全文获取类型
收费全文 | 4027篇 |
免费 | 387篇 |
国内免费 | 158篇 |
出版年
2024年 | 2篇 |
2023年 | 36篇 |
2022年 | 35篇 |
2021年 | 71篇 |
2020年 | 94篇 |
2019年 | 132篇 |
2018年 | 123篇 |
2017年 | 120篇 |
2016年 | 178篇 |
2015年 | 174篇 |
2014年 | 176篇 |
2013年 | 213篇 |
2012年 | 161篇 |
2011年 | 178篇 |
2010年 | 173篇 |
2009年 | 248篇 |
2008年 | 231篇 |
2007年 | 242篇 |
2006年 | 167篇 |
2005年 | 187篇 |
2004年 | 175篇 |
2003年 | 127篇 |
2002年 | 135篇 |
2001年 | 124篇 |
2000年 | 113篇 |
1999年 | 117篇 |
1998年 | 104篇 |
1997年 | 78篇 |
1996年 | 100篇 |
1995年 | 71篇 |
1994年 | 78篇 |
1993年 | 74篇 |
1992年 | 55篇 |
1991年 | 32篇 |
1990年 | 45篇 |
1989年 | 39篇 |
1988年 | 36篇 |
1987年 | 20篇 |
1986年 | 19篇 |
1985年 | 14篇 |
1984年 | 19篇 |
1983年 | 12篇 |
1982年 | 13篇 |
1981年 | 9篇 |
1980年 | 6篇 |
1979年 | 2篇 |
1977年 | 5篇 |
1976年 | 2篇 |
1974年 | 3篇 |
1973年 | 3篇 |
排序方式: 共有4572条查询结果,搜索用时 31 毫秒
91.
Carotenogenic mutants ofPhycomyces, which accumulate excess β-carotene or its intermediates, always failed in zygospore development. No improvement occurred
when such mutants were mated together with a helper wild type of the same mating type against the wild type of the opposite
mating type. Addition of excess synthesized pheromone, trisporin B, also failed to improve the zygospore development, though
the mating response was significantly activated in the early stages and abundant zygophores were formed. Exceptional acceleration
of the zygospore development under these experimental conditions occurred in a regulatory albino mutant (carA), which does not accumulate excess intermediate carotenoids. Chemically- or genetically-induced ovarproduction of β-carotene
or lycopene also inhibited the zygospore development. These results imply that the zygospore development ofPhycomyces is maximal when the intracellular amount of β-carotene is optimal (=wild type), and that pheromones act mainly in the early
stages of mating, while other factors such as the cell-to-cell recognition system may also be involved in the later stages.
Intracellular accumulation of excess β-carotene or its intermediates probably disturb such later-stage factors. 相似文献
92.
This study examines how Choristoneura rosaceana male quality, as determined by larval diet, age and mating history, affects the reproductive success of both sexes. While the size of the spermatophore produced at first mating increased linearly with male age, the frequency of mating was significantly higher for middle-aged males (2–4 days old) than younger (0–2 days old) or older (6–8 days old) individuals, when both sexes were fed on artificial diet. However, the duration of copulation was longer in couples with older than younger males. The observed age-related changes in spermatophore size had no significant effect on female longevity, fecundity or fertility, suggesting no direct relationship between male investment and spermatophore size under these experimental conditions. Different larval food sources (artificial diet, maple and hazelnut) did not affect the proportion of 2-day-old virgin males that mated; however, the proportion that remated was significantly higher for males reared on high-quality food (maple and artificial diet) than those on hazelnut, a poorer food source. There was a 5-fold decline in spermatophore size between the first and second matings on all diets, but female reproductive output was reduced by only 25%. In contrast, while the first spermatophore produced by males on hazelnut was 1.5 times smaller than those produced on maple and artificial diet, the fecundity of their mates was 40% less than those mated with high-quality virgin males. These results provide additional support to the idea that spermatophore size is not a valuable indicator of male quality. Most tethered females placed in the field during the first flight period mated with virgin males (based on the size of the spermatophore), suggesting that female choice exists in this species. These results are discussed in relation to the incidence of polyandry in naturally occurring populations of Choristoneura and the potential use of size and/or chemical cues by females to assess male quality. 相似文献
93.
We determined the fertilization rates achieved by males of the polygynous wrasse Xyrichtys novacula in the northern Mediterranean sea, by collecting all sperms and eggs released in individual spawns from three adjacent male territories that encompassed different numbers of female home ranges. Two males with territories including four and six females respectively, released similar average numbers of sperm/spawn, whereas a male holding at the same time two territories with six females each, released significantly fewer sperm/spawn. Although fertilization rates decreased with absolute numbers of sperm released, the male with the largest harem achieved the highest total daily number of fertilized eggs, as a result of the high number of females in his territory. These results demonstrate a strategy of a male's sperm economy that is in conflict with the female's interests. 相似文献
94.
Females ofSpodoptera littoralis Boisd. (Lepidoptera: Noctuidae) with different feeding experiences during their larval development were tested for their
ovipositional response to methanol extracts of larval frass and semisynthetic diets. The effect of the following frass, diet
and diet component extracts was tested: (a) frass fromS. littoralis orAgrotis segetum larvae fed on a potato-based diet; (b) frass fromS. littoralis larvae fed on a wheat germ-based diet; (c) potato and wheat germ-based diets; and (d) potatoes and wheat germ.
Ovipositing females without prior experience of the potato diet were deterred by extracts of: (1) larval frass from either
species fed on potato diet; (2) the potato-based diet; (3) potato. Also females with experience of the potato diet during
only a part of their larval development were deterred from oviposition by frass of larvae reared on the potato diet and by
the diet itself. However, for females reared on the potato diet for their entire larval development, oviposition was no longer
deterred by either of the three extracts listed above. Extracts of: (1) frass from larvae of either species reared on wheat
germ diet: (2) the wheat germ diet; or (3) wheat germ did not significantly affect oviposition. Females with ablated antennae
were still deterred by frass extracts from larvae fed on potato diet, when they had been reared on the wheat germ diet. In
feeding experiments, larvae of larval stage one and of larval stage three-four reared on either of the two diets preferred
to feed on the wheat germ diet. However, the preference was significantly stronger for larvae with no prior contact with the
potato diet. The effect of larval experience on the loss of oviposition-deterring activity by extracts of larval frass, diets
and diet components is discussed in view of induction and selection. 相似文献
95.
The effect of adult learning through an oviposition experience, and pre-adult learning through development inside a host was
investigated in two strains of an Australian egg parasitoid,Trichogramma nrivelae (Hymenoptera: Trichogrammatidae). Host response was measured in two types of laboratory preference tests. In single host
tests, females reared on three lepidopteran hosts (Heliothis punctigera (Noctuidae),Papilio aegeus (Papilionidae), andHypolimnas bolina (Nymphalinae)) were presented host eggs individually, and allowed two ovipositions. Pre-adult experience affected host preference
in only one strain, while acceptance of a host was increased in both strains when they had previously oviposited in this host
species. An oviposition experience had a stronger effect on host preference than pre-adult experience. In choice tests, pairwise
combinations of hosts were arranged in an alternating grid. The ratio of host acceptances to host contacts was computed for
the whole test and up to the first oviposition. These indicated that effects of rearing host were weak or absent, but the
host chosen initially had a strong effect on host preference. Relative size of the hosts had a strong effect on choice of
the first host. The implications of learning inTrichogramma are discussed in relation to host preference testing procedures and the selection of candidate strains for mass rearing and
inundative release. 相似文献
96.
Mona El Agoze Marylène Poirié Georges Périquet 《Entomologia Experimentalis et Applicata》1995,75(3):251-255
In most insect species where double matings occur, sperm from the second male preferentially fertilize subsequent eggs. However,
we demonstrate here that, as already shown for some other hymenopteran species, this is not the case in the ichneumonid waspDiadromus pulchellus (Wesmeal): sperm from the first male usually father all the female progeny. This precedence of the first male sperm is also
observed in double matings involving an haploid male and a diploid sterile male, whichever is the first mating male. We discuss
the consequences of this phenomenon from an evolutionary point of view. 相似文献
97.
Michael Lynch John Conery Reinhard Bürger 《Evolution; international journal of organic evolution》1995,49(6):1067-1080
Although it is widely acknowledged that the gradual accumulation of mildly deleterious mutations is an important source of extinction for asexual populations, it is generally assumed that this process is of little relevance to sexual species. Here we present results, based on computer simulations and supported by analytical approximations, that indicate that mutation accumulation in small, random-mating monoecious populations can lead to mean extinction times less than a few hundred to a few thousand generations. Unlike the situation in obligate asexuals in which the mean time to extinction (t?e) increases more slowly than linearly with the population carrying capacity (K), t?e increases approximately exponentially with K in outcrossing sexual populations. The mean time to extinction for obligately selfing populations is shown to be equivalent to that for asexual populations of the same size, but with half the mutation rate and twice the mutational effect; this suggests that obligate selfing, like obligate asexuality, is inviable as a long-term reproductive strategy. Under all mating systems, the mean time to extinction increases relatively slowly with the logarithm of fecundity, and mutations with intermediate effects (similar to those observed empirically) cause the greatest risk of extinction. Because our analyses ignore sources of demographic and environmental stochasticity, which have synergistic effects that exacerbate the accumulation of deleterious mutations, our results should yield liberal upper bounds to the mean time to extinction caused by mutational degradation. Thus, deleterious mutation accumulation cannot be ruled out generally as a significant source of extinction vulnerability in small sexual populations or as a selective force influencing mating-system evolution. 相似文献
98.
Seasonal migration promoting assortative mating in Littorina brevicula on a boulder shore in Japan 总被引:1,自引:1,他引:0
Yoshitake Takada 《Hydrobiologia》1995,309(1-3):151-159
Littorina brevicula Philippi is one of the most common snails found in the upper intertidal zone of Japan. In Amakusa, some of the population
of L. brevicula migrate to the lower zone in the winter, while the rest stay in the upper zone. Thus, during the winter, which is its reproductive
season, the population of L. brevicula divides into two sub-populations. This leads to a hypothesis that the migration pattern in winter is genetically controlled
and this behavioural dimorphism is maintained by reproductive isolation between the two sub-populations. In order to test
this hypothesis, the following three points were investigated: (1) whether the same snails migrate in a similar way every
winter, (2) whether there is a significant tidal level preference in snails, and (3) whether reproductive isolation occurs
between the two sub-populations. The results showed (1) the migration behaviour of each snail was consistent over two successive
winters, i.e. the same group of snails migrated downward every winter and the same group of snails stayed in the upper zone
every winter, (2) transplanted snails moved toward the original zones where they were caught, suggesting that the snails actively
selected their tidal zone in winter, and (3) most of the snails copulated within each sub-population. Therefore, reproductive
isolation between the two sub-populations was considered to be established to some extent by the dimorphic migration behaviour.
In conclusion, the migratory behaviour of L. brevicula is determined separately for each individual and might be genetically controlled, and the behavioural dimorphism may be maintained
by partial reproductive isolation between the two sub-populations. 相似文献
99.
We present results on cross-mating experiments using Brachionus plicatilis strains collected in three ponds of a coastal marsh (Torreblanca Marsh, Castellón, Spain). These strains were known to differ widely both in morphology and allozyme patterns from a previous study, where they were grouped into three genetically different clonal groups. Although some of the strains co-occurred in the same pond and sexual periods overlapped, no gene flow was found among them. Our first objective was to determine whether behavioral reproductive isolation was responsible for the absence of interbreeding. A second objective was to explore the relationship between sexual isolation and genetic divergence. We performed two experiments. In Experiment 1, we tested five strains from different clonal groups; in Experiment 2, we added a strain from a congeneric species, and strains from different ponds. We recorded male mating behavior in all possible male-female strain pairings. Our data show that males of a strain tend to mate with females of the same strain or genetically similar strains, regardless of the pond they come from. We also found a high positive correlation between isolation distance and genetic distance. These results support the view that mating behavior acts as an important isolating barrier giving cohesion to clonal groups, and structuring populations of this rotifer, and that Brachionus plicatilis is a taxon composed of more than one biological species. 相似文献
100.
Sand lizard Lacerta agilis females characteristically mate with several males which, in staged mating experiments, results in multiple paternity of the offspring. In order to investigate multiple paternity in a natural population and interpret male reproductive behaviours in terms of sired young, we sampled the blood of females, potential fathers and hatchlings, and determined paternity using multilocus DNA fingerprinting as well as the variation at a single locus detected by the probe (TC) n . The paternity analyses were preceded by a laboratory experiment in which we established that the parental alleles identified by the single locus probe were inherited in a Mendelian way. Our molecular data demonstrated that 12 out of 13 males (92%) that sired offspring were correctly identified from the 56 sexually mature males in the population. Also smaller males were accepted as sexual partners by the females, but sired fewer young in competition with larger males and were less able to maintain prolonged post-copulatory mate guarding. This may result in that some sexually successful males are only observed inside a female's home range, but never in pair-association with the female. 相似文献