Patterns of natural selection on morphology of male and female collared flycatchers (Ficedula albicollis)

Natural selection may act in different directions among years, life stages, or classes of individuals. Fluctuating selection of this kind is potentially an important mechanism by which additive genetic variation for quantitative traits is maintained, and can prevent populations reaching local adaptive peaks. We analysed natural selection acting on three morphological traits of male and female collared flycatchers via both fecundity and survival, using 15 years' data from a large isolated population on Godand, Sweden. We particularly investigated variation in the direction and magnitude of selection acting: (1) among years over the study period; (2) on different life stages and (3) the consistency of observed patterns of selection with sexual size dimorphism (SSD) in this population. We found little evidence of natural selection on these traits over the study period. Evidence for directional, stabilizing and disruptive selection was found for some year-trait combinations, but these patterns were inconsistent with respect to both the magnitude and form of selection found. Consequently, our results, based on the detailed analysis of natural selection in a large wild population over a period of 15 years, provide evidence for the common assumption that forces of selection acting on quantitative traits are generally weak. They are also consistent with the suggestion that environmental stability is an important determinant of the degree to which organisms fit their environment.     

Spatial ecology, habitat and speciation in the Porto Santan land snail genus Heterostoma

The Madeiran land snail genus Heterostoma expresses two forms of genital anatomy. One form (hemiphallic) lacks the distal male organs while the other (euphallic) has fully developed hermaphroditic genitalia. Recent evidence suggests this genital variation characterizes two sibling or incipient species. The spatial ecology and habitat associations of these taxa are described in the light of recent ideas on the role of ecology in speciation. Hemiphallic snails are found at a lower density and show a tendency to be more separated from each other compared to euphallic snails. Both taxa are more likely to be closer to other members of the same taxon. The genus as a whole is not found in the presence of sand or pale rock. However, when only areas actually containing snails are examined, hemiphallics show a positive association with sand and rock while euphallics continue to show a negative association. The differences in population density and spacing may suggest an adaptive explanation for reduction of male organs in hemiphallics since this taxon would be expected to have significantly fewer mating encounters. The observed differences in habitat association may suggest a mechanism for sympatric or parapatric divergence in keeping with current models of habitat-driven adaptive speciation. Both of these factors (selection on the mating system in a marginal environment and small scale separation based on habitat) may have been significant in the divergence of hemiphallic and euphallic taxa.     

Phenotypic plasticity of body size in Drosophila: effects of a daily periodicity of growth temperature in two sibling species

Abstract. Variation of wing and thorax length under thermoperiodic growth conditions was analysed in four strains of two sibling species, Drosophila melanogaster and D. simulans , from two European localities. Results were compared to those obtained with constant temperatures ranging from 12 to 31 °C.
Under constant temperatures the data basically confirmed previous results: concave reaction norms for wing and thorax length; a monotonically decreasing norm for wing : thorax ratio; and an increasing norm for sex dimorphism (female : male ratio). Phenotypic variability was maximum at extreme temperatures and minimum at middle ones. Slight differences were observed according to the geographical origin: the difference between strains from Bordeaux (France) and Cordoba (Spain) was maximum at low temperatures but disappeared at about 28 °C.
According to the temperatures chosen, alternating thermal regimens had either no effect or produced a significant size reduction, probably reflecting a periodic stress. The magnitude of this effect was proportional to the amplitude of the thermoperiod but not to the quality (cold or heat) of the stress. In a similar way, the wing : thorax ratio was either not modified or reduced significantly, indicating that wing length was relatively more affected than thorax length by alternating thermal regimens. Sex dimorphism also showed either no change or a significant increase, indicating that males were relatively more reactive than females to alternating conditions. Finally, regimens of broad amplitudes increased the phenotypic variability, again an indication of stressful effects. All these observations should be taken into account when analysing phenotypic variability in nature and trying to understand natural selection in wild-living populations.     

Biostratigraphic and palaeoecological implications of new fossil felid material from the Plio-Pleistocene site of Tegelen, the Netherlands

The Plio-Pleistocene site of Tegelen in the Netherlands contains some of the oldest evidence for the presence of the medium-sized pantherine felid of the European Pleistocene, Panthera gombaszoegensis , based on published fragmentary dental specimens. Measurements of these specimens, together with those from other localities, suggest that earlier representatives of the species are small, but one of a small number of previously unstudied specimens from the site now indicates the presence of a larger pantherine individual. Such a size difference could point to the presence of a second species, or suggest that the Tegelen deposits encompass a longer time span than has been supposed. Either of these would have serious implications; the first would increase the complexity of the early Pleistocene carnivore guild, and the second would threaten the status of Tegelen as a type site in the European Pleistocene biostratigraphic scheme. However, consideration of the size range in sexed samples of extant pantherines in conjunction with a re-examination of the size distributions in the hypodigm of P. gombaszoegensis suggests that sexual dimorphism offers a more plausible interpretation.     

Functional implications of early sexual growth dimorphism in vundu

Female juvenile vundu Heterobranchus longifilis outweighed males by > 60% ( P < 0·0001) margins when aged 142 or 177 days. Not only did females grow faster ( P =0·0494) than males during the experiment, but they also outweighed them by a 38·5% margin at the start of the experiment (86 days: P =0·0109). Sexual growth dimorphism (SGD) was estimated as having arisen at 15 ± 10 days. Vundu aquaculture could be improved substantially by monosex female rearing. Because the modelled emergence of SGD corresponded precisely with the start of type II cannibalism, it was suggested that cannibalism among juvenile vundu be exerted essentially by females, and might have different impacts on faster-growing females and slower-growing males.     

A new species of Hyphessobrycon (Characiformes,Characidae) from the upper Rio Jequitinhonha basin,Minas Gerais,Brazil, with comments on morphological similarities with Stethaprioninae species from headwater environments

A new species of Hyphessobrycon is described from a marshland area in the headwaters of Rio Jequitinhonha basin, Minas Gerais, Brazil. The new species differs from congeners by presenting a single well-delimited conspicuous humeral blotch, rounded to vertically oval, restricted to the area dorsal to the lateral-line row of scales, without a narrower downward extension, greatest body depth anterior to dorsal-fin origin, bony processes in anal and pectoral-fin rays of males and four teeth in the inner row of the premaxillary bone. The new species presents a set of morphological features shared by some species currently assigned to Hasemania, Myxiops and to the Astyanax scabripinnis complex. Some of these features are discussed.     

Genetic covariances promote climatic adaptation in Australian Drosophila*

Evolutionary potential for adaptation hinges upon the orientation of genetic variation for traits under selection, captured by the additive genetic variance-covariance matrix (G), as well as the evolutionary stability of G. Yet studies that assess both the stability of G and its alignment with selection are extraordinarily rare. We evaluated the stability of G in three Drosophila melanogaster populations that have adapted to local climatic conditions along a latitudinal cline. We estimated population- and sex-specific G matrices for wing size and three climatic stress-resistance traits that diverge adaptively along the cline. To determine how G affects evolutionary potential within these populations, we used simulations to quantify how well G aligns with the direction of trait divergence along the cline (as a proxy for the direction of local selection) and how genetic covariances between traits and sexes influence this alignment. We found that G was stable across the cline, showing no significant divergence overall, or in sex-specific subcomponents, among populations. G also aligned well with the direction of clinal divergence, with genetic covariances strongly elevating evolutionary potential for adaptation to climatic extremes. These results suggest that genetic covariances between both traits and sexes should significantly boost evolutionary responses to environmental change.     

Subtle,pervasive genetic correlation between the sexes in the evolution of dimorphic hummingbird tail ornaments*

Male hummingbirds have repeatedly evolved sexually dimorphic tails that they use as ornaments during courtship. We examine how male ornament evolution is reflected in female morphology. Lande's two-step model of the evolution of dimorphism predicts that γ (the genetic correlation between the sexes) causes trait elaboration to first evolve quickly in both sexes, then dimorphism evolves more slowly. On the hummingbird phylogeny, tail length does not fit this two-step model; although hummingbirds repeatedly evolved ornamental, elongated tails, dimorphism evolves on the same phylogenetic branch as elongation, implying that γ quickly evolves to be low over phylogenetic timescales. Male “bee” hummingbirds have evolved diverse rectrix shapes that they use to produce sound. Female morphologies exhibit subtle, pervasive correlations with male morphology. No female-adaptive hypotheses explain these correlations, since females do not also make sounds with their tail. Subtle shape similarity has arisen through the genetic correlation with males, and is subject to intralocus sexual conflict. Intralocus sexual conflict may produce increased phenotypic variation of female ornaments. Other evolutionary constraints on tail morphology include a developmental correlation between neighboring tail-feathers, biasing tail elaboration to occur most often at the ends of the feather tract (rectrix 5 or 1) and not the middle.     

Sex‐specific differences in gonopore and gonadal growth trajectories in the brooding sea urchin,Abatus cavernosus (Spatangoida)

The heart urchin Abatus cavernosus shows sexual dimorphism characterized by the development of external brood pouches and the enlargement of gonopores in brooding females. Relationships between body size, gonopore size, and gonadal maturation in each sex were examined for inflection points using piecewise regression models (PRM). Opening of the gonopore occurred at 15.5 mm test length. Inflection points in the gonadal growth and gonopore diameter trajectories were clustered at smaller sizes in males (23 and 24.2 mm, respectively) than in females (25.1 and 25.9 mm), indicating sex‐specific differences in sexual maturation. Gonopore growth showed positive allometry at pre‐adult stages of development in both sexes, but isometry and negative allometry in adult females and males, respectively. Gonadal growth was initiated at smaller sizes and proceeded at a higher rate with increasing body size in males than in females. Identification of inflection points in gonopore and gonadal growth trajectories, using objective PRM, allows the determination of life stages and sexual maturation for individuals, thus providing a complementary tool for population studies.