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Sex Ratio
A sex ratio is the ratio of males to females in a population. As explained by Fisher's principle, for evolutionary reasons this is typically about 1:1 in species which reproduce sexually. However, many species deviate from an even sex ratio, either periodically or permanently. These include parthenogenic and androgenetic species, periodically mating organisms such as aphids, some eusocial wasps, bees, ants, and termites. Types In most species, the sex ratio varies according to the age profile of the population. It is generally divided into four subdivisions: * — ratio at fertilization * — ratio at birth * — ratio in sexually mature organisms ** The tertiary sex ratio is equivalent to the (ASR), which is defined as the ratio of adult males to females in a population. ** The operational sex ratio (OSR) is the ratio of ''sexually active'' males to females in a population, and is therefore derived from a subset of the individuals included when calculating the ASR. ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Sex-selective Abortion
Sex-selective abortion is the practice of terminating a pregnancy based upon the predicted sex of the infant. As the practice overwhelmingly targets female fetuses, sex-selective abortion often specifically refers to female-selective abortion. Sex-selective abortion is closely linked to female infanticide, and is recognized by many human rights organizations as an act of violence against women. The selective abortion of female fetuses is most common where male children are valued over female children, especially in parts of East Asia and South Asia (particularly in countries such as People's Republic of China, India and Pakistan), as well as in the Caucasus, Western Balkans, and to a lesser extent North America. Based on the third National Family and Health Survey, results showed that if both partners, mother and father, or just the father, preferred male children, sex-selective abortion was more common. In cases where only the mother prefers sons, this is likely to result i ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Human Sex Ratio
The human sex ratio is the ratio of males to females in a population in the context of anthropology and demography. In humans, the natural sex ratio at birth is slightly biased towards the male sex. It is estimated to be about 1.05 worldwide or within a narrow range from 1.03 to 1.06 males per female at birth. The sex ratio for the entire world population (all ages) is approximately 101 males to 100 females . The sex ratios at birth and of the total population are affected by various factors including natural factors, exposure to pesticides and environmental contaminants, war casualties, returning soldier effect, effects of war on men, sex-selective abortions, infanticides, aging, gendercide, problems with birth registration and Life expectancy#Sex differences, sex differences in life expectancy. Human sex ratios, either at birth or in the population as a whole, can be reported in any of four ways: the ratio of males to females, the ratio of females to males, the proportion of ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Ratio
In mathematics, a ratio () shows how many times one number contains another. For example, if there are eight oranges and six lemons in a bowl of fruit, then the ratio of oranges to lemons is eight to six (that is, 8:6, which is equivalent to the ratio 4:3). Similarly, the ratio of lemons to oranges is 6:8 (or 3:4) and the ratio of oranges to the total amount of fruit is 8:14 (or 4:7). The numbers in a ratio may be quantities of any kind, such as counts of people or objects, or such as measurements of lengths, weights, time, etc. In most contexts, both numbers are restricted to be Positive integer, positive. A ratio may be specified either by giving both constituting numbers, written as "''a'' to ''b''" or "''a'':''b''", or by giving just the value of their quotient Equal quotients correspond to equal ratios. A statement expressing the equality of two ratios is called a ''proportion''. Consequently, a ratio may be considered as an ordered pair of numbers, a Fraction (mathematic ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Macrotera Portalis
''Macrotera portalis'' is a species of communal, ground nesting, partially bivoltine bees found in arid grasslands and desert regions of North America. An oligolectic bee, ''M. portalis'' gathers pollen only from plants in the genus ''Sphaeralcea'' and has patterns of seasonal emergence to survive the harsh conditions of the desert, with emergence delayed until monsoon rains arrive. Taxonomy and phylogenetics ''Macrotera portalis'' is in the order Hymenoptera, family Andrenidae, genus ''Macrotera''. This species was formerly in ''Perdita (bee), Perdita'', a very large genusTimberlake"Taxonomy" ‘’Taxonomy Details for ''Perdita portalis''’’, Retrieved on 15 October 2015. that now contains around 600 species of bees which are located primarily in northern Mexico and the arid southwestern United States. Around 30 species removed from ''Perdita'' were placed in the genus ''Macrotera''. Description and identification ''Macrotera portalis'' is a small (~7 mm), dark bee spe ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Dichogamy
Sequential hermaphroditism (called dichogamy in botany) is one of the two types of hermaphroditism, the other type being simultaneous hermaphroditism. It occurs when the organism's sex changes at some point in its life. A sequential hermaphrodite produces eggs (female gametes) and sperm (male gametes) at different stages in life. Sequential hermaphroditism occurs in many fish, gastropods, and plants. Species that can undergo these changes do so as a normal event within their reproductive cycle, usually cued by either social structure or the achievement of a certain age or size. In animals, the different types of change are male to female (protandry or protandrous hermaphroditism), female to male (protogyny or protogynous hermaphroditism), and bidirectional (serial or bidirectional hermaphroditism). Both protogynous and protandrous hermaphroditism allow the organism to switch between functional male and functional female. Bidirectional hermaphrodites have the capacity for sex chan ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Peafowl
Peafowl is a common name for two bird species of the genus '' Pavo'' and one species of the closely related genus '' Afropavo'' within the tribe Pavonini of the family Phasianidae (the pheasants and their allies). Male peafowl are referred to as peacocks, and female peafowl are referred to as peahens. The two Asiatic species are the blue or Indian peafowl originally from the Indian subcontinent, and the green peafowl from Southeast Asia. The third peafowl species, the Congo peafowl, is native only to the Congo Basin. Male peafowl are known for their piercing calls and their extravagant plumage. The latter is especially prominent in the Asiatic species, which have an eye-spotted "tail" or "train" of covert feathers, which they display as part of a courtship ritual. The functions of the elaborate iridescent coloration and large "train" of peacocks have been the subject of extensive scientific debate. Charles Darwin suggested that they served to attract females, and the ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Temperature-dependent Sex Determination
Temperature-dependent sex determination (TSD) is a type of environmental sex determination in which the temperatures experienced during embryonic/larval development determine the sex of the offspring. It is observed in reptiles and teleost fish, with some reports of it occurring in species of shrimp. TSD differs from the chromosomal sex-determination systems common among vertebrates. It is the most studied type of environmental sex determination (ESD). Some other conditions, e.g. density, pH, and environmental background color, are also observed to alter sex ratio, which could be classified either as temperature-dependent sex determination or temperature-dependent sex differentiation, depending on the involved mechanisms. As sex-determining mechanisms, TSD and genetic sex determination (GSD) should be considered in an equivalent manner, which can lead to reconsidering the status of fish species that are claimed to have TSD when submitted to extreme temperatures instead of the temp ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Copepods
Copepods (; meaning 'oar-feet') are a group of small crustaceans found in nearly every freshwater and saltwater habitat. Some species are planktonic (living in the water column), some are benthic (living on the sediments), several species have parasitic phases, and some continental species may live in limnoterrestrial habitats and other wet terrestrial places, such as swamps, under leaf fall in wet forests, bogs, springs, ephemeral ponds, puddles, damp moss, or water-filled recesses of plants ( phytotelmata) such as bromeliads and pitcher plants. Many live underground in marine and freshwater caves, sinkholes, or stream beds. Copepods are sometimes used as biodiversity indicators. As with other crustaceans, copepods have a larval form. For copepods, the egg hatches into a nauplius form, with a head and a tail but no true thorax or abdomen. The larva molts several times until it resembles the adult and then, after more molts, achieves adult development. The nauplius form is so ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Arthropod
Arthropods ( ) are invertebrates in the phylum Arthropoda. They possess an arthropod exoskeleton, exoskeleton with a cuticle made of chitin, often Mineralization (biology), mineralised with calcium carbonate, a body with differentiated (Metamerism (biology), metameric) Segmentation (biology), segments, and paired jointed appendages. In order to keep growing, they must go through stages of moulting, a process by which they shed their exoskeleton to reveal a new one. They form an extremely diverse group of up to ten million species. Haemolymph is the analogue of blood for most arthropods. An arthropod has an open circulatory system, with a body cavity called a haemocoel through which haemolymph circulates to the interior Organ (anatomy), organs. Like their exteriors, the internal organs of arthropods are generally built of repeated segments. They have ladder-like nervous systems, with paired Anatomical terms of location#Dorsal and ventral, ventral Ventral nerve cord, nerve cord ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Wolbachia
''Wolbachia'' is a genus of gram-negative bacteria infecting many species of arthropods and filarial nematodes. The symbiotic relationship ranges from parasitism to obligate mutualism. It is one of the most common parasitic microbes of arthropods, and is possibly the most widespread reproductive parasite bacterium in the biosphere. Its interactions with hosts are complex and highly diverse across different host species. Some host species cannot reproduce, or even survive, without ''Wolbachia'' colonisation. One study concluded that more than 16% of neotropical insect species carry bacteria of this genus, and as many as 25 to 70% of all insect species are estimated to be potential hosts. History The first organism classified as ''Wolbachia'' was discovered in 1924 by Marshall Hertig and Simeon Burt Wolbach in the common house mosquito. They described it as "a somewhat pleomorphic, rodlike, Gram-negative, intracellular organism hatapparently infects only the ovaries and te ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Bacterium
Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among the first life forms to appear on Earth, and are present in most of its habitats. Bacteria inhabit the air, soil, water, acidic hot springs, radioactive waste, and the deep biosphere of Earth's crust. Bacteria play a vital role in many stages of the nutrient cycle by recycling nutrients and the fixation of nitrogen from the atmosphere. The nutrient cycle includes the decomposition of dead bodies; bacteria are responsible for the putrefaction stage in this process. In the biological communities surrounding hydrothermal vents and cold seeps, extremophile bacteria provide the nutrients needed to sustain life by converting dissolved compounds, such as hydrogen sulphide and methane, to energy. Bacteria also live in mutualistic, commensal and ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
