Fecundity Selection Research Articles

Genetic and demographic responses of mercury-exposed mosquitofish (Gambusia holbrooki) populations: Temporal stability and reproductive components of fitness

Two previous mesocosm studies showed changes in glucosephosphate isomerase-2 (Gpi-2) allele frequencies in mosquitofish populations exposed to mercury for 111 d or two years. A previous selection component analysis of single-generation populations exposed for 111 d to 18 microg/L Hg suggested that female sexual selection and fecundity selection could contribute to changes in Gpi-2 allele frequencies. The present multigeneration study was conducted to determine the stability of Gpi-2 allele frequencies over four years of mercury exposure, measure the reproductive fitness of Gpi-2 genotypes inhabiting control and mercury-contaminated mesocosms to determine a mechanism explaining changes in Gpi-2 allele frequencies, investigate differences in the demographic characteristics of mercury-exposed and control populations, and investigate the water quality of the mesocosms to determine if variables other than mercury show concordant patterns among mesocosms. Differences in Gpi-2 allele frequencies between control and mercury-exposed populations were stable over four years (approximately eight generations) of mercury exposure. Mercury-exposed female mosquitofish had a lower probability of being gravid than control females (p = 0.001). Mercury-exposed females also had lower fecundity (total number of eggs and embryos) than control females (p = 0.036). Unlike the results of the more intense mercury exposures in the single generation study, no strong evidence was found that Gpi-2 genotype influenced fecundity or the probability of being gravid in both control and mercury-exposed females. The quantification of fitness components is difficult but has the potential to enhance our understanding of how toxicants alter allele frequencies in exposed populations.

Sexual dimorphism in lizard body shape: the roles of sexual selection and fecundity selection.

Sexual dimorphism is widespread in lizards, with the most consistently dimorphic traits being head size (males have larger heads) and trunk length (the distance between the front and hind legs is greater in females). These dimorphisms have generally been interpreted as follows: (1) large heads in males evolve through male-male rivalry (sexual selection); and (2) larger interlimb lengths in females provide space for more eggs (fecundity selection). In an Australian lizard (the snow skink, Niveoscincus microlepidotus), we found no evidence for ongoing selection on head size. Trunk length, however, was under positive fecundity selection in females and under negative sexual selection in males. Thus, fecundity selection and sexual selection work in concert to drive the evolution of sexual dimorphism in trunk length in snow skinks.

Nuptial gifts and the evolution of male body size.

In many insect systems, males donate nuptial gifts to insure an effective copulation or as a form of paternal investment. However, if gift magnitude is both body size-limited and positively related to fitness, then the opportunity exists for the gift to promote the evolution of large male size. In the striped ground cricket, Allonemobius socius, males transfer a body size-limited, somatic nuptial gift that is comprised primarily of hemolymph. To address the implications of this gift on male size evolution, we quantified the intensity and direction of natural (fecundity) and sexual (mating success) selection over multiple generations. We found that male size was under strong positive sexual selection throughout the breeding season. This pattern of selection was similar in successive generations spanning multiple years. Male size was also under strong natural selection, with the largest males siring the most offspring. However, multivariate selection gradients indicated that gift size, and not male size, was the best predictor of female fecundity. In other words, direct fecundity selection for larger gifts placed indirect positive selection on male body size, supporting the hypothesis that nuptial gifts can influence the evolution of male body size in this system. Although female size was also under strong selection due to a size related fecundity advantage, it did not exceed selection on male size. The implications of these results with regard to the maintenance of the female-biased size dimorphic system are discussed.

POPULATION VARIATION IN SEXUAL SELECTION AND ITS EFFECT ON SIZE ALLOMETRY IN TWO DUNG FLY SPECIES WITH CONTRASTING SEXUAL SIZE DIMORPHISM

Body size is one of the most important quantitative traits under evolutionary scrutiny. Sexual size dimorphism (SSD) in a given species is expected to result if opposing selection forces equilibrate differently in both sexes. We document variation in the intensity of sexual and fecundity selection, male and female body size, and thus SSD among 31 and 27 populations of the two dung fly species, Scathophaga stercoraria and Sepsis cynipsea, across Switzerland. Whereas in S. cynipsea females are larger, the SSD is reversed in S. stercoraria. We comprehensively evaluated Fairbairn and Preziosi's (1994) general, three-tiered scenario, hypothesizing that sexual selection for large male size is the major driving force of SSD allometry within these two species. Sexual selection intensity on male size in the yellow dung fly, S. stercoraria, was overall positive, greater, and more variable among populations than fecundity selection on females. Also, sexual selection intensity in a given population correlated positively with mean male body size of that population for both the field-caught fathers and their laboratory-reared sons, indicating a response to selection. In S. cvnipsea, sexual selection intensity on males was lower overall and significantly positive, about equal in magnitude, but more variable than fecundity selection on females. However, there was no correlation between the intensity of sexual selection and mean male body size among populations. In both species, the laboratory-reared offspring indicate genetic differentiation among populations in body size. Despite fulfillment of all key prerequisites, at least in S. stercoraria, we did not find hypoallometry for SSD (Rensch's rule, i.e., greater evolutionary divergence in male size than female size) for the field-caught parents or the laboratory-reared offspring: Female size was isometric to male size in both species. We conclude that S. cynipsea does not fit some major requirements of Fairbairn and Preziosi's (1994) scenario, whereas for S. stercoraria we found partial support for it. Failure to support Rensch's rule within the latter species may be due to phylogenetic or other constraints, power limitations, erroneous estimates of sexual selection, insufficient genetic isolation of populations, or sex differences in viability selection against large size.

NUPTIAL GIFTS AND THE EVOLUTION OF MALE BODY SIZE

In many insect systems, males donate nuptial gifts to insure an effective copulation or as a form of paternal investment. However, if gift magnitude is both body size-limited and positively related to fitness, then the opportunity exists for the gift to promote the evolution of large male size. In the striped ground cricket, Allonemobius socius, males transfer a body size-limited, somatic nuptial gift that is comprised primarily of hemolymph. To address the implications of this gift on male size evolution, we quantified the intensity and direction of natural (fecundity) and sexual (mating success) selection over multiple generations. We found that male size was under strong positive sexual selection throughout the breeding season. This pattern of selection was similar in successive generations spanning multiple years. Male size was also under strong natural selection, with the largest males siring the most offspring. However, multivariate selection gradients indicated that gift size, and not male size, was the best predictor of female fecundity. In other words, direct fecundity selection for larger gifts placed indirect positive selection on male body size, supporting the hypothesis that nuptial gifts can influence the evolution of male body size in this system. Although female size was also under strong selection due to a size related fecundity advantage, it did not exceed selection on male size. The implications of these results with regard to the maintenance of the female-biased size dimorphic system are discussed.Corresponding Editor: S. Pitnick

The effects of selection for larval behavior on adult life-history features in Drosophila melanogaster.

Selection for late-life fecundity and longevity in adult Drosophila melanogaster is well known to modify numerous characteristics of life history and physiology. We report experiments here in which selection applied to behavior affects features in an identical fashion. Selection for feeding rate of larval D. melanogaster modifies caloric intake, as measured by the uptake and incorporation of labeled glucose. Selection for slow larval feeding produced lines of D. melanogaster in which larvae synthesized significantly less lipid prior to pupation and eclosed to have low early-life fecundity and a long life as adults. They also had greater lifetime fecundity, but lower viability of egg to hatched adult. Alternatively, fast-feeding larvae incorporated more lipid before pupation and eclosed with high early-fecundity that declined rapidly throughout their short adult life. Slow-feeding populations also had a significantly enhanced expression of the stress-resistance genes CuZn-SOD, CATALASE, and HSP70. Selection on larval feeding behavior reproduced the antagonistic evolutionary trade-off found under selection for adult life span and mimicked the physiological response in life span as seen in many species when dietary restriction is imposed on adults. Thus, nutrient acquisition during development appears to share a common evolutionary and genetic basis with the allocation processes that determine adult life-history traits and the related phenotypic dietary restriction phenomena.

Plumage Color as a Composite Trait: Developmental and Functional Integration of Sexual Ornamentation

Most studies of condition-dependent sexual ornaments have treated such ornaments as single traits. However, sexual ornaments are often composites of several components, each produced by partially independent developmental pathways. Depending on environmental and individual condition, components of these ornaments may reflect different behavioral or physiological properties of an individual. One of the best-known, condition-dependent ornaments is carotenoid-based plumage coloration, which has at least four distinct components: pigment elaboration, patch area, pigment symmetry, and patch area symmetry. Here we examined fitness consequences of variation in individual components of carotenoid ornamentation in male house finches (Carpodacus mexicanus). Over 5 yr and several selection episodes, we studied variation in the plumage components in a large sample (n = 498) of males from a Montana population. The ornament components were partially independent of each other and had distinct fitness consequences. Selection for higher fecundity favored an increase in redness of coloration and a decrease in pigment asymmetry and patch area asymmetry but did not act on patch area itself. In contrast, viability selection favored larger and more symmetrical ornamental patches but did not act on pigment elaboration. Developmental and functional interrelationships among individual components of ornamentation strongly differed between house finch populations. Distinct patterns of selection on individual components of condition-dependent ornaments, combined with partially independent development of components, should favor the evolution of composite sexual traits whose components reliably reflect condition across a wide array of environments.

Sexual differences in life‐history traits in the butterfly Lycaena tityrus: a comparison between direct and diapause development

AbstractDuring direct development the butterfly Lycaena tityrus was previously found to display sex‐related reaction norms in response to temperature. Based on selection for protandry in males and fecundity selection for larger females, males favoured early emergence over large size, leading to a dramatic weight loss at higher temperatures, whereas females maintained similar weights throughout. Because males were able to avoid a weight reduction relative to females in spite of their shorter development at lower temperatures, sexual size dimorphism existed at higher temperatures only. In the present paper we compare sexual differences in life‐history traits in L. tityrus between direct and diapause development at 25 °C. We demonstrate that, regardless of developmental pathway, protandry persisted and relative sexual size dimorphism, with females being larger, remained unchanged. Although diapausing individuals were less time‐constrained, allowing them to grow to considerably higher final weights in both sexes, males were not able to reduce their weight loss relative to females. This is explained by the pressure to gain a developmental advantage solely during post‐diapause development, whereas direct developing males may spread the burden over the whole larval period. Our results highlight the importance of considering sexual differences in selective pressures, which may influence central life‐history traits in manifold ways.

Sexual size dimorphism in a natural population of Callicorixa vulnerata (Hemiptera: Corixidae)

Sexual size dimorphism occurs in many species. Differences between males and females, in size or other characteristics, may result from sexual selection, fecundity selection, natural selection, non-adaptive processes, or a combination of these pressures (Darwin 1874; Selander 1966; Trivers 1976; Slatkin 1984; Shine 1989). In insects, females with large body size often produce more eggs than smaller females, and femalebiased sexual size dimorphism is commonly attributed to such fecundity selection (e.g., Preziosi and Fairbairn 1997; but see Leather 1988). Water boatmen are detrivorous or zoophagous aquatic insects often inhabiting small ponds of the Northern Hemisphere (Hungerford 1948; Nosil and Reimchen 2001). Female water boatmen are generally larger than males. In this note, I quantify the nature and magnitude of a previously undescribed sexual size dimorphism in a natural population of the water boatman Callicorixa vulnerata Uhler (Hemiptera: Corixidae). I tested for differences between males and females in mean trait size (body length, body weight, mid-leg tarsal length, mid-leg tarsal spine number), and also tested for sexual dimorphism in allometric relationships between tarsal traits and body length.

Sexual cannibalism in fishing spiders (Dolomedes triton): an evaluation of two explanations for female aggression towards potential mates

I manipulated food availability in juvenile and adult fishing spiders (Dolomedes triton) to test two hypotheses for sexual cannibalism. The adaptive foraging hypothesis posits that sexual cannibalism is an economic, adaptive foraging strategy on the part of the adult female. In contrast, the aggressive-spillover hypothesis suggests that precopulatory sexual cannibalism is misplaced aggression favoured in previous life-history phases. Several results indicated support for the adaptive foraging hypothesis. First, increased adult food availability produced marginally nonsignificant fecundity benefits in female's first egg sacs and highly significant fecundity benefits in female's second egg sacs. Second, while consumption of a male did not result in more offspring in either egg sac, it did significantly increase the probability a female would successfully hatch an egg sac. Finally, mating trials revealed mixed support for the adaptive foraging hypothesis as, for the most part, female mating behaviour (attack or mate) was not determined by the adaptive value a male represented (food item or sperm donor). Specifically, the likelihood of a precopulatory attack was not determined by male size, date (an indirect estimate of male availability) or female nutrient load. However, mated females did tend to attack courting males more often than virgin females. The aggressive-spillover hypothesis was supported by several findings. For juveniles, food availability had a significant positive effect on fixed female size (cephalothorax area at final moult) which, in turn, had a significant positive effect on fecundity. Thus, the spillover hypothesis' assertion that strong fecundity selection acts on juvenile feeding and fixed adult size was supported. The possibility that the spillover and adaptive foraging hypotheses are not mutually exclusive is discussed, especially in light of the presence of high levels of sexual cannibalism both before and after mating.

Reproductive Patterns of Two Sympatric Rhacophorid Frogs, Buergeria japonica and B. robusta, with Comments on Anuran Breeding Seasons in Taiwan

Present study tested a hypothesis that the seasonal reproductive pattern would be similar between two sympatric congeneric species of frogs, Buergeria japonica and B. robusta, by examining, fat body, liver and gonadal cycle. In total, 132 and 148 adult frogs, respectively, were collected from Taichung, Central Taiwan during July 1996 and July 1997. Both species were classified as prolonged breeders, breeding from March to August. This breeding season differs from those in conspecific populations from other localities. In both species, adult females attained larger snout-vent length (SVL), body mass (BM), head length (HL), and head width (HW) than males. The smaller size of male in these species is inconsistent with a previous energetic constraint hypothesis that smaller males are caused by the costs of advertising, maintaining territories, and lower food intake in prolonged breeding species. Neither B. japonica nor B. robusta showed a significant positive correlation between female ovary mass and female SVL, indicating that fecundity selection does not directly affect female body size. Also, timing of reproduction in three other anurans from the present study site is also discussed.

THE EFFECTS OF SELECTION FOR LARVAL BEHAVIOR ON ADULT LIFE-HISTORY FEATURES IN DROSOPHILA MELANOGASTER

Selection for late-life fecundity and longevity in adult Drosophila melanogaster is well known to modify numerous characteristics of life history and physiology. We report experiments here in which selection applied to behavior affects features in an identical fashion. Selection for feeding rate of larval D. melanogaster modifies caloric intake, as measured by the uptake and incorporation of labeled glucose. Selection for slow larval feeding produced lines of D. melanogaster in which larvae synthesized significantly less lipid prior to pupation and eclosed to have low early-life fecundity and a long life as adults. They also had greater lifetime fecundity, but lower viability of egg to hatched adult. Alternatively, fast-feeding larvae incorporated more lipid before pupation and eclosed with high early-fecundity that declined rapidly throughout their short adult life. Slow-feeding populations also had a significantly enhanced expression of the stress-resistance genes CuZn-SOD, CATALASE, and HSP70. Selection on larval feeding behavior reproduced the antagonistic evolutionary trade-off found under selection for adult life span and mimicked the physiological response in life span as seen in many species when dietary restriction is imposed on adults. Thus, nutrient acquisition during development appears to share a common evolutionary and genetic basis with the allocation processes that determine adult life-history traits and the related phenotypic dietary restriction phenomena.Corresponding Editor: S. Pitnick

The Evolution of Body Size: What Keeps Organisms Small?

It is widely agreed that fecundity selection and sexual selection are the major evolutionary forces that select for larger body size in most organisms. The general, equilibrium view is that selection for large body size is eventually counterbalanced by opposing selective forces. While the evidence for selection favoring larger body size is overwhelming, counterbalancing selection favoring small body size is often masked by the good condition of the larger organism and is therefore less obvious. The suggested costs of large size are: (1) viability costs in juveniles due to long development and/or fast growth; (2) viability costs in adults and juveniles due to predation, parasitism, or starvation because of reduced agility, increased detectability, higher energy requirements, heat stress, and/or intrinsic costs of reproduction; (3) decreased mating success of large males due to reduced agility and/or high energy requirements; and (4) decreased reproductive success of large females and males due to late reproduction. A review of the literature indicates a substantial lack of empirical evidence for these various mechanisms and highlights the need for experimental studies that specifically address the fitness costs of being large at the ecological, physiological, and genetic levels. Specifically, theoretical investigations and comprehensive case studies of particular model species are needed to elucidate whether sporadic selection in time and space is sufficient to counterbalance perpetual and strong selection for large body size.

The evolution of sexual size dimorphism in the house finch. II. Population divergence in relation to local selection.

Recent colonization of ecologically distinct areas in North America by the house finch (Carpodacus mexicanus) was accompanied by strong population divergence in sexual size dimorphism. Here we examined whether this divergence was produced by population differences in local selection pressures acting on each sex. In a long-term study of recently established populations in Alabama, Michigan, and Montana, we examined three selection episodes for each sex: selection for pairing success, overwinter survival, and within-season fecundity. Populations varied in intensity of these selection episodes, the contribution of each episode to the net selection, and in the targets of selection. Direction and intensity of selection strongly differed between sexes, and different selection episodes often favored opposite changes in morphological traits. In each population, current net selection for sexual dimorphism was highly concordant with observed sexual dimorphism--in each population, selection for dimorphism was the strongest on the most dimorphic traits. Strong directional selection on sexually dimorphic traits, and similar intensities of selection in both sexes, suggest that in each of the recently established populations, both males and females are far from their local fitness optimum, and that sexual dimorphism has arisen from adaptive responses in both sexes. Population differences in patterns of selection on dimorphism, combined with both low levels of ontogenetic integration in heritable sexually dimorphic traits and sexual dimorphism in growth patterns, may account for the close correspondence between dimorphism in selection and observed dimorphism in morphology across house finch populations.

Temporal and microspatial variation in the intensities of natural and sexual selection in the yellow dung fly Scathophaga stercoraria

Abstract Studies of phenotypic selection in natural populations often concentrate only on short time periods and do not quantify selection intensities. We quantified temporal and microspatial variation in the intensities of natural and sexual selection for body size in the yellow dung fly over 2 years. Female fecundity selection intensity remained approximately constant over the season with an overall mean ± SE of 0.187 ± 0.014. Selection intensity for male reproductive success, defined as eggs obtained by mating males, did not differ from zero, indicating there was no assortative mating by size. Sexual selection intensity for male mating success favouring large males was variable but overall strong in the two years (0.499 ± 0.053 and 0.510 ± 0.051). As theoretically expected for male–male competition, sexual selection intensity increased with competitor density and reached an asymptote at about 250 males per pat; it also decreased with time in spring and increased again in autumn as a function of density. Small males had the best chance of obtaining a female at very low male densities. Greater selection intensity for large size in males than females is consistent with, and might be responsible for, the observed sexual size dimorphism in this species, as males are larger. The seasonal pattern of mean male body size (smallest at the beginning and end of the season) most likely reflects mere environmental (primarily temperature) influences on phenotypic size.

Paternal Investment in the Seed Beetle <I>Callosobruchus maculatus</I> (Coleoptera: Bruchidae): Variation Among Populations

Ejaculate size in seed beetles (Coleoptera: Bruchidae) is subject to both sexual and fecundity selection. We examined interpopulation variation and inheritance of ejaculate size in the seed beetle Callosobruchus maculatus (F.). There was significant variation among three populations in both body mass and the proportion of a male's body mass that was transferred to females during mating. The seed upon which beetles were raised had a small effect on male body size but not the size of their ejaculates. To investigate the inheritance of ejaculate size, we performed inter- and intrapopulation crosses with two of these populations. The progeny of interpopulation crosses were intermediate between the intrapopulation (parental) crosses, suggesting additive genetic autosomal inheritance. This result differs from an earlier study in which ejaculate size of a different population was maternally inherited. This study demonstrates that there is indeed genetic variation in ejaculate size, but that the loci exhibiting within-population variation may be different than the loci producing among-population variation.

Reproductive versus ecological advantages to larger body size in female snakes, Vipera aspis

Body size can influence an organism's microevolutionary fitness either via ecological factors (ecological selection) or changes in reproductive output (sexual or fecundity selection). Published studies on sexual dimorphism in reptiles have generally focussed on sexual‐selective forces on males, under the implicit assumption that the intensity of fecundity selection in females (and hence, overall selection on female body size) is likely to be relatively consistent among lineages. In this paper, we explore the degree to which larger body size enhances ecological attributes (e.g., food intake, growth, survival) and reproductive output (reproductive frequency, litter size, offspring size, offspring viability) in free‐ranging female aspic vipers, Vipera aspis. The less‐than‐annual reproductive frequency of these animals allows us to make a direct comparison between females in years during which they concentrate on “ecological” challenges (especially building energy reserves) versus reproductive challenges (producing a litter). Because female snakes have limited abdominal space to hold the clutch (litter), we expect that fecundity should depend on body size. However, our data show that larger body size had a more consistent effect on ecological attributes (such as feeding rates and “costs of reproduction”) than on reproductive output per se. Indeed, total reproductive output was maximised at intermediate body sizes. These results suggest that variation in female body size among and within species (and hence, in the degree of sexual dimorphism) may be driven by the ecological as well as reproductive consequences of body size variation in both sexes.

Effect of One- and Two-Eared Selection on Stalk Strength and Other Characters in Maize

Prolificacy associated with higher grain yield in maize (Zea mays L.) has been widely documented. However, one serious concern is that prolificacy appears associated with poor stalk strength and plant standability. The objective of this research was to compare stalk strength and other agronomic characters of one- and two-eared subpopulations derived from three maize populations (MoSQA(S7-H)C8 × Georgia Cow Corn [ACC]; MoSQB(S8-H)C8 × Georgia Cow Corn [BCC]:SI171). Entries were evaluated by using nine combinations of three levels of nitrogen (N) application (90, 180, and 270 kg ha 1 N) and three levels of plant density (35 800, 47 800, and 59 800 plants ha -1 ). Stalk crushing strength showed significant differences between one- and two-eared subpopulations for ACC and BCC, but it was not significant for SI171. One-eared subpopulations had higher rind penetrometer resistance than two-eared subpopulations for all populations. Two-eared selections generally resulted in poorer root and stalk strength. However, total grain yield of the two-eared subpopulations was significantly higher than that of the one-eared subpopulations for BCC and SI171, but not for ACC. Prolificacy has significant potential as a novel character for grain yield improvement in the future. Selection for prolificacy alone, representing indirect selection for grain yield, would produce undesirable effects on other agronomic characters, especially root and stalk strength. Concurrent improvement for total grain yield, prolificacy, and root and stalk strength by using a standardized, weighted selection index should be used to extract the real benefit of the prolific character.

Vertebral numbers in male and female snakes: the roles of natural, sexual and fecundity selection

Abstract The relative numbers of trunk (body) and caudal (tail) vertebrae in snakes might be influenced by at least four processes: (1) natural selection for crawling speed, (2) fecundity selection for larger trunk size in females, (3) sexual selection for longer bodies or tails in males and/or (4) developmental constraints (if an increase in the number of body vertebrae requires a decrease in the number of tail vertebrae, or vice versa). These four hypotheses generate different predictions about the relationship between sex differences in the numbers of body vertebrae vs. tail vertebrae. I collated published data to test these predictions, both with raw data and using phylogenetically independent contrasts. Some snake lineages show a negative correlation between the magnitude of sex disparities in trunk vs. caudal vertebrae whereas other lineages show the reverse pattern, or no correlation. Thus, different selective pressures seem to have been important in different lineages. Vertebral numbers in snakes may offer a useful model system in which to explore the conflicts between natural, fecundity and sexual selection.

LIFETIME SELECTION ON ADULT BODY SIZE AND COMPONENTS OF BODY SIZE IN A WATERSTRIDER: OPPOSING SELECTION AND MAINTENANCE OF SEXUAL SIZE DIMORPHISM

Abstract.— Sexual size dimorphism (SSD), the difference in body size between males and females, is common in almost all taxa of animals and is generally assumed to be adaptive. Although sexual selection and fecundity selection alone have often been invoked to explain the evolution of SSD, more recent views indicate that the sexes must experience different lifetime selection pressures for SSD to evolve and be maintained. We estimated selection acting on male and female adult body size (total length) and components of body size in the waterstrider Aquarius remigis during three phases of life history. Opposing selection pressures for overall body size occurred in separate episodes of fitness for females in both years and for males in one year. Specific components of body size were often the targets of the selection on overall body size. When net adult fitness was estimated by combining each individual's fitnesses from all episodes, we found stabilizing selection in both sexes. In addition, the net optimum overall body size of males was smaller than that of females. However, even when components of body size had experienced opposing selection pressures in individual episodes, no components appeared to be under lifetime stabilizing selection. This is the first evidence that contemporary selection in a natural population acts to maintain female size larger than male size, the most common pattern of SSD in nature.