Variance In Male Fitness Research Articles

Increased adaptability to sudden environmental change can more than make up for the two-fold cost of males(a)(a)Contribution to the Focus Issue Evolutionary Modeling and Experimental Evolution edited by José Cuesta, Joachim Krug and Susanna Manrubia.

The famous “two-fold cost of sex” is really the cost of anisogamy – why should females mate with males who do not contribute resources to offspring, rather than isogamous partners who contribute equally? In typical anisogamous populations, a single very fit male can have an enormous number of offspring, far larger than is possible for any female or isogamous individual. If the sexual selection on males aligns with the natural selection on females, anisogamy thus allows much more rapid adaptation via super-successful males. We show via simulations that this effect can be sufficient to overcome the two-fold cost and favor anisogamy over isogamy in populations adapting to environmental change. The key quantity is the variance in male fitness – if this exceeds what is possible in an isogamous population, anisogamous populations can win out in direct competition by adapting faster. This advantage in adaptability is particularly effective at maintaining anisogamy against invasion by isogamy. We discuss the extensions to our simple model needed to determine how important this mechanism is to overcoming the two-fold cost in natural populations.

The Opportunity for Post-Copulatory Sexual Selection in the Ectoparasitic Pea Crab, Dissodactylus primitivus (Brachyura: Pinnotheridae).

Pea crabs, Dissodactylus primitivus, inhabit multiple echinoid (heart urchin) hosts. Male and female crabs move among hosts in search for mates, and both sexes mate multiple times, creating opportunities for post-copulatory sexual selection. For such selection to occur, only a fraction of the males who succeed in mating can also succeed in siring progeny. Jossart et al. 2014 used 4 microsatellite loci to document parentage and mating frequencies of both sexes in D. primitivus. From these data we identified the mean and variance in female offspring numbers, as well as the proportions of the female population that were gravid and not bearing offspring. We next identified the proportions of the male population who had (1) mated and sired offspring, (2) mated but failed to sire offspring, and (3) failed to mate altogether. We used these results to estimate the opportunity for selection on males and females in terms of mate numbers and offspring numbers, and estimated the sex difference in the opportunity for selection (i.e., the opportunity for sexual selection) using both forms of data. We then partitioned the total variance in male fitness into pre- and post-copulatory components and identified the fraction of the total opportunity for selection occurring in each context. Our results show that the opportunity for selection on each sex was of similar magnitude (0.69–0.98), consistent with this polyandrogynous mating system. We also found that 37% of the total opportunity for sexual selection on males occurred within the context of post-copulatory sexual selection. However, the fraction of the total opportunity for selection that was due to sexual selection, estimated using both mate numbers and offspring numbers, was 9% and 23% respectively. Thus, we further reduced our estimate of the opportunity for post-copulatory sexual selection in D. primitivus to less than 10% of the total opportunity for selection (0.37 of 0.09 and 0.23 = 0.03 and 0.09). Our results provide the first estimate of the maximum possible strength of post-copulatory sexual selection in crustaceans using this approach.

EXTRA-PAIR PATERNITY AND THE VARIANCE IN MALE FITNESS IN SONG SPARROWS (MELOSPIZA MELODIA)

The variance in fitness across population members can influence major evolutionary processes. In socially monogamous but genetically polygynandrous species, extra-pair paternity (EPP) is widely hypothesized to increase the variance in male fitness compared to that arising given the socially monogamous mating system. This hypothesis has not been definitively tested because comprehensive data describing males' apparent (social) and realized (genetic) fitness have been lacking. We used 16 years of comprehensive social and genetic paternity data for an entire free-living song sparrow (Melospiza melodia) population to quantify and compare variances in male apparent and realized fitness, and to quantify the contribution of the variances in within-pair reproductive success (WPRS) and extra-pair reproductive success (EPRS) and their covariance to the variance in realized fitness. Overall, EPP increased the variance in male fitness by only 0-27% across different fitness and variance measures. This relatively small effect reflected the presence of socially unpaired males with zero apparent and low realized fitness, small covariance between WPRS and EPRS, and large variance in WPRS that was relatively unaffected by EPP. Therefore, although EPP altered individual males' contributions to future generations, its impact on population-level parameters such as the opportunity for selection and effective population size was limited.

SEXUAL SELECTION AND THE GENETIC MATING SYSTEM OF WILD TURKEYS

Molecular measures of parentage provide important insights into the opportunity for sexual selection; in birds, such studies have been conducted almost exclusively on pair-bonded passerines. Here I employ a multitiered parentage analysis involving 10-locus microsatellite genotypes to characterize the genetic mating system of a population of Wild Turkeys (Meleagris gallopavo), a promiscuous species in which males and potentially females have been thought to mate multiply. Young in almost half of nests (48%, n = 15) were each apparently the product of a single male and female parent. Fourteen broods (45%) resulted from multiple paternity; seven contained eggs from multiple females, four of which appeared to be cases of quasi parasitism, in which the offspring of the brood parasitic female were fathered by the same male that sired at least one of the host females' offspring. Bateman gradients for males and females indicate that males experience a significantly greater gain in reproductive success from additional mates; the trend for females to benefit from multiple matings disappeared when the small “clutches” of parasitic females were excluded from the analysis. Of the components of variance in male fitness, number of mates was the most important determinant of male reproductive success. Somewhat surprisingly, when considering only reproductively successful males, the proportion of a female's offspring that a male sired also explained a substantial proportion of the total variance in male reproductive success. Incomplete sampling of offspring could mean that these estimates, particularly the importance of mate number, may be underestimated. Regardless, these results suggest that multiple mating by females may be an important and overlooked component of sexual selection in species with lek-like mating systems, and that selection may act independently on males to attract more mates and increase their share of paternity with those mates.

Integrating population demography, genetics and self-incompatibility in a viability assessment of the Wee Jasper Grevillea (Grevillea iaspicula McGill., Proteaceae)

Grevillea iaspicula is an endangered shrub known from only eight small populations (<250 individuals) in south-eastern Australia. The species is threatened by combined ecological and genetic factors, e.g. land conversion, weed invasion, low recruitment and low gene flow among populations. The populations also show large variance in male fitness and limited mate availability which are thought to arise as a consequence of gametophytic self-incompatibility (GSI). This study has used an individual-based, spatially explicit simulation model to explore the interaction between GSI and mate limitation in this species, as well as its effect on long-term population viability. The model was parameterised with demographic and genetic data obtained from 2 years of population monitoring. Simulation results identified extremely low establishment rates as the most critical factor currently influencing the persistence of G. iaspicula populations and indicated that the extant populations are at serious risk of extinction in the near future unless this is altered by, at very least, an order of magnitude higher. SI was shown to affect the magnitude of variation in establishment but this effect was masked when establishment was critically low. Disassortative mating, owing to low allelic richness at the S-locus, had the negative demographic effect of restricting mating to relatively few compatible plants. Restricted mate availability imposed additional limitations to the viability of populations but, given a 20-fold increase in establishment rate, population fluctuations stabilised. The long-term viability of G. iaspicula is bleak without artificial augmentation of the populations but management planning must also consider genetic processes, including SI, to ensure such strategies optimise the benefits gained.

Extrapair paternity and the opportunity for sexual selection in long-distant migratory passerines

Extrapair fertilizations (EPFs) are frequently documented in songbirds; however, the extent to which this reproductive tactic contributes to variance in male reproductive success and hence the strength of sexual selection on males remains little studied. Using 2 approaches, intraspecific and comparative, we test the hypothesis that the contribution of EPFs to variance in male fitness increases with migration distance in north temperate songbirds. Using data on the genetic mating system of the scarlet rosefinch Carpodacus erythrinus, a long-distance migrant, we show that the number of extrapair mates and within-pair paternity are the most important components of variance in male reproductive success. There was no evidence of a trade-off between extrapair and within-pair success of individual males as males successful in procuring EPFs were less likely to be cuckolded. Comparative data reveal that the opportunity for sexual selection due to EPFs is positively associated with both migration distance and breeding synchrony in north temperate passerines, and we discuss several mechanisms that could extend these relationships. In general, these data suggest that EPFs have a potential to play an important role in the evolution of sexually selected traits in long-distance migratory songbirds such as rosefinches. Key words: comparative analysis, mate choice, parentage assignment, promiscuity, scarlet rosefinch Carpodacus erythrinus, within-pair paternity. [Behav Ecol]

Sexual selection: harem size and the variance in male reproductive success.

Sexual selection is potentially stronger than natural selection when the variance in male reproductive fitness exceeds all other components of fitness variance combined. However, measuring the variance in male reproductive fitness is difficult when nonmating males are absent, inconspicuous, or otherwise difficult to find. Omitting the nonmating males inflates estimates of average male reproductive success and diminishes the variance, leading to underestimates of the potential strength of sexual selection. We show that, in theory, the proportion of the total variance in male fitness owing to sexual selection is approximately equal to H, the mean harem size, as long as H is large and females are randomly distributed across mating males (i.e., Vharem=H). In this case, mean harem size not only provides an easy way to estimate the potential strength of sexual selection but also equals the opportunity for sexual selection, I(mates). In nature, however, females may be overdispersed with Vharem<H or more aggregated than random with Vharem>H. We show that H+(k-1) is a good measure of the opportunity for sexual selection, where k is the ratio Vharem/H. A review of mating system data reveals that in nature the median ratio for Vharem/H is 1.04, but as H increases, females tend to become more aggregated across mating males with V(harem) two to three times larger than H.

Determinants of effective population size for loci with different modes of inheritance.

Here we report an assessment of the determinants of effective population size (N(e)) in species with overlapping generations. Specifically, we used a stochastic demographic model to investigate the influence of different life-history variables on N(e)/N (where N = population census number) and the influence of sex differences in life-history variables on N(e) for loci with different modes of inheritance. We applied an individual-based modeling approach to two datasets: one from a natural population of savannah baboons (Papio cynocephalus) in the Amboseli basin of southern Kenya and one from a human tribal population (the Gainj of Papua New Guinea). Simulation-based estimates of N(e)/N averaged 0.329 for the Amboseli baboon population (SD = 0.116, 95% CI = 0.172 - 0.537) and 0.786 for the Gainj (SD = 0.184, 95% CI = 0.498 - 1.115). Although variance in male fitness had a substantial impact on N(e)/N in each of the two primate populations, ratios of N(e) values for autosomal and sex-linked loci exhibited no significant departures from Poisson-expected values. In each case, similarities in sex-specific N(e) values were attributable to the unexpectedly high variance in female fitness. Variance in male fitness resulted primarily from age-dependent variance in reproductive success, whereas variance in female fitness resulted primarily from stochastic variance in survival during the reproductive phase.

Components of male fitness in relation to body size in Epirrita autumnata (Lepidoptera, Geometridae)

Abstract. 1. The effect of body size on different components of male fitness was studied for Epirrita autumnata, a geometrid known for its eruptive population dynamics. Body size is the main determinant of female fecundity in this species.2. Longevity of males was found to have a weak negative correlation with body size at low temperatures. No significant correlation was found at higher temperatures.3. We found no correlation between male size and female fecundity or egg size which is consistent with the small size of spermatophores in this species.4. Small and large males were equally successful when allowed to compete for females in laboratory conditions.5. In one or two field collections, males found mating were larger than males found singly. Large males also had an advantage in finding of virgin females, offered experimentally. No size‐assortative mating was recorded.6. We conclude that size‐dependent mate location ability is the factor accounting for most of the variance in male fitness in E.autumnata. The dependence of fitness on body size may well be equally strong in males and females.

Experimentally Induced Density-Dependent Mating Patterns in Phyllobius maculicornis Germ. (Coleoptera: Curculionidae)

The mating pattern of the weevil Phyllobius maculicornis Germ. was monitored in the field following experimental reduction of population density. As density decreased, a tendency for size-assortative mating emerged. Body size of mating and nonmating animals was not significantly different within the sexes. However, on average, mating females were significantly larger than nonmating females when population density decreased, indicating male selection under low density conditions. When mating pairs were isolated in the laboratory, durations of the copulations were positively associated with female body size. Males in this way put extra effort into fertilizing eggs of large females. INTRODUCTION For insect species in which males perform no parental care, the number of inseminated females is the prime determinant of male fitness (Trivers, 1972). Sexual selection theory predicts intrasexual competition among the low investment sex (typically males), and usually the competitive ability of male insects is related to their body size (Mason, 1980; McLain, 1982; Johnson, 1983; Thornhill and Alcock, 1983; Lawrence, 1986; Pinto and Mayor, 1986; Simmons, 1986). Theory also includes the possibility that females choose among possible mates, thereby enforcing selective pressures on males (Majerus, 1986). Since female body size in insects usually is positively correlated with fecundity (Freeman, 1981; Evans, 1982; O'Neill, 1984; Butlin and Day, 1985; Juliano, 1985), males are expected to prefer large females. Such a choice has been demonstrated in tropical weevils (Johnson and Hubbell, 1984). Several studies of insects have revealed size-assortative mating patterns, in which body size of male and female is correlated in mating pairs (Thornhill and Alcock, 1983; Partridge and Halliday, 1984). Few studies, however, offer more detailed analyses of variations in recorded mating patterns. Numerous selective pressures obviously influence fitness of insects (e.g, habitat changes, food availability, parasite/predator incidence, or the dynamics within the population itself). Differences in male behavior and mating success, for example, may depend upon local sex ratios (Lawrence, 1986), interspecific interference (McLain, 1981), weather conditions (Banks and Thomson, 1985) or differences in female oviposition behavior (Fincke, 1985). Density-dependent differences in mating patterns have been reported in soldier beetles. Positive phenotypic assortative mating, based on various body size measurements, was more pronounced in low density populations (McLain, 1982, 1984). In general, the level of male competition and the variance in male fitness are thought to be positively related to increased density, as the disparity between successful and unsuccessful males increases (Wade, 1980). The occurrence of alternative male mating patterns at high densities is reported in several insect orders (Alcock et al., 1977; Greenfield and Shelly, 1985). I measured the direct effects of experimental changes in population density on mating pattern in a field study of the weevil Phyllobius maculicornis Germ. Density manipulations were conducted in the field, and additional observations of mating behavior were performed in the laboratory. The basic hypothesis tested was an expected change in mating pattern as density was reduced. MATERIALS AND METHODS An isolated population of Phyllobius maculicornis was monitored during the mating

Primate population structure: evaluation of models.

Genetic markers among macaques on Cayo Santiago island were analyzed in an attempt to infer aspects of mating structure. Several models that included high levels of gene flow among groups could not be distinguished, but the data are clearly incompatible with group endogamy and with high variance in male fitness. Drift effective size is approximately one half census size in this population.