ that sexually selected traits are condition dependent. I performed four main experiments using the guppy, Poecilia reticulata. This small tropical fresh-water fish is well suited for my purposes as traits subject to both pre- (male ornamentation, size, and behaviour) and postcopulatory selection (sperm number, velocity, and viability) exhibit high levels of phenotypic and additive genetic variation. With the first experiment (manuscript 1) I characterized, for the first time, the selection acting in a whole on both pre- and postcopulatory traits. I then measured the long term cost of sperm production (manuscript 2) with the aim of determine the trade-offs present between pre- and postcopulatory traits. With the last two experiments (manuscripts 3 and 4) I tested condition dependence of a wide set of sexually selected traits. My results suggest that in this species non linear selection may be more important than previously estimated and, in particular, that disruptive and correlational selection can contribute to maintain polymorphisms in sexually selected traits. Moreover investment in ejaculate is traded off with investment in obtaining mating, in agreement with sperm competition theory. Lastly, both pre- and postcopulatory sexually selected traits show a strong condition dependence, thus confirming one assumption of the ‘genic capture hypothesis’.
La selezione sessuale, descritta da Darwin nella sua opera “The Descent of Man and Selection in Relation to Sex”(1871) è una delle forze trainanti in natura e, nella maggior parte degli esseri viventi, determina fortemente la loro evoluzione. Negli ultimi tre decenni, la ricerca scientifica nell’ambito della selezione sessuale ha visto una rapida crescita, e numerosissimi lavori sia teorici che sperimentali hanno chiarito numerosi aspetti della selezione sessuale sia pre- che postcopulatoria. Nonostante ciò, la coesistenza di due fondamentali condizioni in natura fa si che esista ancora un paradosso evolutivo irrisolto: nella maggior parte dei caratteri si osserva una grande variabilità genetica nonostante la presenza di una forte selezione, sia naturale che sessuale. Poiché la selezione dovrebbe esaurire la variabilità genetica, queste due condizioni (variabilità da una parte e selezione dall’altra) sono in diretto conflitto. Poiché questa situazione riguarda la maggior parte dei tratti che formano il fenotipo di un organismo, o almeno tutti quei caratteri sotto una qualche forma di selezione, è facilmente immaginabile come questo problema abbia attirato l’attenzione di moltissimi ricercatori. Durante il mio dottorato di ricerca ho mi sono dedicato allo studio di una parte di questo problema, concentrandomi sui caratteri maschili selezionati sessualmente. Questo campo è stato ampliamente studiato ma la maggior parte degli sforzi fatti per comprendere questa contraddizione è stata compiuta esclusivamente in un contesto di selezione precopulatoria, ed in particolare per quanto riguarda un suo caso particolare, quello del paradosso del lek. Tuttavia, nel caso in cui le femmine di una specie siano sessualmente promiscue (situazione quasi totalmente diffusa nel regno animale), ci si aspetta la presenza di una selezione direzionale per i caratteri maschili legati alla competizione spermatica. Ed infatti quello che si osserva è che le caratteristiche dell’eiaculato hanno un ruolo importante nel determinare il successo riproduttivo maschile. La selezione su questi tratti è quindi forte ma, come per i caratteri precopulatori, numerose evidenze sperimentali dimostrano la presenza di un’elevata variabilità sia genetica che fenotipica in caratteri soggetti a selezione postcopulatoria. Molte ipotesi sono state formulate per spiegare il mantenimento della variabilità genetica nei tratti selezionati sessualmente. In particolare, durante il mio dottorato ho testato le previsioni di tre di queste principali teorie, applicabili sia ai tratti pre- che postcopulatori. Per prima cosa ho verificato la presenza di selezione non lineare disruptiva e correlazionale prendendo in considerazione un ampio set di caratteri maschili. In secondo luogo, ho verificato che esistano dei trade-off di tipo energetico tra i tratti pre- e postcopulatori, come proposto nella ‘teoria della competizione spermatica’ di Parker. Infine, ho testato una delle condizioni fondamentali della teoria della ‘cattura genica’ proposta da Rowe e Houle (1996) e cioè che i tratti selezionati sessualmente siano condizione-dipendenti. Durante il dottorato ho svolto quattro esperimenti principali utilizzando come specie modello Poecilia reticulata, comunemente chiamata guppy. Questo piccolo pesce tropicale d'acqua dolce è particolarmente adatto per i miei scopi. I maschi presentano infatti caratteristiche soggette sia a selezione precopulatoria (ornamenti, dimensioni e comportamento sessuale) che a selezione postcopulatoria (numero, velocità e vitalità degli spermatozoi) ed inoltre si osserva in questi caratteri un’elevata varianza genetica additiva. Appare quindi evidente la presenza della contraddizione prima descritta. Con il primo esperimento (primo articolo) ho descritto, per la prima volta, la selezione non lineare che agisce sull’insieme dei tratti sia pre- che postcopulatori. Ho poi misurato il costo a lungo termine imposto dalla produzione di spermi (secondo articolo) con l'obiettivo di determinare i trade-off presenti tra i tratti pre- e postcopulatori. Negli ultimi due articoli (terzo e quarto) ho testato l’ipotesi di condizione-dipendenza in un ampio set di tratti selezionati sessualmente. I miei risultati suggeriscono che in questa specie la selezione non lineare può essere più importante di quanto stimato in precedenza e, in particolare, che la selezione disruptiva e correlazionale possono contribuire a mantenere il polimorfismo osservato nei tratti selezionati sessualmente. Inoltre l’investimento a livello postcopulatorio nell’eiaculato presenta per i maschi di Poecilia un costo in termini di successo precopulatorio (possibilità di accoppiarsi), in accordo con la teoria della competizione spermatica. Infine, i tratti sessualmente selezionati, sia pre- che postcopulatori, mostrano una forte dipendenza dalla condizione del maschio, confermando così uno degli assunti dell’ ipotesi della cattura genica.

Sexual selection is ubiquitous in all sexually reproducing species and a powerful evolutionary force. The effect of sexual selection on population fitness has caused wide debate and has been proposed to both enhance adaptation rates, but also possibly increase extinction risk. Using experimental evolution, the strength of sexual selection was altered by biasing adult sex ratios in replicated populations of the Indian meal moth, Plodia interpunctella. Under increasing temperatures strong sexual selection and polyandry was associated with increased population fitness. Suggesting sexual selection could provide a buffer against climate change by increasing adaptation rates. However, no effect on male mating success was observed. In contrast, under stable temperatures male mating success was increased by strong sexual selection, however, this did not translate to increased population fitness. These results indicate that female choice is a potentially strong selective pressure in P. interpunctella. Moreover, under stabilising and directional selection the costs and benefits of sexual selection change. In a field study, dung beetle species richness and abundance were compared across a gradient of habitat disturbance, ranging from old-growth rainforest to oil palm plantation. Species persistence within altered habitats was positively associated with expressing horns and relative horn lengths, suggesting male-male competition increases a species ability to persist within modified landscapes. There was not a strong effect of relative testes mass or sexual size dimorphism on the abundance of species. Additionally, by examining the expression of sexually selected traits within species, horn length and testes mass appear to be condition dependent, but only the expression of horns was effected by habitat change. Overall, from both laboratory and field studies it was found that sexual selection can increase adaptation rates and the persistence of species within altered and changing environments. Moreover, both sets of studies suggest pre-copulatory sexual selection to be an important aspect of sexual selection in driving this adaptation.

The problem of the maintenance of the additive genetic variability underlying sexually selected traits has been typically investigated in a precopulatory context (see Radwan et al. 2015). However, whenever females are sexually promiscuous within the same reproductive cycle, sperm competition can also generate directional selection on traits involved in fertilisation success, in particular on sperm number (Parker and Pizzari 2010). The selection acting on traits associated with competitive fertilization success should erode their genetic and phenotypic variability. In contrast, observed genetic variability for postcopulatory traits is unexpectedly high (reviewed by Evans and Simmons 2008). Among the several hypotheses that have been proposed to explain this paradox, environmental variation may play a crucial role in preventing one genotype from producing the optimal phenotype across all the possible environments (the so-called genotype-by-environment interaction, GEI). Under this hypothesis, genetic variability may be maintained because different phenotypes are favoured in a continuously changing environment (reviewed by Hunt and Hosken 2014). While the role of GEI in maintaining the genetic variability in precopulatory traits has been investigated in a relatively large number of cases, yet postcopulatory traits have been very little studied in this context so far. The aim of my study was to investigate whether environmental variations contribute to the maintenance of the genetic variability in sperm number in the guppy (Poecilia reticulata). Guppies are particularly appropriate to investigate the effects of environmental variations on postcopulatory sexual traits for a number of reasons. First, their natural habitats vary along multiple environmental gradients (Magurran 2005). Secondly, postcopulatory mechanisms are relatively well understood in this species (Evans and Pilastro 2011). Sperm number was chosen as target trait, since it shows a surprisingly high additive genetic variability (Gasparini et al. 2013) although sperm competition leads to strong directional selection on this trait (Boschetto et al. 2011). I used males selected for producing high and low number of sperm (Di Nisio 2014) to investigate GEIs. The use of artificially selected lines is a powerful method to investigate GEIs as it allows me to estimate the average effect size of this interaction in a group of individuals, giving a good representation of the “average” genetic background in the population. I performed three experiments by exposing male guppies from the two selected lines to different environmental manipulations. No evidence of GEI was found under dietary restriction both in sperm production and in the expression of other covarying pre- and postcopulatory sexual traits. The occurrence of GEIs was also evaluated in the offspring of males and females from the selected lines, as GEIs might arise in a trans-generational context. Again, no significant effects were detected at this level. On the contrary, evidence for GEIs was found when I manipulated the social conditions in which males and females interacted. In particular, the reproductive success of males from the selected lines significantly differed accordingly to sex ratio variations, suggesting that social environments appear to be especially relevant in generating GEIs for reproductive fitness. Although this, I found no evidence of GEI for sperm production adjustment in response to different perceived mating opportunities showed by males of the two selected lines. An experimental manipulation of the perceived male mating opportunities was also used to investigate the investment in mating acquisition associated to an increase in sperm investment. I found a trade-off between sperm investment and courtship behaviour, highlighting the importance of social interactions in potentially maintaining genetic variability in sperm number. In the final experiment, I found that diet had a significant effect on social context by altering the mating rate and hence the variance in mating and reproductive success. Furthermore, my results indicate that harsh environmental conditions tend to reduce the relative importance of postcopulatory sexual selection relatively to precopulatory processes. This suggests a synergetic effect of multiple environmental factors that could prevent one genotype to be the best across the environments and, as a consequence, maintaining genetic variability. My study provides a comprehensive picture of mechanisms responsible for the maintenance of genetic variability in a trait, sperm number, which was supposedly exposed to continuous, strong directional selection. Moreover, I have shown that it is not necessary to investigate particularly harsh or stressful environmental conditions in order to reveal significant GEIs (Hoffmann and Merila 1999), since the environmental manipulations I employed are within the normal bounds that the guppy might encounter, especially those affect social interactions. My results therefore highlight the potential of environmental fluctuations in the maintenance of sexually selected traits and the importance of studying sexual selection across varying environmental conditions.
Uno dei grandi problemi irrisolti nell’ambito della biologia evoluzionistica consiste nello spiegare come possa essere mantenuta variabilità genetica in tratti che sono sottoposti a selezione direzionale. Il problema è stato inizialmente investigato solo per quei tratti sottoposti a selezione naturale ma recentemente la problematica è stata applicata anche per quei caratteri sottoposti a selezione sessuale. Nelle specie in cui le femmine possono accoppiarsi con più di un maschio nello stesso ciclo riproduttivo, le femmine sono cioè poliandriche, si creano le condizioni per cui gli spermi di due o più maschi competano per fecondate lo stesso gruppo di uova. La competizione spermatica genera quindi una forte pressione selettiva in tutti quei caratteri che possono aumentare in qualche modo la probabilità di un maschio di fecondare le uova della femmina, vincendo quindi la competizione spermatica con gli altri maschi (Parker 1970). Tra questi, il numero di spermi prodotti è uno dei caratteri che maggiormente risponde alla forte pressione selettiva esercitata dalla competizione spermatica (Birkhead and Moller 1998). La selezione che agisce sul numero di spermi dovrebbe, generazione dopo generazione, eroderne la variabilità genetica ma, al contrario, la variabilità genetica di tale carattere, così come di altri caratteri postcopulatori, è elevata (Evans and Simmons 2008). Numerosi sono i meccanismi proposti per spiegare questo paradosso evolutivo (Radwan 2008) e la mia ricerca si è focalizzata principalmente su uno di questi: le interazioni tra il genotipo e l’ambiente (GEI). Le variazioni ambientali possono generare cambiamenti molto evidenti nell’espressione di molti caratteri sottoposti a selezione sessuale e, inoltre, genotipi diversi possono rispondere in maniera diversa a condizioni ambientali diverse (Ingleby et al. 2010). Secondo questa ipotesi, la performance relativa di diversi genotipi dipende dalle condizioni ambientali nelle quali sono espressi. E se l’ambiente è sufficientemente variabile nello spazio e/o nel tempo la variabilità genetica in caratteri sessuali può essere mantenuta a dispetto della selezione direzionale che agisce su questi, in quanto non vi è un genotipo con una performance relativa migliore in tutti gli ambienti (si veda il recente libro di Hunt and Hosken 2014). Lo scopo del mio lavoro, quindi, è stato quello di indagare se e come i cambiamenti ambientali, sia ecologici che relativi al contesto sociale, possano contribuire a mantenere la variabilità genetica in un tratto sottoposto a selezione direzionale quale il numero di spermi. Il lavoro è stato svolto usando un piccolo pesce tropicale d’acqua dolce, il guppy (Poecilia reticulata). Questa specie è particolarmente appropriata per studiare gli effetti di cambiamenti ambientali sul numero di spermi, un carattere coinvolto nella selezione sessuale postcopulatoria. Innanzitutto l’habitat naturale dei guppies, originari di Trinidad and Tobago (nei Caraibi), è soggetto a modificazioni ambientali che comprendono molteplici fattori, tra i quali la disponibilità di cibo, la torbidità delle acque, la sex ratio e la densità (Magurran 2005). Inoltre in questa specie sono stati studiati approfonditamente i meccanismi coinvolti nella selezione postocopulatoria (Evans and Pilastro 2011). Il numero di spermi trasferiti alla femmina è stato identificato come il responsabile più importante del successo postcopulatorio di un maschio, cosa che fa del numero di spermi un tratto sottoposto ad elevata pressione selettiva direzionale (Boschetto et al. 2011). Nonostante ciò, questo carattere possiede un’elevata variabilità genetica (Gasparini et al. 2013), il che rende tale carattere un candidato ideale per studiare i meccanismi che ne mantengono la variabilità. A questo scopo ho svolto sei esperimenti, descritti dettagliatamente nei sei manoscritti presentati in questa tesi. Buona parte degli esperimenti è stata svolta utilizzando maschi provenienti da due linee di selezione artificiale presenti nel mio laboratorio, in cui i maschi sono stati selezionati per un’elevata (High) e una bassa (Low) produzione di spermi (Di Nisio 2014). Nel corso del primo esperimento ho indagato se un’alterazione nella disponibilità di cibo possa modificare l’espressione del carattere “numero di spermi” e di altri caratteri importanti per la fitness riproduttiva dei maschi in maniera diversa tra i maschi delle due linee di selezione. Ho quindi sottoposto i maschi delle due linee in età giovanile ad una manipolazione della dieta, creando un gruppo sottoposto a dieta ad libitum e l’altro sottoposto ad una dieta ristretta per poi valutare l’espressione dei caratteri nelle due condizioni e nelle due linee (Manuscript 1). Tuttavia in questo primo esperimento non ho evidenziato alcuna interazione tra le linee di selezione (il genotipo) e la disponibilità di cibo (l’ambiente), confermando quanto già trovato in precedenti esperimenti: il numero di spermi prodotti sembra essere correlato alla qualità genetica dei maschi. Inoltre, ho indagato se eventuali GEI emergessero nelle femmine derivate dalle linee di selezione (sottoposte anch’esse a manipolazione della dieta) e nella prole nata da maschi e femmine delle linee di selezione precedentemente sottoposti a manipolazione della dieta. Anche per quanto riguarda questo secondo esperimento non ho riscontrato interazioni tra il genotipo e l’ambiente significative che possano contribuire al mantenimento della variabilità genetica nel numero di spermi (Manuscript 2). Se quindi, la disponibilità di cibo non sembrerebbe essere implicata nel mantenimento della variabilità genetica, così non sembra per il contesto sociale. Infatti i maschi delle due linee di selezione hanno dimostrato un diversa risposta alla presenza di una femmina stimolo: i maschi Low innalzano la loro produzione di spermi maggiormente dei maschi High, anche se questa differenza non è abbastanza elevata da generare una significativa interazione genotipo-ambiente (Manuscript 3). Al contrario il successo riproduttivo dei maschi delle due linee è risultato essere significativamente diverso al variare della sex ratio. Infatti sottoponendo coppie di maschi delle due linee a due diverse sex ratio, una in cui potevano interagire con solo due femmine (equilibrium) e l’altra con ben sei femmine (female-biased), ho riscontrato un maggior successo riproduttivo dei maschi Low quando le femmine erano in numero minore. L’interazione genotipo-ambiente che ho riscontrato tramite questo esperimento sottolinea come il contesto sociale in cui i maschi si trovano possa contribuire a mantenere il mantenimento della variabilità genetica nel numero di spermi. Il mio quinto esperimento è stato svolto a Perth, in collaborazione con l’Università della Western Australia. Questa collaborazione mi ha permesso di effettuare uno studio sulla popolazione di guppies australiana, su cui ho indagato come l’investimento nel numero di spermi (questa volta fenotipico anziché genetico) possa alterare l’investimento nel comportamento sessuale, importante carattere precopulatorio. Infatti maschi che plasticamente aumentano la loro produzione di spermi in risposta alla presenza di una femmina stimolo, adottano la tattica di accoppiamento meno dispendiosa dal punto di vista energetico. Viceversa un minor investimento in spermi è accompagnato da un maggior investimento in corteggiamento, carattere sessuale dimostrato essere molto costoso (Manuscript 5). Infine nell’ultimo esperimento ho indagato come la modificazione di un parametro ecologico, quale la disponibilità di cibo, possa ripercuotersi sulla selezione sessuale nel suo complesso. Nonostante nei primi due esperimenti non abbia riscontrato alcuna interazione genotipo-ambiente significativa in risposta ad un cambiamento della disponibilità di cibo, in quest’ultimo esperimento ho riscontrato come un cambiamento della disponibilità di cibo possa influenzare il contesto sociale, modificando quindi le interazioni maschio-femmine con importanti ripercussioni sull’importanza relativa della selezione pre- e postcopulatoria (Manuscript 6). Ho sottoposto a dieta ad libitum e ristretta popolazioni composte da sei maschi e sei femmine e poi analizzato, non solo l’espressione dei loro tratti, ma anche il loro successo di accoppiamento e riproduttivo. I miei risultati indicano che in condizioni di ambientali sfavorevoli (poco cibo) gli individui si accoppiano meno ma aumenta significativamente la varianza nel successo di accoppiamento. Nel complesso ho riscontrato come l’importanza relativa dei due episodi di selezione sessuale, pre- e postcopulatorio, cambi relativamente alle condizioni ambientali. Questo processo potrebbe essere cruciale nel mantenere la variabilità genetica nei tratti sessuali in quanto la selezione è più forte alternativamente sui tratti pre- o postcopulatori al variare delle condizioni ambientali. In conclusione questa tesi fornisce un quadro più completo dei meccanismi coinvolti nel mantenimento della variabilità genetica nei tratti sottoposti a selezione sessuale, in particolare il numero di spermi, carattere sottoposto a forte selezione direzionale. I miei risultati sottolineano come sia sufficiente generare condizioni ambientali “reali” per riscontrare delle interazioni genotipo-ambiente significative (Hoffmann and Merila 1999). Concludendo quindi, penso sia necessario che i fattori ambientali, siano essi ecologici o sociali, vengano presi in considerazione nei futuri studi di selezione sessuale visto il loro potenziale ruolo nel mantenimento della variabilità genetica dei tratti sessuali.

(1) This study investigated the maintenance of variation in the black throat patch or 'badge' of the male house sparrow. This sexually dimorphic trait is thought to be a sexually selected ornament, with previous workers providing evidence of a role in both mate choice of males by females and male-male competition. The study was conducted in 1995 and 1996 in a closed population of approximately 40 breeding pairs on Lundy Island, in the Bristol Channel, England. (2) Genetic analysis of paternity using PCR-based microsatellite genotyping revealed a very low level of extra-pair paternity in both years and no intra-specific brood parasitism. Just three extra-pair chicks (1.0% offspring in 2.5% of broods) were discovered among 305 chicks in 112 broods. This low frequency of extra-pair paternity is significantly lower than the rates reported in three other populations of house sparrows and provides further evidence for a low level of extra-pair paternity occurring in isolated populations. (3) The very low frequency of extra-pair paternity in this population allowed an examination of the costs and benefits that may be gained by a female exhibiting a preference for a large-badged male, unconfounded by the effects of extra-pair behaviour. (4) The direct benefits models of sexual selection were tested by assessing male help in provisioning chicks at the nest. Counter to the predictions of these models, large-badged males contributed relatively fewer feeds than males with smaller badges. Similarly, large-badged males, and the females that chose them as maters, had lower annual fecundity and were predicted to recruit significantly less offspring into the breeding population. 5) A female preference might be driven by the indirect benefits of obtaining genes for either viability or attractiveness for the female's offspring. A cross-fostering experiment revealed that variation in badge size had a large environmental component with a strong correlation between offspring badge size and that of their foster father, with no discernible additive genetic variation. This mechanism for the determination of badge size cannot support a process of Fisherian 'runaway' selection and is consistent with those models which require a sexual ornament to be phenotypically plastic and therefore provide an honest signal.

This thesis investigated whether there was evidence for sexual selection in the great tit Parus major. The sexually dimorphic badge carried by the great tit was negatively associated with survival of both sexes. This badge, the black chin area, has previously been shown to have a role in male-male competition. The potential benefits to a female great tit of mating with large-badged males were assessed to investigate whether this trait also has a role in sexual selection. Large males produced a greater response to territorial intrusion than their smaller counter parts, a behaviour which may be a form of parental investment. Chicks were transferred between nests in a cross-foster experiment to distinguish between direct and indirect consequences of a male's badge size on offspring development. A potential direct benefit that covaried with badge size was identified, male badge-size was positively related to condition and number of (unrelated) chicks raised. An indirect benefit was also suggested: male's chin area was related to the survival of his offspring raised away from his nest. Large-badged males tended to produce more surviving chicks. When chicks were raised by their own father, male chin area explained variation in chick leukocyte count. Large-badged males produced chicks with low leukocyte counts and low leukocyte count improved chick survival. Female great tits, therefore, may experience both direct and indirect benefits according to the badge size of her mate. Females may also adjust their behaviour according to the phenotype of their mate. For example, there was evidence that the sex ratio of the brood may vary according to male size, with larger males producing more sons. These results are discussed with respect to the current theories of sexual selection.

Barriers to interbreeding limit gene flow between sister taxa, leading to reproductive isolation and the maintenance of distinct species. These barriers come in many forms, and can act at different stages in the reproductive process. Pre-copulatory barriers may be due to individuals discriminating against heterospecifics in mate choice decisions. These decisions may be informed through a range of sensory modalities. If a female is mated and inseminated, then there may be multiple postmating-prezygotic barriers that affect the success of heterospecific sperm in attaining fertilisations. Post-zygotic barriers can be very early acting, resulting in embryonic fatality, or may be later acting, affecting the fitness of hybrid offspring. In this thesis I investigate potential reproductive barriers between the interbreeding field crickets Gryllus bimaculatus and G. campestris. I find that females of both species show only weak preference for conspecific calling song, and may even respond phonotactically to songs typical of heterospecific males. Female G. bimaculatus are repeatable in their preferences and strength of response. G. bimaculatus females presented with synthetic songs prefer those with longer inter-pulse intervals, whereas G. campestris show no discrimination between these songs. Upon meeting, G. campestris females strongly discriminate against heterospecific males, behaving aggressively towards them. This is likely driven by females responding to close range species recognition cues, including chemoreception. The species differ in their cuticular hydrocarbon profiles, and females that are no longer able to use their antennae to receive chemosensory information reduced their aggressive behaviour towards heterospecific males. G. bimaculatus females will mate with heterospecific males, though less readily than to conspecifics. When sequentially mated to both conspecific and heterospecific males, these females will preferentially take up and store sperm from the conspecific male, and sperm from conspecific males is more likely to sire offspring than would be predicted from the proportion of sperm in storage. Eggs from inter-species mating pairs are less likely to begin embryogenesis, and are more likely to suffer developmental arrest during the early stages of embryogenesis. However hybrid embryos that survive to later stages of development have hatching success similar to that of pure-bred embryos. After mating, phonotaxis of G. bimaculatus females towards male songs follows a pattern of suppression and subsequent recovery, likely triggered through detection of seminal proteins transferred in the male ejaculate, or detection of mechanical filling of the spermatheca. This pattern of suppression and recovery of phonotaxis does not differ between females mated to conspecific or heterospecific males. Females that lay few or no eggs do not experience a refractory period.

In this thesis I set out to further our understanding of two broad questions, 1) why it is that sexual selection favours the evolution of particular traits and 2) why do the patterns of sexual selection on such traits differ between groups and populations? Specifically, I focused on the role that variation in intrasexual competitive structure, the non-random distribution of socio-competitive environments across individuals, plays in shaping variation in the operation of sexual selection both within and across populations. I explore the roles of three main determinants of competitive structure, namely; population structure, polyandry and non-random variation in the distribution of the intensity of competition. To achieve this, I used a combination of empirical and theoretical tools, using the model system Red junglefowl, Gallus gallus. Throughout this thesis I both develop and employ network quantitative tools as a framework to describe variation in intrasexual competitive structure. Overall, this thesis demonstrates a complex relationship between competitive structure and the operation of sexual selection. This structure can modify the strength and direction of sexual selection operating on phenotypic traits, obscure the operation of selection at the population level and influence the relative roles of pre- and post-copulatory sexual selection. Furthermore, this thesis explores how differences in local ecology can shape competitive structure itself and in turn shape sexual selection. In doing so, this thesis sheds some light on the role that variation in competitive structure may play in shaping the operation of sexual selection both within and between populations and generating the great diversity in sexually-selected traits and behaviours between populations.

This dissertation investigates the evolutionary consequences of three very different behavioral mechanisms by which males may bias female mating decisions in their favor, elaborate male displays (Chapters I and II) and sexual coercion and resource brokering (Chapter III). The results presented here suggest that sexual selection is an important force in evolution. In Chapters I and II, I investigate the relationship between male courtship displays and speciation. Chapter I utilizes the multiple sister-taxa comparison method to test the hypothesis that sexual dimorphism is correlated with increased species diversity in teleost fishes. In 21 of 27 sister-group comparisons, the lineage with the greater degree of sexual dimorphism was more species-rich than its hypothesized sister taxa. The pattern holds across taxonomic levels and sensory modalities, and whether the male, or the female, is the displaying sex. Additional data supporting the sexual selection-diversity hypothesis in other taxa are also discussed. Chapter II investigates how variation in the signals displayed during social and sexual interactions affect reproductive isolation and may facilitate subsequent speciation, utilizing both field and laboratory experiments with the marine fish, Acanthemblemaria crockeri, a chaenopsid blenny endemic to the Gulf of California, Mexico. The anterior portion of the body, the "signal organ", displayed during social interactions, was found not only to be the most variable but was also the most geographically informative. The behavioral responses of the fish themselves, in both male courtship discrimination trials and in female spawning trials, reinforce these geographical differences and results suggest that variation in socially selected traits may accelerate reproductive isolation. In Chapter III, I examine how, if some males in a population use force to bias female mating decisions, protection can become a valuable resource that other males can use to attract females. I term this the bodyguard hypothesis of female mate choice. I present data illustrating the effectiveness of protective males in reducing the probability of aggression from other males and suggest the importance of protective mating alliances in the evolution of a diversity of animal mating systems including mate guarding, leks, "harems", monogamy, polygyny, and pair-bonding in humans.

Thesis (Ph. D.)--Florida State University, 2009.
Advisor: Joseph Travis, Florida State University, College of Arts and Sciences, Dept. of Biological Science. Title and description from dissertation home page (viewed on May 4, 2010). Document formatted into pages; contains xi, 137 pages. Includes bibliographical references.

The function of the complex song of the European Starling (Sturnus vulgaris) was examined. Song playback experiments showed that both male and female starlings were attracted by song, and complex song inhibited males from entering nestboxes. Wild starlings (at least two years old when first recorded) showed extensive changes in the composition of their song phrase repertoires and most also increased the size of their repertoires, which will result in a correlation between age and repertoire size in this species. Females prefer males that have more complex song, and this preference remained significant when preferences for certain nest sites were controlled. Males with larger repertoires did not spend more time incubating or make more feeding visits to nestlings than did males with smaller repertoires. The evolution of complex song in the European starling is consistent with an age-indicator model of sexual selection, in which aspects of male quality correlated with age are advertised by the complexity of male song.