Assessing the roles of population density and predation risk in the evolution of offspring size in populations of a placental fish

Population density is an ecological variable that is hypothesized to be a major agent of selection on offspring size. In high-density populations, high levels of intraspecific competition are expected to favor the production of larger offspring. In contrast, lower levels of intraspecific competition and selection for large offspring should be weaker and more easily overridden by direct selection for increased fecundity in low-density populations. Some studies have found associations between population density and offspring size consistent with this hypothesis. However, their interpretations are often clouded by a number of issues. Here, we use data from a 10-year study of nine populations of the least killifish, Heterandria formosa, to describe the associations of offspring size with habitat type, population density, and predation risk. We found that females from spring populations generally produced larger offspring than females from ponds; however, the magnitude of this difference varied among years. Across all populations, larger offspring were associated with higher densities and lower risks of predation. Interestingly, the associations between the two ecological variables (density and predation risk) and offspring size were largely independent of one another. Our results suggest that previously described genetic differences in offspring size are due to density-dependent natural selection.     

Lack of reproductive isolation between populations of the grasshopper Myrmeleotettix maculatus across a steep cline in B-chromosome frequency

The rapid cline in B-chromosome frequency in populations of the grasshopper Myrmeleotettix maculatus in East Anglia, reported by Hewitt & Brown (1970), has not been satisfactorily explained. Test were made of the hypothesis that the cline represented the contact of two groups of populations with differently co-adapted genomes, between which there was reproductive isolation. There was no evidence that females in a cage discriminated between the courtship of males from the same or the opposite side of the cline. Progeny from crosses between populations on opposite sides of the cline showed no reduced survival to hatching, survived to adulthood and had normal meiosis. Thus the evidence available does not support the hypothesis that the cline is the interface of two incompatible genomes. The problems of other models accounting for the cline are discussed.     

The influence of a positive correlation between clutch size and offspring fitness on the optimal offspring size

Summary The effect is modeled of a positive relationship between clutch size and offspring fitness on the optimal investment in offspring. In species which meet the assumptions of the model, the model predicts a positive correlation between maternal resource level and offspring size. If larger mothers are able to allocate more resources to offspring, then the model would also predict a positive correlation between maternal size and offspring size when the assumptions of the model are met. Thus, this model may help explain both among and within individual variation in offspring size. When offspring are produced in groups and the number of offspring killed per clutch is limited by predator satiation, offspring in larger clutches may experience a higher probability of survival. Such a life style may be found in animals such as sea turtles. Offspring size is positively correlated with maternal size in some members of this group.     

Cannibalism as a selective force on offspring size in fish

Cannibalism may cause considerable mortality on juvenile fish and it has been hypothesised that it may exercise selection on offspring size in that larger offspring may enjoy a size refuge. For this to be evolutionarily advantageous the survival of individual offspring must compensate for the reduced fecundity implied by larger offspring size. We develop a model which combines standard assumptions of size‐dependent mortality with adult cannibalism to investigate the potential for cannibalism to act as selective force on offspring size. We find that for this potential to be realised, the mortality due to cannibalism must exceed a threshold value that is a decreasing function of non‐cannibalistic predation intensity, cannibalized size range width and the average cannibalized size. If cannibalism exceeds this threshold, the model predicts evolution of offspring size towards refuges above or below cannibalized size range depending on initial offspring size. Cannibalistic mortality cannot be so great that the population is non‐viable, however, the range of parameter values describing cannibalistic intensity allowed within these boundaries is wide. On this basis, we suggest that cannibalism is a potential mechanism for offspring size selection.     

Macrogeographic clines in fecundity, reproductive allocation, and offspring size of the forest tent caterpillar Malacosoma disstria

1. The fecundity of the forest tent caterpillar varies considerably across its geographic range. Field data indicate that populations in the southern United States (Gulf States) produce nearly twice as many eggs as females from Canada or the Lake States, with little or no difference in the size of adult females. 2. In controlled rearing experiments, female forest tent caterpillar from the southern United States (Louisiana) had much larger clutch sizes than same sized females from northern populations in Michigan or Manitoba, Canada. Increased fecundity in Louisiana females was achieved through a significant reduction in egg size and a concomitant increase in the allocation of resources to egg production. 3. Comparison of 10 forest tent caterpillar populations spanning a 27° latitudinal gradient, validated the results of detailed comparisons among the three populations above by confirming the strong negative correlation between latitude and clutch size. 4. Neonate forest tent caterpillars from Manitoba were significantly larger than larvae from either Michigan or Louisiana. Michigan larvae were intermediate in size. It is postulated that large neonates are advantageous in thermally limiting environments. More than three times as many degree‐days are available to Louisiana neonates during the first 2 weeks after hatching. A consistently favourable climate during the vulnerable post‐hatching period may have allowed the evolution of larger clutches at the expense of neonate size in southern populations.     

Clutch size, offspring performance, and intergenerational fitness

It is now generally recognized that clutch size affects morethan offspring number. In particular, clutch size affects asuite of traits associated with offspring reproductive performance.Optimal clutch size is therefore determined not by the numericallymost productive clutch but by the clutch that maximizes collectiveoffspring reproductive success. Calculation of optimal clutchsize thus requires a consideration of ecological factors operatingduring an intergenerational time frame, spanning the lifetimeof the egglaying adult and the lifetimes of her offspring. Theoptimal clutch cannot define reproductive values in advance,but instead requires that the strategy chosen is the best responseto the set of reproductive values that it itself generates.In this article, we introduce methods for solving this problem,based on an iterative solution of the equation characterizingexpected lifetime reproductive success. We begin by consideringa semelparous organism, in which case lifetime reproductivesuccess is a function only of the state of the organism. Foran iteroparous organism, lifetime reproductive success dependsupon both state and time, so that our methods extend the usualstochastic dynamic programming approach to the evaluation oflifetime reproductive success. The methods are intuitive andeasily used. We consider both semelparous and iteroparous organisms,stable and varying environments, and describe how our methodscan be employed empirically.     

Variation in offspring size within a population of the web-building spiderAgelena limbata

Within a population of the web-building spiderAgelena limbata, the weight of the first instar nymphs ranged from 1.187 to 6.559 mg. Both intraclutch and interclutch variation were recorded. The mean weights were different among clutches and the coefficients of variation within a clutch ranged from 3.3 to 29.2%. Variation in the nymphal weight was certainly derived from variation in the egg weight because there was a high correlation between the two weights. Factors affecting interclutch variation in nymphal weight were examined by multiple regression analysis. Nymphal weight was positively correlated with the body size and food conditions of female parents, and negatively correlated with the clutch size. Among these three factors, the food conditions of female parents had the largest apparent effect on the interclutch variation. The results suggest that females with larger body size and more food produce larger offspring, and that there is a trade-off between offspring size and clutch size. Heavier nymphs had larger body size (carapace width) and may have larger energy reserves. Heavier nymphs survived experimental starvation for a significantly longer period.     

The role of sexual isolation during rapid ecological divergence: Evidence for a new dimension of isolation in Rhagoletis pomonella

The pace of divergence and likelihood of speciation often depends on how and when different types of reproductive barriers evolve. Questions remain about how reproductive isolation evolves after initial divergence. We tested for the presence of sexual isolation (reduced mating between populations due to divergent mating preferences and traits) in Rhagoletis pomonella flies, a model system for incipient ecological speciation. We measured the strength of sexual isolation between two very recently diverged (~170 generations) sympatric populations, adapted to different host fruits (hawthorn and apple). We found that flies from both populations were more likely to mate within than between populations. Thus, sexual isolation may play an important role in reducing gene flow allowed by early-acting ecological barriers. We also tested how warmer temperatures predicted under climate change could alter sexual isolation and found that sexual isolation was markedly asymmetric under warmer temperatures – apple males and hawthorn females mated randomly while apple females and hawthorn males mated more within populations than between. Our findings provide a window into the early speciation process and the role of sexual isolation after initial ecological divergence, in addition to examining how environmental conditions could shape the likelihood of further divergence.     

The positive correlation between maternal size and offspring size: fitting pieces of a life‐history puzzle

The evolution of investment per offspring (I) is often viewed through the lens of the classic theory, in which variation among individuals in a population is not expected. A substantial departure from this prediction arises in the form of correlations between maternal body size and I, which are observed within populations in virtually all taxonomic groups. Based on the generality of this observation, we suggest it is caused by a common underlying mechanism. We pursue a unifying explanation for this pattern by reviewing all theoretical models that attempt to explain it. We assess the generality of the mechanism upon which each model is based, and the extent to which data support its predictions. Two classes of adaptive models are identified: models that assume that the correlation arises from maternal influences on the relationship between I and offspring fitness [w(I)], and those that assume that maternal size influences the relationship between I and maternal fitness [W(I)]. The weight of evidence suggests that maternal influences on w(I) are probably not very general, and even for taxa where maternal influences on w(I) are likely, experiments fail to support model predictions. Models that assume that W(I) varies with maternal size appear to offer more generality, but the current challenge is to identify a specific and general mechanism upon which W(I) varies predictably with maternal size. Recent theory suggests the exciting possibility that a yet unknown mechanism modifies the offspring size–number trade‐off function in a manner that is predictable with respect to maternal size, such that W(I) varies with size. We identify two promising avenues of inquiry. First, the trade‐off might be modified by energetic costs that are associated with the initiation of reproduction (‘overhead costs’) and that scale with I, and future work could investigate what specific overhead costs are generally associated with reproduction and whether these costs scale with I. Second, the trade‐off might be modified by virtue of condition‐dependent offspring provisioning coupled with metabolic factors, and future work could investigate the proximate cause of, and generality of, condition‐dependent offspring provisioning. Finally, drawing on the existing literature, we suggest that maternal size per se is not causatively related to variation in I, and the mechanism involved in the correlation is instead linked to maternal nutritional status or maternal condition, which is usually correlated with maternal size. Using manipulative experiments to elucidate why females with high nutritional status typically produce large offspring might help explain what specific mechanism underlies the maternal‐size correlation.     

Relationship between fluctuating asymmetry and fitness within and between stressed and unstressed populations of the wolf spider Pirata piraticus

Although developmental instability, measured as fluctuating asymmetry (FA), is expected to be positively related to stress and negatively to fitness, empirical evidence is often lacking or contradictory when patterns are compared at the population level. We demonstrate that two important properties of stressed populations may mask such relationships: (i) a stronger relationship between FA and fitness, resulting in stronger selection against low quality (i.e. developmental unstable) individuals and (ii) the evolution of adaptive responses to environmental stress. In an earlier study, we found female wolf spiders Pirata piraticus from metal exposed populations to be characterized by both reduced clutch masses and increased egg sizes, the latter indicating an adaptive response to stress. By studying the relationship between these two fitness related traits and levels of FA at individual level, we here show a significant negative correlation between FA and clutch mass in metal stressed populations but not in unstressed reference populations. As a result, levels of population FA may be biased downward under stressful conditions because of the selective removal of developmentally unstable (low quality) individuals. We further show that females that produced larger eggs in stressed populations exhibited lower individual FA levels. Such interaction between individual FA and fitness with stress may confound the effect of metal stress on FA, resulting in an absence of relationships between FA, fitness and stress at the population level.     

Evolution of mating isolation between populations of Drosophila ananassae

Prezygotic mating isolation has been a major interest of evolutionary biologists during the past several decades because it is likely to represent one of the first stages in the transition from populations to species. Mate discrimination is one of the most commonly measured forms of prezygotic isolation and appears to be relatively common among closely related species. In some cases, it has been used as a measure to distinguish populations from subspecies, races, and sister species, yet the influences of various evolutionary mechanisms that may generate mate discrimination are largely unknown. In this study, we measured the level and pattern of mate discrimination among 18 populations of a cosmopolitan drosophilid species, Drosophila ananassae , from throughout its geographical range and its sister species, Drosophila pallidosa, which has a restricted geographical distribution in the South Pacific Islands. In addition, we measured genetic differentiation between all 18 populations using mitochondrial DNA polymorphism data. Mate discrimination varies considerably throughout the species range, being higher among populations outside the ancestral Indonesian range, and highest in the South Pacific. Our results suggest that colonization and genetic differentiation may have an influence on the evolutionary origin of mate discrimination. Our phylogeographical approach clarifies the ancestral relationships of several populations from the South Pacific that show particularly strong mate discrimination and suggests that they may be in the early stages of speciation. Furthermore, both the genetic and behavioral results cast doubt on the status of D. pallidosa as a good species.     

Relationships between oviposition date, hatch date, and offspring size in the grasshopper Chorthippus brunneus

Abstract 1. The grasshopper Chorthippus brunneus has been shown to increase egg size with maternal age under constant laboratory conditions, such that late-laid eggs are larger than early laid eggs. In this study, an increase in the size of eggs (and hatchlings from those eggs) was recorded with date of oviposition in a field cage population. This suggests that the relationship between maternal age and egg size, observed previously in the laboratory, also occurs in the field. There was, however, some evidence that the behaviour of the maternal females in field cages was modified by the onset of autumnal weather at the end of the breeding season. Only a small number of eggs was laid in the last 3 weeks of the summer but these yielded relatively small hatchlings.
2. The date on which eggs were laid was correlated positively with their date of hatch in the following year. A consequence of this relationship, and that between oviposition date and egg size, was that size at hatch increased with date of hatch through most of the spring but then declined as the last few eggs hatched. Temporal variation in size at hatch parallelled temporal variation in the maternal females' investment in egg size in the previous year.
3. An undisturbed field population was monitored to assess whether the temporal variation in size at hatch resulted in size variation at eclosion in subsequent developmental stages. There were negative correlations between size at eclosion and date of eclosion at the third and fourth instars, suggesting that despite their smaller size at hatch, early hatchers experienced more favourable conditions for juvenile growth than late hatchers. The larger size at hatch of late hatchers may enhance their survival and compensate (albeit incompletely) for their reduced opportunity for growth. Female reproductive behaviour may represent an adaptive response to a predictable seasonal decline in offspring fitness at hatch.     

Differences in style length confer prezygotic isolation between two dioecious species of Silene in sympatry

One fundamental signature of reinforcement is elevated prezygotic reproductive isolation between related species in sympatry relative to allopatry. However, this alone is inadequate evidence for reinforcement, as traits conferring reproductive isolation can occur as a by‐product of other forces. We conducted crosses between Silene latifolia and S. diclinis, two closely related dioecious flowering plant species. Crosses with S. latifolia mothers from sympatry exhibited lower seed set than mothers from five allopatric populations when S. diclinis was the father. However, two other allopatric populations also exhibited low seed set. A significant interaction between style length and sire species revealed that seed set declined as style length increased when interspecific, but not intraspecific, fathers where used. Moreover, by varying the distance pollen tubes had to traverse, we found interspecific pollen placement close to the ovary resulted in seed set in both long‐ and short‐styled S. latifolia mothers. Our results reveal that the long styles of S. latifolia in sympatry with S. diclinis contribute to the prevention of hybrid formation. We argue that forces other than reinforcing selection are likely to be responsible for the differences in style length seen in sympatry.     

Conflict between optimal clutch size for mothers and offspring in the leaf miner, Leucoptera sinuella

Abstract. 1. Clutch size in a leaf‐mining moth, Leucoptera sinuella (Reutti), was examined to determine whether the clutch size in natural populations meets the prediction of an optimal strategy, through comparisons between the optimal clutch sizes for offspring and for a mother. 2. A field experiment revealed that premature leaf abscission, egg dropping, and larval competition were important selective forces in determining the clutch size of this leaf miner on its host plant, Salix miyabeana. Then, optimal clutch size was predicted using the theoretical model of Weis et al. (1983 ), from the data obtained in the field experiment. 3. The model predicts that the clutch size that maximises offspring fitness is two, and that the clutch size that maximises reproductive success of the female varies from two to four, depending on the female's survival rate between oviposition events. The predicted clutch size (two) was identical to the clutch size observed most frequently in the field, assuming > 95% survival rate of females. Suitability of the model of Weis et al. (1983 ) was discussed based on these results.     

Pre-reproductive isolation as a consequence of allopatric differentiation between populations of Drosophila melanogaster

While pre-reproductive isolations are more and more frequently described between closely related species or within species, very little is known about their conditions of emergence. In Brazzaville, two populations (Kronenbourg and Loua) of Drosophila melanogaster show a premating isolation. Two hypotheses were proposed to explain such a situation: a local sympatric differentiation or an allopatric divergence followed by a secondary contact. A microsatellite analysis, using markers on all chromosomes, strongly suggests that the Kronenbourg population has a European origin. Therefore, the allopatric divergence between Kronenbourg and Loua populations is probably responsible for the sexual isolation observed today in sympatry, after a recent introduction of a European propagule in Brazzaville.     

Intrinsic reproductive isolation between Trinidadian populations of the guppy, Poecilia reticulata

Although Trinidadian populations of the guppy, Poecilia reticulata, show considerable adaptive genetic differentiation, they have been assumed to show little or no reproductive isolation. We tested this assumption by crossing Caroni (Tacarigua River) and Oropuche (Oropuche R.) drainage populations from Trinidad's Northern Range, and by examining multiple aspects of reproductive compatibility in the F1, F2 and BC1 generations. In open-aquarium experiments, F1 males performed fewer numbers of mating behaviours relative to parental population controls. This is the first documentation of hybrid behavioural sterility within a species, and it suggests that such sterility may feasibly be involved in causing speciation. The crosses also uncovered hybrid breakdown for embryo viability, brood size and sperm counts. In contrast, no reductions in female fertility were detected, indicating that guppies obey Haldane's rule for sterility. Intrinsic isolation currently presents a much stronger obstacle to gene flow than behavioural isolation, and our results indicate that Trinidadian populations constitute a useful model for investigating incipient speciation.     

Density‐dependent offspring interactions do not explain macroevolutionary scaling of adult size and offspring size

Most life forms exhibit a correlated evolution of adult size (AS) and size at independence (SI), giving rise to AS–SI scaling relationships. Theory suggests that scaling arises because relatively large adults have relatively high reproductive output, resulting in strong density‐dependent competition in early life, where large size at independence provides a competitive advantage to juveniles. The primary goal of our study is to test this density hypothesis, using large datasets that span the vertebrate tree of life (fishes, amphibians, reptiles, birds, and mammals). Our secondary goal is to motivate new hypotheses for AS–SI scaling by exploring how subtle variation in life‐histories among closely related species is associated with variation in scaling. Our phylogenetically informed comparisons do not support the density hypothesis. Instead, exploration of AS–SI scaling among life‐history variants suggests that steeper AS–SI scaling slopes are associated with evolutionary increases in size at independence. We suggest that a positive association between size at independence and juvenile growth rate may represent an important mechanism underlying AS–SI scaling, a mechanism that has been underappreciated by theorists. If faster juvenile growth is a consequence of evolutionary increases in size at independence, this may help offset the cost of delayed maturation, leading to steeper AS–SI scaling slopes.     

Genetics of wing size asymmetry in Drosophila buzzatii

Contemporary approaches that use fluctuating asymmetry (FA) as a possible target for natural and sexual selection are based on the premise that FA is a quantifiable expression of developmental instability (DI) that is inherited. Previous work with Drosophila buzzatii found that male mating success was correlated positively to body size (wing length) and negatively to FA, but these relationships seem to be environmentally induced. Heritability of FA was low and not significantly different from zero, but statistical power was also estimated to be very low and, hence, no conclusive evidence could be obtained. A large half‐sib mating design is used here to examine the relationships of different aspects of development for wing size. Consistently with previous findings, I found high heritabilities for wing length (WL) and wing width (WW), and positive correlations between both traits. Heritabilities of FA (FA_WL, FA_WW) were low (0.037) but significantly different from zero, and the genetic correlation between FA_WL and FA_WW was estimated as ?1 because the absolute value for the genetic covariance was similar in magnitude or even larger than the estimated genetic variances of both traits. This suggests that these two traits should be considered to be the same character. The between‐trait phenotypic correlation in FA, which reduces to the repeatability in this situation, was positive and statistically significant thus rendering an estimate of heritability for DI in D. buzzatii of . Nevertheless, the fact that left/right wing sizes were found to be determined by the same set of genes is difficult to reconcile with the presence of special genetic mechanisms that stabilize left/right development in this species. A qualitatively different pattern for asymmetry was observed when the nonlinear composite character wing area (WA ≈ WL × WW) was used, and . Although the results could be made compatible with the existence of a diallelic locus with antagonistic pleiotropic effects on FA_WL and FA_WW that combine multiplicatively to produce overdominance for FA_WA, the available evidence is extremely weak at best. Finally, a test to the null hypothesis of a nongenetic basis of FA, particularly relevant to those situations when directional asymmetry may be heritable, is suggested.     

Smaller amphipod mothers show stronger trade-offs between offspring size and number

Trade-offs between embryo mass and number were studied in 10 populations of the freshwater amphipod Gammarus minus . Trade-offs were stronger in populations with small brooding females than in those with larger brooding females. Relationships between embryo mass and maternal body mass were also stronger in populations dominated by small versus large brooding females. These patterns are likely the result of morphological constraints, at least in part. Embryo size is more affected by brood size and maternal size in small mothers, probably because of offspring-packaging constraints associated with small brood pouches. Energy constraints appear to be less important. These results suggest that body size may not only affect the magnitude of individual life-history traits, as is well known, but also the covariance between traits.