Genus

Genus

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"Genera" redirects here. For the operating system, see Genera (operating system).

For other uses, see Genus (disambiguation).

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In biology, a genus (plural: genera) is a low-level taxonomic rank used in the classification of living and fossil organisms, which is an example of definition by genus and differentia. The term comes from Latin genus "descent, family, type, gender",^[1] cognate with Greek: γένος – genos, "race, stock, kin".^[2]

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The hierarchy of biological classification's eight major taxonomic ranks, which is an example of definition by genus and differentia. A family contains one or more genera. Intermediate minor rankings are not shown.

The composition of a genus is determined by a taxonomist. The standards for genus classification are not strictly codified, and hence different authorities often produce different classifications for genera. In the hierarchy of the binomial classification system, genus comes above species and below family.

Contents

[hide]

• 1 Generic name
  • 1.1 Binomial nomenclature
  • 1.2 Pivotal in binomial nomenclature
  • 1.3 The problem of identical names used for different genera
  • 1.4 Types and genera
• 2 Guidelines
  • 2.1 Nomenclature
• 3 See also
• 4 References
• 5 External links

[edit] Generic name

The scientific name of a genus may be called the generic name or generic epithet: it is always capitalized. It plays a pivotal role in binomial nomenclature, the system of biological nomenclature.

[edit] Binomial nomenclature

The rules for scientific names are laid down in the Nomenclature Codes; depending on the kind of organism and the Kingdom it belongs to, a different Code may apply, with different rules, laid down in a different terminology. The advantages of scientific over common names are that they are accepted by speakers of all languages, and that each species has only one name. This reduces the confusion that may arise from the use of a common name to designate different things in different places (example elk), or from the existence of several common names for a single species.

It is possible for a genus to be assigned to a kingdom governed by one particular Nomenclature Code by one taxonomist, while other taxonomists assign it to a kingdom governed by a different Code, but this is the exception, not the rule.

[edit] Pivotal in binomial nomenclature

The generic name often is a component of the names of taxa of lower rank. For example, Canis lupus is the scientific name of the Gray wolf, a species, with Canis the generic name for the dog and its close relatives, and with lupus particular (specific) for the wolf (lupus is written in lower case). Similarly, Canis lupus familiaris is the scientific name for the domestic dog.

Taxonomic units in higher ranks often have a name that is based on a generic name, such as the family name Canidae, which is based on Canis. However, not all names in higher ranks are necessarily based on the name of a genus: for example, Carnivora is the name for the order to which the dog belongs.

[edit] The problem of identical names used for different genera

A genus in one kingdom is allowed to bear a scientific name that is in use as a generic name (or the name of a taxon in another rank) in a kingdom that is governed by a different Nomenclature Code. Although this is discouraged by both the International Code of Zoological Nomenclature and the International Code of Botanical Nomenclature, there are some five thousand such names that are in use in more than one kingdom. For instance, Anura is the name of the order of frogs but also is the name of a genus of plants (although not current: it is a synonym); Aotus is the genus of golden peas and night monkeys; Oenanthe is the genus of wheatears and water dropworts, Prunella is the genus of accentors and self-heal, and Proboscidea is the order of elephants and the genus of devil's claws.

Within the same kingdom one generic name can apply to only one genus. This explains why the platypus genus is named Ornithorhynchus—George Shaw named it Platypus in 1799, but the name Platypus had already been given to a group of ambrosia beetles by Johann Friedrich Wilhelm Herbst in 1793. Names with the same form but applying to different taxa are called homonyms. Since beetles and platypuses are both members of the kingdom Animalia, the name Platypus could not be used for both. Johann Friedrich Blumenbach published the replacement name Ornithorhynchus in 1800.

[edit] Types and genera

Because of the rules of scientific naming, or "binomial nomenclature", each genus should have a designated type, although in practice there is a backlog of older names that may not yet have a type. In zoology this is the type species (see Type (zoology)); the generic name is permanently associated with the type specimen of its type species. Should this specimen turn out to be assignable to another genus, the generic name linked to it becomes a junior synonym, and the remaining taxa in the former genus need to be reassessed.

See scientific classification and Nomenclature Codes for more details of this system. Also see type genus.

[Guidelines

There are no hard and fast rules that a taxonomist has to follow in deciding what does and what does not belong in a particular genus. This does not mean that there is no common ground among taxonomists in what constitutes a "good" genus. For instance, some rules-of-thumb for delimiting a genus are outlined in Gill.^[3] According to these, a genus should fulfill three criteria to be descriptively useful:

1. monophyly – all descendants of an ancestral taxon are grouped together;
2. reasonable compactness – a genus should not be expanded needlessly; and
3. distinctness – in regards of evolutionarily relevant criteria, i.e. ecology, morphology, or biogeography; note that DNA sequences are a consequence rather than a condition of diverging evolutionary lineages except in cases where they directly inhibit gene flow (e.g. postzygotic barriers).

[edit] Nomenclature

...difficulties occurring in generic nomenclature: similar cases abound, and become complicated by the different views taken of the matter by the various taxonomists.

Prof. C. S. Rafinesque. 1836^[4]

None of the Nomenclature Codes require such criteria for defining a genus, because these are concerned with the nomenclature rules, not with taxonomy. These regulate formal nomenclature, aiming for universal and stable scientific names.

mud

As habitat

This section requires expansion.

Mud plastered home in Pakistan

Dried mud with wind-blown stones

A lorry stuck in mud

Mud can provide a home for numerous types of animals, including varieties of worms, frogs, snails, clams, and crayfish. Other animals, such as pigs and elephants, bathe in mud in order to cool off and protect themselves from the sun. Humans have also used mud as a building material, or a sealant material.

[edit] Problems

Clay soil can pose problems for traffic when moisture is present. A road built upon such soil may become stable over time as the packing of the soil will make it more water-resistant. However, any attempt to grade it can be disastrous, since excess water can then enter the surface and will be worked in by traffic, transforming portions of the road into a mud bog that can trap vehicles. The typical solution in road building is to add layers of crushed stone. The stone particles will interlock and distribute the weight of a vehicle over a larger surface area. Proper drainage is also essential when low spots are encountered by the road, usually requiring the addition of culverts to pass water underneath the elevation of the street.

Buildings constructed upon clay soil must also be properly drained around their perimeter, particularly where a perimeter foundation (rather than a monolithic slab) is used. As clay will expand and soften when moisture is added, the resultant mud will squeeze out from underneath the foundation, however, in the next dry cycle it will contract, but the clay squeezed out will not return. Over a number of such cycles the foundation can sink in the moisture-cycled locations, possibly causing both wall and foundation cracks. Maintaining a constant moisture level in firm soil is important and can be effected by appropriate landscaping and landscaping maintenance. Where drainage is toward a building a French drain may be installed to route water around the building.

[edit] As food

Haiti consumes a large variety of different non-traditional foods in an attempt to quelch hunger pains. Mud cakes are traditionally fashioned and consumed, but items such as clay and chalk can also be eaten. Due to recent increases in food prices and growing starvation in Haiti, this habit has been extended and received much media attention.^[1]

Outside of hunger, mud and dirt can be consumed accidentally during sports and other outdoor activities. This has led to dysphemisms for poor-tasting food such as "tastes like dirt", based on the experience of getting mud, dirt, etc. in one's teeth.

There also exist children's recipes for "mud", which is generally a chocolate or cornstarch-based sludge used more for visual appeal than actual taste. Never does this confectionery mud actually contain real mud.^[2]

Petroleum{ زبان اصلی1 }

Petroleumthe temperature t is in Celsius and d is the specific gravity at 15 °C.^[15]

[edit] Formation

Structure of vanadium porphyrin compound extracted from petroleum by Alfred E. Treibs, father of organic geochemistry. Treibs noted the close structural similarity of this molecule and chlorophyll a.

According to generally accepted theory, petroleum is derived from ancient biomass.^[16] It is a fossil fuel derived from ancient fossilized organic materials. The theory was initially based on the isolation of molecules from petroleum that closely resemble known biomolecules.

More specifically, crude oil and natural gas are products of heating of ancient organic materials (i.e. kerogen) over geological time. Formation of petroleum occurs from hydrocarbon pyrolysis, in a variety of mostly endothermic reactions at high temperature and/or pressure.^[17] Today's oil formed from the preserved remains of prehistoric zooplankton and algae, which had settled to a sea or lake bottom in large quantities under anoxic conditions (the remains of prehistoric terrestrial plants, on the other hand, tended to form coal). Over geological time the organic matter mixed with mud, and was buried under heavy layers of sediment resulting in high levels of heat and pressure (diagenesis). This process caused the organic matter to change, first into a waxy material known as kerogen, which is found in various oil shales around the world, and then with more heat into liquid and gaseous hydrocarbons via a process known as catagenesis.

There were certain warm nutrient-rich environments such as the Gulf of Mexico and the ancient Tethys Sea where the large amounts of organic material falling to the ocean floor exceeded the rate at which it could decompose. This resulted in large masses of organic material being buried under subsequent deposits such as shale formed from mud. This massive organic deposit later became heated and transformed under pressure into oil.^[18]

Geologists often refer to the temperature range in which oil forms as an "oil window"^[19]—below the minimum temperature oil remains trapped in the form of kerogen, and above the maximum temperature the oil is converted to natural gas through the process of thermal cracking. Sometimes, oil which is formed at extreme depths may migrate and become trapped at much shallower depths than where it was formed. The Athabasca Oil Sands is one example of this.

[edit] Abiogenic origin

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Main article: Abiogenic petroleum origin

A small number of geologists adhere to the abiogenic petroleum origin hypothesis and maintain that hydrocarbons of purely inorganic origin exist within Earth's interior. Chemists Marcellin Berthelot and Dmitri Mendeleev, as well as astronomer Thomas Gold championed the theory in the Western world by supporting the work done by Nikolai Kudryavtsev and Vladimir Porfiriev in the 1950s. It is currently supported primarily by Jack F. Kenney, Vladilen Krayushkin, and Vladimir Kutcherov.^[20][21]

The abiogenic origin hypothesis has not yet been ruled out, but it has little support among modern petroleum geologists.^[22] Its advocates consider that it is "still an open question"^[21] Extensive research into the chemical structure of kerogen has identified algae as the primary source of oil. The abiogenic origin hypothesis fails to explain the presence of these markers in kerogen and oil, as well as failing to explain how inorganic origin could be achieved at temperatures and pressures sufficient to convert kerogen to graphite. It has not been successfully used in uncovering oil deposits by geologists, as the hypothesis lacks any mechanism for determining where the process may occur.^[23] More recently scientists at the Carnegie Institution for Science have found that ethane and heavier hydrocarbons can be synthesized under conditions of the upper mantle.^[24]

[edit] Crude oil

[edit] Crude oil reservoirs

Hydrocarbon trap.

Three conditions must be present for oil reservoirs to form: a source rock rich in hydrocarbon material buried deep enough for subterranean heat to cook it into oil; a porous and permeable reservoir rock for it to accumulate in; and a cap rock (seal) or other mechanism that prevents it from escaping to the surface. Within these reservoirs, fluids will typically organize themselves like a three-layer cake with a layer of water below the oil layer and a layer of gas above it, although the different layers vary in size between reservoirs. Because most hydrocarbons are lighter than rock or water, they often migrate upward through adjacent rock layers until either reaching the surface or becoming trapped within porous rocks (known as reservoirs) by impermeable rocks above. However, the process is influenced by underground water flows, causing oil to migrate hundreds of kilometres horizontally or even short distances downward before becoming trapped in a reservoir. When hydrocarbons are concentrated in a trap, an oil field forms, from which the liquid can be extracted by drilling and pumping.

The reactions that produce oil and natural gas are often modeled as first order breakdown reactions, where hydrocarbons are broken down to oil and natural gas by a set of parallel reactions, and oil eventually breaks down to natural gas by another set of reactions. The latter set is regularly used in petrochemical plants and oil refineries.

Wells are drilled into oil reservoirs to extract the crude oil. "Natural lift" production methods that rely on the natural reservoir pressure to force the oil to the surface are usually sufficient for a while after reservoirs are first tapped. In some reservoirs, such as in the Middle East, the natural pressure is sufficient over a long time. The natural pressure in many reservoirs, however, eventually dissipates. Then the oil must be pumped out using “artificial lift” created by mechanical pumps powered by gas or electricity. Over time, these "primary" methods become less effective and "secondary" production methods may be used. A common secondary method is “waterflood” or injection of water into the reservoir to increase pressure and force the oil to the drilled shaft or "wellbore." Eventually "tertiary" or "enhanced" oil recovery methods may be used to increase the oil's flow characteristics by injecting steam, carbon dioxide and other gases or chemicals into the reservoir. In the United States, primary production methods account for less than 40% of the oil produced on a daily basis, secondary methods account for about half, and tertiary recovery the remaining 10%. Extracting oil (or “bitumen”) from oil/tar sand and oil shale deposits requires mining the sand or shale and heating it in a vessel or retort, or using “in-situ” methods of injecting heated liquids into the deposit and then pumping out the oil-saturated liquid.

[edit] Unconventional oil reservoirs

See also: Unconventional oil, Oil sands, and Oil shale reserves

Oil-eating bacteria biodegrades oil that has escaped to the surface. Oil sands are reservoirs of partially biodegraded oil still in the process of escaping and being biodegraded, but they contain so much migrating oil that, although most of it has escaped, vast amounts are still present—more than can be found in conventional oil reservoirs. The lighter fractions of the crude oil are destroyed first, resulting in reservoirs containing an extremely heavy form of crude oil, called crude bitumen in Canada, or extra-heavy crude oil in Venezuela. These two countries have the world's largest deposits of oil sands.

On the other hand, oil shales are source rocks that have not been exposed to heat or pressure long enough to convert their trapped hydrocarbons into crude oil. Technically speaking, oil shales are not really shales and do not really contain oil, but are usually relatively hard rocks called marls containing a waxy substance called kerogen. The kerogen trapped in the rock can be converted into crude oil using heat and pressure to simulate natural processes. The method has been known for centuries and was patented in 1694 under British Crown Patent No. 330 covering, "A way to extract and make great quantityes of pitch, tarr, and oyle out of a sort of stone." Although oil shales are found in many countries, the United States has the world's largest deposits.^[25]

[edit] Classification

See also: Benchmark (crude oil)

A sample of medium heavy crude oil

The petroleum industry generally classifies crude oil by the geographic location it is produced in (e.g. West Texas Intermediate, Brent, or Oman), its