Natural Selection of Optimal Reproductive Tactics

Acting through differential reproductive success, natural selectionhas produced a great diversity of existing reproductive tactics,each of which presumably corresponds to a local optimum thatmaximizes an individual organism's lifetime reproductive successin its particular environment. A body of theory on so-calledreproductive "strategies" has yet to be adequately related toan independent theoretical framework on optimal foraging tactics.Some of the possible interactions and constraints between ananimal's input of matter and energy via foraging and its outputin offspring using these same materials are briefly considered.For example, storage and utilization of lipids allow an organismto gather and sequester matter and energy during a period thatis not suitable for successful reproduction, but enable theorganism to expend those materials at a later, more satisfactorytime. Such interactions between foraging and reproduction leadto a sort of temporal integration, which greatly complicatesestimation of reproductive effort (current investment in seed,eggs, or progeny) in variable environments. An optimal reproductivetactic maximizes an individual's reproductive value (the sumof all present plus the expected probable number of all futureoffspring) at every age. Reproductive effort should vary inverselywith residual reproductive value (expectation of future offspring);moreover, the precise form of the trade-off between presentoffspring versus future progeny, which is itself sensitive toa multitude of environmental influences including resource availabilityand the immediate environmental conditions for reproductionand survival, dictates the optimal tactic at any given age.Simple graphical models of optimal reproductive tactics arepresented and discussed. Finally, some promising directionsfor future work, as well as certain potential difficulties,are noted.     

The Structure of Bonobo Copulation Calls During Reproductive and Non‐Reproductive Sex

Copulation calls in primates are usually identified as sexually selected signals that promote the reproductive success of the caller. In this study, we investigated the acoustic structure of copulation calls in bonobos (Pan paniscus), a great ape known for its heightened socio‐sexuality. Throughout their cycles, females engage in sexual relations with both males and other females and produce copulation calls with both partners. We found that calls produced during sexual interactions with male and female partners could not be reliably distinguished in terms of their acoustic structure, despite major differences in mating behaviour and social context. Call structure was equally unaffected by the size of a female’s sexual swelling and by the rank of her mating partner. Rank of the partner did affect call delivery although only with male, but not female partners. The only strong effect on call structure was because of caller identity, suggesting that these signals primarily function to broadcast individual identity during sexual interactions. This primarily social use of an evolved reproductive signal is consistent with a broader trend seen in this species, namely a transition of sexual behaviour to social functions.     

Sex Pheromones and Reproductive Isolation in Five Mirid Species

Mate location in many mirid bugs (Heteroptera: Miridae) is mediated by female-released sex pheromones. To elucidate the potential role of the pheromones in prezygotic reproductive isolation between sympatric species, we investigated differences in the pheromone systems of five mirid species, Apolygus lucorum, Apolygus spinolae, Orthops campestris, Stenotus rubrovittatus and Taylorilygus apicalis. GC/MS analyses of metathoracic scent gland extracts of virgin females showed that all five species produced mixtures of hexyl butyrate, (E)-2-hexenyl butyrate and (E)-4-oxo-2-hexenal, but in quite different ratios. (E)-2-hexenyl butyrate was the major component of A. spinolae, while hexyl butyrate was the most abundant component in the pheromone blends of the other four species. In addition to the three compounds, a fourth component, (E)-2-octenyl butyrate, was present in the gland extracts of A. lucorum and T. apicalis females. Field tests suggest that the ternary blends of hexyl butyrate, (E)-2-hexenyl butyrate and (E)-4-oxo-2-hexenal as found in the extracts of the females of each species do not inhibit attraction of conspecific males but ensure species-specificity of attraction between A. lucorum, O. campestris and T. apicalis. Furthermore, (E)-2-octenyl butyrate was essential for attraction of A. lucorum and T. apicalis males, but strongly inhibited attraction of male A. spinolae, O. campestris and S. rubrovittatus. The combined results from this study and previous studies suggest that the minor component and pheromone dose in addition to the relative ratio of the major components play an important role in reproductive isolation between mirid species.     

Sex Steroids and the Differentiation of Avian Reproductive Behavior

Experiments in which avian embryos are treated with sex steroidsor steroid antagonists suggest that sexual differentiation ofreproductive behavior (and thus differentiation of the brainmechanisms for such behavior) is controlled by steroids producedby the embryonic gonads. In chickens and Japanese quail, maleshatched from eggs treated with estradiol or testosterone duringincubation are feminized (demasculinized); they fail to exhibitmasculine sexual behavior as adults, and no longer are behaviorallydistinguishable from females. Some evidence suggests that testosteronemay mimic the feminizing action of estradiol by being convertedto an estrogen in the embryonic brain. Genetic female quailexposed to an antiestrogen during embryonic development aremasculinized; they exhibit an increased ability to display themasculine copulatory pattern. Thus the behavior of these speciesis feminized by embryonic exposure to sex steroids, the anhormonal(neutral) sex for behavioral differentiation appears to be themale, and females appear to result from estrogen produced bythe embryonic ovaries. In contrast, sex steroid treatment ofmammals early in development masculinizes behavior, the femaleis the neutral sex, and males result from fetal androgen secretion.These opposite patterns of psychosexual differentiation in birdsand mammals are correlated with a major difference between theavian and mammalian sex-determining mechanism. Implicationsfor other vertebrates are discussed.     

Sex Chromosome Translocations in the Evolution of Reproductive Isolation

Haldane's rule states that in organisms with differentiated sex chromosomes, hybrid sterility or inviability is generally expressed more frequently in the heterogametic sex. This observation has been variously explained as due to either genic or chromosomal imbalance. The fixation probabilities and mean times to fixation of sex-chromosome translocations of the type necessary to explain Haldane's rule on the basis of chromosomal imbalance have been estimated in small populations of Drosophila melanogaster. The fixation probability of an X chromosome carrying the long arm of the Y(X·Y^L) is approximately 30% greater than expected under the assumption of no selection. No fitness differences associated with the attached Y^L segment were detected. The fixation probability of a deficient Y chromosome is 300% greater than expected when the X chromosome contains the deleted portion of the Y. It is suggested that sex-chromosome translocations may play a role in the establishment of reproductive isolation.     

Lineage Selection and the Maintenance of Sex

Sex predominates in eukaryotes, despite its short-term disadvantage when compared to asexuality. Myriad models have suggested that short-term advantages of sex may be sufficient to counterbalance its twofold costs. However, despite decades of experimental work seeking such evidence, no evolutionary mechanism has yet achieved broad recognition as explanation for the maintenance of sex. We explore here, through lineage-selection models, the conditions favouring the maintenance of sex. In the first model, we allowed the rate of transition to asexuality to evolve, to determine whether lineage selection favoured species with the strongest constraints preventing the loss of sex. In the second model, we simulated more explicitly the mechanisms underlying the higher extinction rates of asexual lineages than of their sexual counterparts. We linked extinction rates to the ecological and/or genetic features of lineages, thereby providing a formalisation of the only figure included in Darwin''s “The origin of species”. Our results reinforce the view that the long-term advantages of sex and lineage selection may provide the most satisfactory explanations for the maintenance of sex in eukaryotes, which is still poorly recognized, and provide figures and a simulation website for training and educational purposes. Short-term benefits may play a role, but it is also essential to take into account the selection of lineages for a thorough understanding of the maintenance of sex.     

Autonomy and Reproductive Rights of Married Ikwerre Women in Rivers State,Nigeria

A woman’s lack of or limited reproductive autonomy could lead to adverse health effects, feeling of being inferior, and above all being unable to adequately care for her children. Little is known about the reproductive autonomy of married Ikwerre women of Rivers State, Nigeria. This study demonstrates how Ikwerre women understand the terms autonomy and reproductive rights and what affects the exercise of these rights. An exploratory research design was employed for this study. A semi-structured interview schedule was used to conduct thirty-four in-depth interviews and six focus group discussions with purposively sampled educated, semi-educated, and uneducated Ikwerre women in monogamous or polygynous marriages. The collected data was analysed qualitatively with MAXQDA 11 using open and axial coding. The interviews and focus group responses reveal a low level of awareness of autonomy and reproductive rights amongst the Ikwerre women in Nigeria. While some educated women were aware of their reproductive rights, cultural practices were reported to limit the exercise of these rights. Participants reported that Ikwerre culture is a patriarchal one where married women are expected to submit and obey their husbands in all matters; and a good married woman according to Ikwerre standard is one who complies with this culture. Women’s refusal of sexual advances from their husbands is described as not being acceptable in this culture; and hence rape in marriage is not recognized in Ikwerre culture. Education and awareness creation on the importance of women’s reproductive autonomy could improve their reproductive rights and autonomy in marital settings. Overcoming the patriarchal aspects of Ikwerre culture—for example, the greater value placed on male children than female children and treating women as incompetent individuals—is necessary to promote gender equality as well as help improve women’s reproductive autonomy.     

Policy Resistance Undermines Superspreader Vaccination Strategies for Influenza

Theoretical models of infection spread on networks predict that targeting vaccination at individuals with a very large number of contacts (superspreaders) can reduce infection incidence by a significant margin. These models generally assume that superspreaders will always agree to be vaccinated. Hence, they cannot capture unintended consequences such as policy resistance, where the behavioral response induced by a new vaccine policy tends to reduce the expected benefits of the policy. Here, we couple a model of influenza transmission on an empirically-based contact network with a psychologically structured model of influenza vaccinating behavior, where individual vaccinating decisions depend on social learning and past experiences of perceived infections, vaccine complications and vaccine failures. We find that policy resistance almost completely undermines the effectiveness of superspreader strategies: the most commonly explored approaches that target a randomly chosen neighbor of an individual, or that preferentially choose neighbors with many contacts, provide at best a relative improvement over their non-targeted counterpart as compared to when behavioral feedbacks are ignored. Increased vaccine coverage in super spreaders is offset by decreased coverage in non-superspreaders, and superspreaders also have a higher rate of perceived vaccine failures on account of being infected more often. Including incentives for vaccination provides modest improvements in outcomes. We conclude that the design of influenza vaccine strategies involving widespread incentive use and/or targeting of superspreaders should account for policy resistance, and mitigate it whenever possible.     

Signatures of Sex-Antagonistic Selection on Recombining Sex Chromosomes

Sex-antagonistic (SA) selection has major evolutionary consequences: it can drive genomic change, constrain adaptation, and maintain genetic variation for fitness. The recombining (or pseudoautosomal) regions of sex chromosomes are a promising setting in which to study SA selection because they tend to accumulate SA polymorphisms and because recombination allows us to deploy the tools of molecular evolution to locate targets of SA selection and quantify evolutionary forces. Here we use coalescent models to characterize the patterns of polymorphism expected within and divergence between recombining X and Y (or Z and W) sex chromosomes. SA selection generates peaks of divergence between X and Y that can extend substantial distances away from the targets of selection. Linkage disequilibrium between neutral sites is also inflated. We show how the pattern of divergence is altered when the SA polymorphism or the sex-determining region was recently established. We use data from the flowering plant Silene latifolia to illustrate how the strength of SA selection might be quantified using molecular data from recombining sex chromosomes.     

Reproductive Value and Fluctuating Selection in an Age-Structured Population

Fluctuations in age structure caused by environmental stochasticity create autocorrelation and transient fluctuations in both population size and allele frequency, which complicate demographic and evolutionary analyses. Following a suggestion of Fisher, we show that weighting individuals of different age by their reproductive value serves as a filter, removing temporal autocorrelation in population demography and evolution due to stochastic age structure. Assuming weak selection, random mating, and a stationary distribution of environments with no autocorrelation, we derive a diffusion approximation for evolution of the reproductive value weighted allele frequency. The expected evolution obeys an adaptive topography defined by the long-run growth rate of the population. The expected fitness of a genotype is its Malthusian fitness in the average environment minus the covariance of its growth rate with that of the population. Simulations of the age-structured model verify the accuracy of the diffusion approximation. We develop statistical methods for measuring the expected selection on the reproductive value weighted allele frequency in a fluctuating age-structured population.THE evolutionary dynamics of age-structured populations were formalized by Charlesworth (1980, 1994) and Lande (1982) on the basis of earlier ideas of Fisher (1930, 1958), Medawar (1946, 1952), and Hamilton (1966), showing that the strength of selection on genes affecting the vital rates of survival or fecundity depends on their age of action (reviewed by de Jong 1994; Charlesworth 2000). Fisher defined the reproductive value of individuals of a given age as their expected contribution to future population growth, determined by the age-specific vital rates. This has the property that in a constant environment the total reproductive value in a population always increases at a constant rate. The total population size, however, undergoes transient fluctuations as the stable age distribution is approached, and the total population size only asymptotically approaches a constant growth rate (Caswell 2001).Environmental stochasticity creates continual fluctuations in age structure, producing temporal autocorrelation in population size and in allele frequencies, which seriously complicate demographic and evolutionary analyses. Fisher (1930, 1958, p. 35) suggested for analysis of genetic evolution that individuals should be weighted by their reproductive value to compensate for deviations from the stable age distribution. Here we apply this suggestion to study weak fluctuating selection in an age-structured population in a stochastic environment.One of the central conceptual paradigms of evolutionary biology was described by Wright (1932). His adaptive topography represents a population as a point on a surface of population mean fitness as a function of allele frequencies. Assuming weak selection, random mating, and loose linkage (implying approximate Hardy–Weinberg equilibrium within loci and linkage eqilibrium among loci), natural selection in a constant environment causes the population to evolve uphill of the mean fitness surface (Wright 1937, 1945, 1969; Arnold et al. 2001; Gavrilets 2004). Evolution by natural selection thus tends to increase the mean fitness of a population in a constant environment.Lande (2007, 2008) generalized Wright''s adaptive topography to a stochastic environment, allowing density-dependent population growth but assuming density-independent selection, showing that the expected evolution maximizes the long-run growth rate of the population at low density, . Here r is population growth rate at low density in the average environment and is the environmental variance in population growth rate among years, which are standard parameters in stochastic demography (Cohen 1977, 1979; Tuljapurkar 1982; Caswell 2001; Lande et al. 2003). In this model of stochastic evolution the adaptive topography describing the expected evolution is derived by expressing r and as functions of allele frequencies with parameters being the mean Malthusian fitnesses of the genotypes and their temporal variances and covariances. These results are based on diffusion approximations for the coupled stochastic processes of population size and allele frequencies in a fluctuating environment.Diffusion approximations are remarkably accurate for many problems in evolution and ecology (Crow and Kimura 1970; Lande et al. 2003). Because a diffusion process is subject to white noise with no temporal autocorrelation, the approximation is most accurate if the noise in the underlying biological process is approximately uncorrelated among years. Despite temporal autocorrelation in total population size produced by age-structure fluctuations, the stochastic demography of age-structured populations over timescales of a generation or more can nevertheless be accurately approximated by a diffusion process (Tuljapurkar 1982; Lande and Orzack 1988; Engen et al. 2005a, 2007). The success of the diffusion approximation for total population size occurs because the noise in the total reproductive value is nearly white, with no temporal autocorrelation to first order, and the log of total population size fluctuates around the log of reproductive value with a return time to equilibrium on the order of a few generations (Engen et al. 2007). Hence the diffusion approximation is well suited to describe the stochastic dynamics of total reproductive value as well as total population size.This article extends Lande''s (2008) model of fluctuating selection without age structure by deriving a diffusion approximation for the evolution of an age-structured population in a stochastic environment. Assuming weak selection at all ages, random mating, and a stationary distribution of environments with no temporal autocorrelation, we show that the main results of the model remain valid, provided that the model parameters are expressed in terms of means, variances, and covariances of age-specific vital rates and that allele frequencies are defined by weighting individuals of different age by their reproductive value, as suggested by Fisher (1930, 1958). We perform simulations to verify the accuracy of the diffusion approximation and outline statistical methods for estimating the expected selection acting on the reproductive value weighted allele frequency.