Sexual selection in males and females

Research on sexual selection shows that the evolution of secondary sexual characters in males and the distribution of sex differences are more complex than was initially suggested but does not undermine our understanding of the evolutionary mechanisms involved. However, the operation of sexual selection in females has still received relatively little attention. Recent studies show that both intrasexual competition between females and male choice of mating partners are common, leading to strong sexual selection in females and, in extreme cases, to reversals in the usual pattern of sex differences in behavior and morphology.     

Sexual selection,temperature, and the lion's mane

The mane of the African lion (Panthera leo) is a highly variable trait that reflects male condition and ambient temperature. We examined the consequences of this variation in a long-term study of lions in the Serengeti National Park, Tanzania. Mane darkness indicates nutrition and testosterone and influences both female choice and male-male competition. Mane length signals fighting success and only appears to influence male-male assessment. Dark-maned males enjoy longer reproductive life-spans and higher offspring survival, but they suffer higher surface temperatures, abnormal sperm, and lower food intake during hot months of the year. Maned males are hotter than females, and males have lighter and/or shorter manes in hotter seasons, years, and habitats. This phenotypic plasticity suggests that the mane will respond to forecasted increases in ambient temperature.     

Sexual recombination in a homothallic,antibiotic producing fungus

The presence of a perfect or sexual stage in the fungus Emericellopsis salmosynnemata has made possible an investigation of the effect of meiotic recombination on yields of antibiotic. While most of the fruiting bodies produced by this organism are the result of selffertilization, conclusive evidence of cross-fertilization and recombination between two mutants was obtained. Cross-fertilization occurred rarely.     

Positive natural selection in the human lineage

Positive natural selection is the force that drives the increase in prevalence of advantageous traits, and it has played a central role in our development as a species. Until recently, the study of natural selection in humans has largely been restricted to comparing individual candidate genes to theoretical expectations. The advent of genome-wide sequence and polymorphism data brings fundamental new tools to the study of natural selection. It is now possible to identify new candidates for selection and to reevaluate previous claims by comparison with empirical distributions of DNA sequence variation across the human genome and among populations. The flood of data and analytical methods, however, raises many new challenges. Here, we review approaches to detect positive natural selection, describe results from recent analyses of genome-wide data, and discuss the prospects and challenges ahead as we expand our understanding of the role of natural selection in shaping the human genome.     

Sexual selection can resolve sex-linked sexual antagonism

Sexual selection is a potent evolutionary force. However, very few models have considered the evolution of female preferences for traits expressed in both sexes. Here we explore how female preferences coevolve with sexually antagonistic traits, which involve alleles that are beneficial to one sex but harmful to the other. We show that with a sexually antagonistic trait on the X chromosome (males XY, females XX), females evolve to prefer mates carrying alleles beneficial to daughters. In contrast, with a Z-linked trait (males ZZ, females ZW), females more often evolve mating preferences for mates carrying alleles beneficial to sons (that is, flashy displays).     

Ecological genetics and natural selection in molluscs

Polymorphic snails of the genus Cepaea have been widely used for research in ecological genetics. Natural selection by selective predation is important in controlling morph frequencies in some populations of C. nemoralis in England. The importance of environmental selection in affecting other patterns of local genetic differentiation of population structure (area effects) is a matter of controversy. Some authors emphasize divergent evolution of whole gene pools between area effects, while others feel that climatic selection acting on individual loci is important. Analysis of 500,000 C. nemoralis snails from throughout Europe shows that there is a strong positive association between gene frequencies at the shell color locus and mean summer temperature, but that no climatic correlations are obvious at other loci. Another species, C. vindobonensis, which has a much simpler system of polymorphism than does C. nemoralis, was investigated in Yugoslavia, in a region where there is known to be intense microclimatic differentiation because of the accumulation of cold air in frost hollows. There was a striking tendency for snails with lightly pigmented shell bands to be found in places with a warm microclimate. Physiological and behavioral experiments demonstrate that this is due primarily to differential energy absorption from sunshine by the different shell phenotypes. As in C. nemoralis, other C. vindobonensis phenotypes show no detectable association with the environment. It is possible that genes whose frequencies cannot be related to environmental selection may have evolved strong linkage interactions with other genes in the population's gene pool. Selection by the ecological environment and the genetic environment may therefore both be important in controlling the genetic structure of snail populations.     

Frequency-dependent natural selection in the handedness of scale-eating cichlid fish

Frequency-dependent natural selection has been cited as a mechanism for maintaining polymorphisms in biological populations, although the process has not been documented conclusively in field study. Here, it is demonstrated that the direction of mouth-opening (either left-handed or right-handed) in scale-eating cichlid fish of Lake Tanganyika is determined on the basis of simple genetics and that the abundance of individuals with left- or right-handedness depends on frequency-dependent natural selection. Attacking from behind, right-handed individuals snatched scales from the prey's left flank and left-handed ones from the right flank. Within a given population, the frequency of the two phenotypes oscillated around unity. This phenomenon was effected through frequency-dependent selection exerted by the prey's alertness. Thus, individuals of the rare phenotype had more success as predators than those of the more common phenotype.     

Rapid temporal reversal in predator-driven natural selection

As the environment changes, will species be able to adapt? By conducting experiments in natural environments, biologists can study how evolutionary processes such as natural selection operate through time. We predicted that the introduction of a terrestrial predator would first select for longer-legged lizards, which are faster, but as the lizards shifted onto high twigs to avoid the predator, selection would reverse toward favoring the shorter-legged individuals better able to locomote there. Our experimental studies on 12 islets confirmed these predictions within a single generation, thus demonstrating the rapidity with which evolutionary forces can change during times of environmental flux.     

Density-dependent natural selection and trade-offs in life history traits

Theories of density-dependent natural selection state that at extreme population densities evolution produces alternative life histories due to trade-offs. The trade-offs are presumed to arise because those genotypes with highest fitness at high population densities will not also have high fitness at low density and vice-versa. These predictions were tested by taking samples from six populations of Drosophila melanogaster kept at low population densities (r-populations) for nearly 200 generations and placing them in crowded cultures (K-populations). After 25 generations in the crowded cultures, the derived K-populations showed growth rate and productivity that at high densities were elevated relative to the controls, but at low density were depressed.     

电厂锅炉天然气点火装置的研制

根据天然气的特性 ,采用强制预混燃烧方式 ,设计了点火装置的喷嘴、引射器及烧嘴 ,并对稳定火焰的方法也进行了研究 .在空燃比一定的条件下 ,对天然气的流量与点火参数的关系进行了试验 .得到了天然气的最佳流量为 0 0 0 172m3 ·s-1,火焰温度高达 182 0℃ ,火焰长达 1 11m .结果表明 ,该点火装置具有火焰温度高、适应性强、操作方便等优点 .与以前用油点燃装置相比 ,预燃时间为燃油的 1/ 4 ,而成本仅为燃油的 1/ 80 0 .     

A molecular basis for natural selection at the timeless locus in Drosophila melanogaster

Diapause is a protective response to unfavorable environments that results in a suspension of insect development and is most often associated with the onset of winter. The ls-tim mutation in the Drosophila melanogaster clock gene timeless has spread in Europe over the past 10,000 years, possibly because it enhances diapause. We show that the mutant allele attenuates the photosensitivity of the circadian clock and causes decreased dimerization of the mutant TIMELESS protein isoform to CRYPTOCHROME, the circadian photoreceptor. This interaction results in a more stable TIMELESS product. These findings reveal a molecular link between diapause and circadian photoreception.     

Allometric engineering: a causal analysis of natural selection on offspring size

Techniques of offspring size manipulation, "allometric engineering," were used in combination with studies of natural selection to elucidate the causal relation between egg size and offspring survival of lizards. The results experimentally validate premises underlying theories of optimal egg size: fecundity selection favoring the production of large clutches of small eggs was balanced by survival selection favoring large offspring. However, large hatchlings did not always have the highest survival, contrary to most theoretical expectations. Optimizing selection on offspring size per se was the most common pattern. Moreover, matches between average and optimal egg size were qualitative, not quantitative, perhaps reflecting known functional constraints on the production of large eggs.