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Talpinaria

Talpinaria

More than 1200, see text Pleurothallis, or the bonnet orchid, is a genus of orchids. The genus name is derived from the Greek word 'pleurothallos', meaning 'riblike branches'. This refers to the rib-like stems of many species. This was a huge genus, which used to contain more than 1,200 species - the second largest in the Orchidaceae after Bulbophyllum. In 2004 it has been more than halved by incorporating many species into new genera (Monogr. Syst. Bot. Missouri Bot. Gard. 95: 256. Feb 2004).

Distribution

It is a completely New World group and occurs from the southeast United States down into the tropics of South America, with most species in the high Andes chain of cloud forest in Colombia. They grow in dry or wet, tropical or temperate climates.

Morphology

As a group they show a huge range in vegetative form, terrestrial or epiphytic, and can be found as tall cane-like plants a metre or so high, clumped or trailing, pendent or climbing, erect or creeping, tufted and tiny, delicate moss-like species that can grow on the thinnest of twigs. But they have one common denominator : they all have two pollinia. They have reduced their pseudobulbs and instead, some species have thick succulent leaves. Their flowers are among the most diverse and unusual, although often very small, and specialise in using tiny insects such as gnats or Chalcid wasps for pollination.

Taxonomy

To bring some order in this extremely diverse genus, 29 subgenera and 25 sections had been created. Much of this work has been done by Dr. C. Luer of MOBOT. A new analysis, based on DNA testing, has confirmed on the whole the classification of the subtribe Pleurothallidinae, with however Pleurothallis as the main difference. This should not surprise anyone, since this somewhat artificial genus has been the dumping ground for species that did not fit into any other genus. New genetic insight has broken up this huge genus and made it more consistent with the principles of monophyletic genera and evolutionary relationships. Several times before, a splitting up has been attempted, but there were always too many intermediate forms. In 2004 the new genera Acianthera, Ancipitia, Antilla, Apoda-prorepentia, Areldia, Atopoglossum, Brenesia, Crocodeilanthe have been created, incorporating a large number of former Pleurothallis species (Monogr. Syst. Bot. Missouri Bot. Gard. 95: 255. 2004 [Feb 2004]). The subgenera Pleurothallis and Specklinia are becoming separate genera, and the subgenus Acuminatae might become the proposed genus Anathallis. Other allied genera include: Dracula, Masdevallia, Restrepia and Stelis.

Synonymy

The following taxa: Acronia C.Presl, Andreettaea Luer, Centranthera Scheidw., Colombiana Ospina, Cryptophoranthus Barb.Rodr., Geocalpa Brieger (nom. inval.), Kraenzlinella Kuntze, Otopetalum Lehm. & Kraenzl., Pabstiella Brieger & Senghas, Palmoglossum Klotzsch ex Rchb.f., Phloeophila Hoehne & Schltr., Physosiphon Lindl., Physothallis Garay, Pleurobotryum Barb.Rodr., Pseudoctomeria Kraenzl., Pseudostelis Schltr., Rhynchopera Klotzsch, Sarracenella Luer, Specklinia Lindl. and Talpinaria H.Karst. are synonyms of Pleurothallis.

Species

This is a long list, with about 1240 species. See List of Pleurothallis species. A few have common names :
- Pleurothallis angustifolia : Wilson's Bonnet Orchid
- Pleurothallis appendiculata : Hand Bonnet Orchid (now a synonym of Antilla appendiculata (Cogn.) Luer in 2004)
- Pleurothallis aristata : Forest Bonnet Orchid
- Pleurothallis domingensis : Guadeloupe Bonnet Orchid (now a synonym of Crocodeilanthe domingensis (Cogn.) Luer 2004)
- Pleurothallis immersa : Black Orchid - now a synonym of Specklinia immersa (Linden & Rchb.f.) Luer
- Pleurothallis oblongifolia : Ladies' Bonnet Orchid
- Pleurothallis obovata : South American Bonnet Orchid
- Pleurothallis pruinosa : Waxy Bonnet Orchid
- Pleurothallis pubescens : Hairy Bonnet Orchid
- Pleurothallis ruscifolia : Green Bonnet Orchid
- Pleurothallis segoviensis : Trout-fly Orchid - now synonym of Unciferia segoviensis (Rchb.f.) Luer

Reference

- Luer, Carlyle; 1986 - Icones Pleurothallidinarum III: Systematics of Pleurothallis (Orchidaceae); Missouri Botanical Garden Press.
- Pridgeon, A., and M. Chase. 2001. A phylogenetic reclassification of Pleurothallidinae (Orchidaceae). Lindleyana 16: 235-271.

External link

- [http://www.loujost.com/DNAFrameset/DNAanalysisDocument.htm Analysis of the DNA-Based Reclassification of the Pleurothallidinae]
- [http://www.pleurothallids.com/ Orchids of the Pleurothallid Alliance]
- [http://www.msu.edu/user/zablotn1/pleuros/genera/pleuro/pleuro.htm Photos] Category:Orchid genera

Genus

In biology, a genus (plural genera) is a grouping in the classification of living organisms having one or more related and morphologically similar species. In the common binomial nomenclature, the name of an organism is composed of two parts: its genus (always capitalized) and a species modifier. An example is Homo sapiens, the name for the human species which belongs to the genus Homo. See scientific classification for more details of this system. The type genus of a taxon is usually the first genus to be named and described. Families, and in plants all taxa up to division, are named after the type genus. The genus and these higher taxa are typified by a specimen that shows the characteristics of the genus. The specimen used to describe this species is preserved as the holotype and designated as a generitype in a zoological museum or a herbarium to be available for further study. A generic name in one kingdom is allowed to bear the same name as a genus or other taxon name in another kingdom (though this is discouraged by the International Code of Zoological Nomenclature). For instance, Anura is a genus of plants in the family Asteraceae and the order of frogs; Aotus is the genus of golden peas and night monkeys; Oenanthe is the genus of wheatears and water dropworts, and Prunella is the genus of accentors and self-heal. It is, however, not allowed for two genera within the same kingdom to have the same name. This explains why the platypus genus is Ornithorhynchus — although the name Platypus was chosen by George Shaw in 1799, that name had already been given to the ambrosia beetle by Johann Friedrich Wilhelm Herbst in 1793. Since beetles and platypuses are both member of the kingdom Animalia, the name Platypus could not be used for both. Johann Friedrich Blumenbach published the replacement name Ornithorhynchus in 1800.

Species

In biology, a species is the basic unit of biodiversity. In scientific classification, a species is assigned a two-part name in Latin. The genus is listed first (and capitalized), followed by a specific epithet. For example, humans belong to the genus Homo, and are in the species Homo sapiens. The name of the species is the whole binomial not just the second term (the specific epithet). The binomial, and most other purely formal aspects of the biological codes of nomenclature, were formalized by Carolus Linnaeus in the 1700's and as a result are called the "Linnaean system". At that time, species were thought to represent independent acts of creation by God, and were therefore considered objectively real and immutable. Since the advent of the theory of evolution, the conception of species has undergone vast changes in biology, however no consensus on the definition of the word has yet been reached. The most commonly cited definition of "species" was first coined by Ernst Mayr. By this definition, called the biological species concept or isolation species concept, species are "groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups". However, many other species concepts are also used (see other definitions of species below). The scientific name of a species is properly typeset in italics. When an unknown species is being referred to this may be done by using the abbreviation "sp." in the singular or "spp." in the plural in the place of the second part of the scientific name. Note that the word "specie" is not the singular of "species". It refers to coined money.

Definitions of species

The definition of a species given above as taken from Mayr, is somewhat idealistic. Since it assumes sexual reproduction, it leaves the term undefined for a large class of organisms that reproduce asexually. Biologists frequently do not know whether two morphologically similar groups of organisms are "potentially" capable of interbreeding. Further, there is considerable variation in the degree to which hybridization may succeed under natural and experimental conditions, or even in the degree to which some organisms use sexual reproduction between individuals to breed. Consequently, several lines of thought in the definition of species exist: ; Typological species : A group of organisms in which individuals are members of the species if they sufficiently conform to certain fixed properties. The clusters of variations or phenotypes within specimens (ie: longer and shorter tails) would differentiate the species. This method was used as a "classical" method of determining species, such as with Linnaeus early in evolutionary theory. However, we now know that different phenotypes do not always constitute different species (e.g.: a 4-winged Drosophila born to a 2-winged mother is not a different species). Species named in this manner are called morphospecies. ; Morphological species : A population or group of populations that differs morphologically from other populations. For example, we can distinguish between a chicken and a duck because they have different shaped bills and the duck has webbed feet. Species have been defined in this way since well before the beginning of recorded history. This species concept is much criticised because more recent genetic data reveals that genetically distinct populations may look very similar and, contrarily, large morphological differences sometimes exist between very closely-related populations. Nonetheless, most species known have been described solely from morphology. ; Biological / Isolation species : A set of actually or potentially interbreeding populations. This is generally the most useful formulation for scientists working with living examples of the higher taxa like mammals, fish, and birds, but meaningless for organisms that do not reproduce sexually. It does not distinguish between the theoretical possibility of interbreeding and the actual likelihood of gene flow between populations and is thus impractical in instances of allopatric (geographically isolated) populations. The results of breeding experiments done in artificial conditions may or may not reflect what would happen if the same organisms encountered each other in the wild, making it difficult to gauge whether or not the results of such experiments are meaningful in reference to natural populations. ; Mate-recognition species : A group of organisms that are known to recognise one another as potential mates. Like the isolation species concept above, it applies only to organisms that reproduce sexually. Unlike the isolation species concept, it focuses specifically on pre-mating reproductive isolation. ; Phylogenetic / Evolutionary / Darwinian species : A group of organisms that shares an ancestor; a lineage that maintains its integrity with respect to other lineages through both time and space. At some point in the progress of such a group, members may diverge from one another: when such a divergence becomes sufficiently clear, the two populations are regarded as separate species. ; Microspecies : Species that reproduce without meiosis or mitosis so that each generation is genetically identical to the previous generation. See also apomixis. In practice, these definitions often coincide, and the differences between them are more a matter of emphasis than of outright contradiction. Nevertheless, no species concept yet proposed is entirely objective, or can be applied in all cases without resorting to judgement. Given the complexity of life, some have argued that such an objective definition is in all likelihood impossible, and biologists should settle for the most practical definition. For most vertebrates, this is the biological species concept, and to a lesser extent (or for different purposes) the phylogenetic species concept. Many BSC subspecies are considered species under the PSC; the difference between the BSC and the PSC can be summed up insofar as that the BSC defines a species as a consequence of manifest evolutionary history, while the PSC defines a species as a consequence of manifest evolutionary potential. Thus, a PSC species is "made" as soon as an evolutionary lineage has started to separate, while a BSC species starts to exist only when the lineage separation is complete.

Importance in biological classification

The idea of species has a long history. It is one of the most important levels of classification, for several reasons:
- It often corresponds to what lay people treat as the different basic kinds of organism - dogs are one species, cats another.
- It is the standard binomial nomenclature (or trinomial nomenclature) by which scientists typically refer to organisms.
- It is the only taxonomic level which has empirical content, in the sense that asserting that two animals are of different species is saying something more than classificatory about them. After thousands of years of use, the concept remains central to biology and a host of related fields, and yet also remains at times ill-defined and controversial.

Implications of assignment of species status

The naming of a particular species should be regarded as a hypothesis about the evolutionary relationships and distinguishability of that group of organisms. As further information comes to hand, the hypothesis may be confirmed or refuted. Sometimes, especially in the past when communication was more difficult, taxonomists working in isolation have given two distinct names to individual organisms later identified as the same species. When two named species are discovered to be of the same species, the older species name is usually retained, and the newer species name dropped, a process called synonymization, or convivially, as lumping. Dividing a taxon into multiple, often new, taxons is called splitting. Taxonomists are often referred to as "lumpers" or "splitters" by their colleagues, depending on their personal approach to recognizing differences or commonalities between organisms (see lumpers and splitters). Traditionally, researchers relied on observations of anatomical differences, and on observations of whether different populations were able to interbreed successfully, to distinguish species; both anatomy and breeding behavior are still important to assigning species status. As a result of the revolutionary (and still ongoing) advance in microbiological research techniques, including DNA analysis, in the last few decades, a great deal of additional knowledge about the differences and similarities between species has become available. Many populations which were formerly regarded as separate species are now considered to be a single taxon, and many formerly grouped populations have been split. Any taxonomic level (species, genus, family, etc.) can be synonymized or split, and at higher taxonomic levels, these revisions have been still more profound. From a taxonomical point of view, groups within a species can be defined as being of a taxon hierarchically lower than a species. In zoology only the subspecies is used, while in botany the variety, subvariety, and form are used as well.

The isolation species concept in more detail

In general, for large, complex, organisms that reproduce sexually (such as mammals and birds), one of several variations on the isolation or biological species concept is employed. Often, the distinction between different species, even quite closely related ones, is simple. Horses (Equus caballus) and donkeys (Equus asinus) are easily told apart even without study or training, and yet are so closely related that they can interbreed after a fashion. Because the result, a mule or hinny, is not usually fertile, they are clearly separate species. But many cases are more difficult to decide. This is where the isolation species concept diverges from the evolutionary species concept. Both agree that a species is a lineage that maintains its integrity over time, that is diagnosably different to other lineages (else we could not recognise it), is reproductively isolated (else the lineage would merge into others, given the chance to do so), and has a working intra-species recognition system (without which it could not continue). In practice, both also agree that a species must have its own independent evolutionary history—otherwise the characteristics just mentioned would not apply. The species concepts differ in that the evolutionary species concept does not make predictions about the future of the population: it simply records that which is already known. In contrast, the isolation species concept refuses to assign the rank of species to populations that, in the best judgement of the researcher, would recombine with other populations if given the chance to do so.

The isolation question

There are, essentially, two questions to resolve. First, is the proposed species consistently and reliably distinguishable from other species? Secondly, is it likely to remain so in the future? To take the second question first, there are several broad geographic possibilities.
- The proposed species are sympatric—they occupy the same habitat. Observation of many species over the years has failed to establish even a single instance of two diagnostically different populations that exist in sympatry and have then merged to form one united population. Without reproductive isolation, population differences cannot develop, and given reproductive isolation, gene flow between the populations cannot merge the differences. This is not to say that cross breeding does not take place at all, simply that it has become negligible. Generally, the hybrid individuals are less capable of successful breeding than pure-bred individuals of either species.
- The proposed species are allopatric—they occupy different geographical areas. Obviously, it is not possible to observe reproductive isolation in allopatric groups directly. Often it is not possible to achieve certainty by experimental means either: even if the two proposed species interbreed in captivity, this does not demonstrate that they would freely interbreed in the wild, nor does it always provide much information about the evolutionary fitness of hybrid individuals. A certain amount can be inferred from other experimental methods: for example, do the members of population A respond appropriately to playback of the recorded mating calls of population B? Sometimes, experiments can provide firm answers. For example, there are seven pairs of apparently almost identical marine snapping shrimp (Altheus) populations on either side of the Isthmus of Panama, which did not exist until about 3 million years ago. Until then, it is assumed, they were members of the same seven species. But when males and females from opposite sides of the isthmus are placed together, they fight instead of mating. Even if the isthmus were to sink under the waves again, the populations would remain genetically isolated: therefore they are now different species. In many cases, however, neither observation nor experiment can produce certain answers, and the determination of species rank must be made on a 'best guess' basis from a general knowledge of other related organisms.
- The proposed species are parapatric—they have breeding ranges that abut but do not overlap. This is fairly rare, particularly in temperate regions. The dividing line is often a sudden change in habitat (an ecotone) like the edge of a forest or the snow line on a mountain, but can sometimes be remarkably trivial. The parapatry itself indicates that the two populations occupy such similar ecological roles that they cannot coexist in the same area. Because they do not crossbreed, it is safe to assume that there is a mechanism, often behavioral, that is preventing gene flow between the populations, and that therefore they should be classified as separate species.
- There is a hybrid zone where the two populations mix. Typically, the hybrid zone will include representatives of one or both of the 'pure' populations, plus first-generation and back-crossing hybrids. The strength of the barrier to genetic transmission between the two pure groups can be assessed by the width of the hybrid zone relative to the typical dispersal distance of the organisms in question. The dispersal distance of oaks, for example, is the distance that a bird or squirrel can be expected to carry an acorn; the dispersal distance of Numbats is about 15 kilometres, as this is as far as young Numbats will normally travel in search of vacant territory to occupy after leaving the nest. The narrower the hybrid zone relative to the dispersal distance, the less gene flow there is between the population groups, and the more likely it is that they will continue on separate evolutionary paths. Nevertheless, it can be very difficult to predict the future course of a hybrid zone; the decision to define the two hybridizing populations as either the same species or as separate species is difficult and potentially controversial.
- The variation in the population is clinal; at either extreme of the population's geographic distribution, typical individuals are clearly different, but the transition between them is seamless and gradual. For example, the Koalas of northern Australia are clearly smaller and lighter in colour than those of the south, but there is no particular dividing line: the further south an individual Koala is found, the larger and darker it is likely to be; Koalas in intermediate regions are intermediate in weight and colour. In contrast, over the same geographic range, black-backed (northern) and white-backed (southern) Australian Magpies do not blend from one type to another: northern populations have black backs, southern populations white backs, and there is an extensive hybrid zone where both 'pure' types are common, as are crossbreeds. The variation in Koalas is clinal (a smooth transition from north to south, with populations in any given small area having a uniform appearance), but the variation in magpies is not clinal. In both cases, there is some uncertainty regarding correct classification, but the consensus view is that species rank is not justified in either. The gene flow between northern and southern magpie populations is judged to be sufficiently restricted to justify terming them subspecies (not full species); but the seamless way that local Koala populations blend one into another shows that there is substantial gene flow between north and south. As a result, experts tend to reject even subspecies rank in this case.

The difference question

Obviously, when defining a species, the geographic circumstances become meaningful only if the populations groups in question are clearly different: if they are not consistently and reliably distinguishable from one another, then we have no grounds for believing that they might be different species. The key question in this context, is "how different is different?" and the answer is usually "it all depends". In theory, it would be possible to recognise even the tiniest of differences as sufficient to delineate a separate species, provided only that the difference is clear and consistent (and that other criteria are met). There is no universal rule to state the smallest allowable difference between two species, but in general, very trivial differences are ignored on the twin grounds of simple practicality, and genetic similarity: if two population groups are so close that the distinction between them rests on an obscure and microscopic difference in morphology, or a single base substitution in a DNA sequence, then a demonstration of restricted gene flow between the populations will probably be difficult in any case. More typically, one or other of the following requirements must be met:
- It is possible to reliably measure a quantitative difference between the two groups that does not overlap. A population has, for example, thicker fur, rougher bark, longer ears, or larger seeds than another population, and although this characteristic may vary within each population, the two do not grade into one another, and given a reasonably large sample size, there is a definite discontinuity between them. Note that this applies to populations, not individual organisms, and that a small number of exceptional individuals within a population may 'break the rule' without invalidating it. The less a quantitative difference varies within a population and the more it varies between populations, the better the case for making a distinction. Nevertheless, borderline situations can only be resolved by making a 'best-guess' judgement.
- It is possible to distinguish a qualitative difference between the populations; a feature that does not vary continuously but is either entirely present or entirely absent. This might be a distinctively shaped seed pod, an extra primary feather, a particular courting behaviour, or a clearly different DNA sequence. Sometimes it is not possible to isolate a single difference between species, and several factors must be taken in combination. This is often the case with plants in particular. In eucalypts, for example, Corymbia ficifolia cannot be reliably distinguished from its close relative Corymbia calophylla by any single measure (and sometimes individual trees cannot be definitely assigned to either species), but populations of Corymbia can be clearly told apart by comparing the colour of flowers, bark, and buds, number of flowers for a given size of tree, and the shape of the leaves and fruit. When using a combination of characteristics to distinguish between populations, it is necessary to use a reasonably small number of factors (if more than a handful are needed, the genetic difference between the populations is likely to be insignificant and is unlikely to endure into the future), and to choose factors that are functionally independent (height and weight, for example, should usually be considered as one factor, not two).

Historical development of the species concept

In the earliest works of science, a species was simply an individual organism that represented a group of similar or nearly identical organisms. No other relationships beyond that group were implied. Aristotle used the words genus and species to mean generic and specific categories. Aristotle and other pre-Darwinian scientists took the species to be distinct and unchanging, with an "essence", like the chemical elements. When early observers began to develop systems of organization for living things, they began to place formerly isolated species into a context. To the modern mind, many of the schemes delineated are whimsical at best, such as those that determined consanguinity based on color (all plants with yellow flowers) or behavior (snakes, scorpions and certain biting ants). In the 18th century Carolus Linnaeus classified organisms according to differences in the form of reproductive apparatus. Although his system of classification sorts organisms according to degrees of similarity, it made no claims about the relationship between similar species. At the time, it was still widely believed that there is no organic connection between species, no matter how similar they appear; every species was individually created by God, a view today called creationism. This approach also suggested a type of idealism: the notion that each species exists as an "ideal form". Although there are always differences (although sometimes minute) between individual organisms, Linnaeus considered such variation problematic. He strove to identify individual organisms that were exemplary of the species, and considered other non-exemplary organisms to be deviant and imperfect. By the 19th century most naturalists understood that species could change form over time, and that the history of the planet provided enough time for major changes. As such, the new emphasis was on determining how a species could change over time. Jean-Baptiste Lamarck suggested that an organism could pass on an acquired trait to its offspring, i.e., the giraffe's long neck was attributed to generations of giraffes stretching to reach the leaves of higher treetops (this well-known and simplistic example, however, does not do justice to the breadth and subtlety of Lamarck's ideas). Lamarck's most important insight may have been that species can be extraordinarily fluid; his 1809 Zoological Philosophy contained one of the first logical refutations of creationism. With the acceptance of the work of Charles Darwin in the 1860s, Lamarck's view of evolution was quickly eclipsed. It was not until the late 20th century that his work began to be reexamined, and took its place as a fundamental stepping stone to the modern theory of adaptive mutation. Lamarck's long-discarded ideas of the goal-oriented evolution of species, also known the teleological process, have also received renewed attention, particularly by proponents of artificial selection. Charles Darwin and Alfred Wallace provided what scientists now consider the most powerful and compelling theory of evolution. Basically, Darwin argued that it is populations that evolve, not individuals. His argument relies on a radical shift in perspective from Linnaeus: rather than defining species in ideal terms (and searching for an ideal representative and rejecting deviations), Darwin considered variation among individuals to be natural. He further argued that variation, far from being problematic, actually provides the explanation for the existence of distinct species. Darwin's work drew on Thomas Malthus' insight that the rate of growth of a biological population will always outpace the rate of growth of the resources in the environment, such as the food supply. As a result, Darwin argued, not all the members of a population will be able to survive and reproduce. Those that did will, on average, be the ones possessing variations—however slight—that make them slightly better adapted to the environment. If these variable traits are heritable, then the offspring of the survivors will also possess them. Thus, over many generations, adaptive variations will accumulate in the population, while counter-adaptive will be eliminated. It should be emphasized that whether a variation is adaptive or non-adaptive depends on the environment: different environments favor different traits. Since the environment effectively selects which organisms live to reproduce, it is the environment (the "fight for existence") that selects the traits to be passed on. This is the theory of evolution by natural selection. In this model, the length of a giraffe's neck would be explained by positing that proto-giraffes with longer necks would have had a significant reproductive advantage to those with shorter necks. Over many generations, the entire population would be a species of long-necked animals. In 1859, when Darwin published his theory of natural selection, the mechanism behind the inheritance of individual traits was unknown. Although Darwin made some speculations on how traits are inherited (pangenesis), his theory relies only on the fact that inheritable traits exist, and are variable (which makes his accomplishment even more remarkable.) Although Gregor Mendel's paper on genetics was published in 1866, its significance was not recognized. It was not until 1900 that his work was rediscovered by Hugo de Vries, Carl Correns and Erich von Tschermak, who realised that the "inheritable traits" in Darwin's theory are genes. The theory of the evolution of species through natural selection has two important implications for discussions of species -- consequences that fundamentally challenge the assumptions behind Linnaeus' taxonomy. First, it suggests that species are not just similar, they may actually be related. Some students of Darwin argue that all species are descended from a common ancestor. Second, it supposes that "species" are not homogeneous, fixed, permanent things; members of a species are all different, and over time species change. This suggests that species do not have any clear boundaries but are rather momentary statistical effects of constantly changing gene-frequencies. One may still use Linnaeus' taxonomy to identify individual plants and animals, but one can no longer think of species as independent and immutable. The rise of a new species from a parental line is called speciation. There is no clear line demarcating the ancestral species from the descendant species. Although the current scientific understanding of species suggests there is no rigorous and comprehensive way to distinguish between different species in all cases, biologists continue to seek concrete ways to operationalize the idea. One of the most popular biological definitions of species is in terms of reproductive isolation; if two creatures cannot reproduce to produce fertile offspring, then they are in different species. This definition captures a number of intuitive species boundaries, but nonetheless has some problems, however. It has nothing to say about species that reproduce asexually, for example, and it is very difficult to apply to extinct species. Moreover, boundaries between species are often fuzzy: there are examples where members of one population can produce fertile offspring with a second population, and members of the second population can produce fertile offspring with members of a third population, but members of the first and third population cannot produces fertile offspring. Consequently, some people reject this notion of species. In recent years we have witnessed the drastic reduction in the size of breeding populations and the geographical range of many physically large mammals. In earlier times it was assumed that every species existed in at least a few thousand living individuals, except very rare relic, isolated groups. In the present, many well know mammal & bird species are so stressed by habitat loss, and other effects of the modern world, that only a very few breeding males may contribute the genetic material to a small number of breeding females. In these highly stressed conditions, the likelihood of change is very much greater. Mammals may become smaller, have darker fur, more stripes, more cautious behavior, even over time learn to co-exist with the human world. Very likely, evolution is radically accelerated, and we are only beginning to notice it. Species in transition before our eyes. It is possible that this severe stress is essential to the creation of new species, and may have been a prime factor throughout biological history, from other population reducing influences. Richard Dawkins defines two organisms as conspecific if and only if they have the same number of chromosomes and, for each chromosome, both organisms have the same number of nucleotides (The Blind Watchmaker, p. 118). However, most if not all taxonomists would strongly disagree. For example, in many amphibians, most notably in New Zealand's Leiopelma frogs, the genome consists of "core" chromosomes which are mostly invariable and accessory chromosomes, of which exist a number of possible combinations. Even though the chromosome numbers are highly variable between populations, these can interbreed successfully and form a single evolutionary unit. In plants, polyploidy is extremely commonplace with few restrictions on interbreeding; as individuals with an odd number of chromosome sets are usually sterile, depending on the actual number of chromosome sets present, this results in the odd situation where some individuals of the same evolutionary unit can interbreed with certain others and some cannot, with all populations being eventually linked as to form a common gene pool. The classification of species has been profoundly affected by technological advances that have allowed researchers to determine relatedness based on molecular markers, starting with the comparatively crude blood plasma precipitation assays in the mid-20th century and coming into full swing with Charles Sibley's ground-breaking DNA-DNA hybridisation studies in the 1970s. The results of the technique caused revolutionary changes in the higher taxonomic categories (such as phyla and classes), resulting in the reordering of many branches of the phylogenetic tree (see also: molecular phylogeny). For taxonomic categories below genera, the results have been mixed so far; the pace of evolutionary change on the molecular level is rather slow, yielding clear differences only after considerable periods of reproductive separation. Instances of hybridization can result in misleading molecular data, the Pomarine Skua - Great Skua phenomenon being a famous example. Turtles have been determined to evolve with just one-eighth of the speed of other reptiles on the molecular level, and the rate of molecular evolution in albatrosses is half of what is found in the rather closely related storm-petrels. The hybridization technique is however no longer considered a good technique and more reliable computational techniques for sequence comparison are now used for. Molecular taxonomy does not directly measure the evolutionary processes, but rather the overall change brought upon by these processes. The processes that lead to the generation and maintenance of variation such as mutation, crossover and selection are not uniform (see also molecular clock). DNA is only extremely rarely a direct target of natural selection rather than changes in the DNA sequence enduring over generations being a result of the latter; for example, silent transition-transversion combinations would alter the melting point of the DNA sequence, but not the sequence of the encoded proteins and thus are a possible example where, for example in microorganisms, a mutation confers a change in fitness all by itself.

External links

- http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/S/Speciation.html
- [http://www.sciencedaily.com/releases/2003/12/031231082553.htm 2003-12-31, ScienceDaily: Working On The 'Porsche Of Its Time': New Model For Species Determination Offered] Quote: "...two species of dinosaur that are members of the same genera varied from each other by just 2.2 percent. Translation of the percentage into an actual number results in an average of just three skeletal differences out of the total 338 bones in the body. Amazingly, 58 percent of these differences occurred in the skull alone. "This is a lot less variation than I'd expected", said Novak..."
- [http://www.sciencedaily.com/releases/2003/08/030808081854.htm 2003-08-08, ScienceDaily: Cross-species Mating May Be Evolutionarily Important And Lead To Rapid Change, Say Indiana University Researchers] Quote: "...the sudden mixing of closely related species may occasionally provide the energy to impel rapid evolutionary change..."
- [http://www.sciencedaily.com/releases/2004/01/040109064407.htm 2004-01-09 ScienceDaily: Mayo Researchers Observe Genetic Fusion Of Human, Animal Cells; May Help Explain Origin Of AIDS] Quote: "...The researchers have discovered conditions in which pig cells and human cells can fuse together in the body to yield hybrid cells that contain genetic material from both species... "What we found was completely unexpected", says Jeffrey Platt, M.D."
- [http://www.sciencedaily.com/releases/2000/09/000913211733.htm 2000-09-18, ScienceDaily: Scientists Unravel Ancient Evolutionary History Of Photosynthesis] Quote: "...gene-swapping was common among ancient bacteria early in evolution..."
- [http://plato.stanford.edu/entries/species/ Stanford Encyclopedia of Philosophy entry]
- [http://www.barcodinglife.org/ Barcoding of species] rank22 rank22 ms:Spesies ja:種 (生物) th:สปีชีส์

Bulbophyllum

List of Bulbophyllum species
1805 species, including:
Bulb. barbigerum (Bearded Bulbophylllum)
Bulb. beccarii
Bulb. crassulifolium (Wheat-leaved Bulbophyllum)
Bulb. falcatum
Bulb. flaviflorum (Golden comb orchid)
Bulb. globuliforme
Bulb. macphersonii
Bulb. macraei (Wu-lai cirrous orchid)
Bulb. makayanum (Daisy orchid)
Bulb. medusae
Bulb. pachyrhachis (Rattail orchid)
Bulb. purpurea (Lizard orchid)
Bulb. retusiusculum (Yellow comb orchid)
Bulb. transarisanense (Lily orchid)
Bulbophyllum Thouars 1822, is the largest genus in the orchid family Orchidaceae. With 1805 species, it is also one of the largest genera in the entire plant kingdom, exceeded only by Senecio and Euphorbia. This genus covers an incredible range of vegetative form, from tall plants with cane like stems, to root climbers that wind or creep their way up tree trunks. Other members are pendulous epiphytes (growing on other plants), and quite a number that have developed succulent foliage to a greater or lesser degree. One species has almost become leafless and uses its pseudobulbs as the organs of photosynthesis. The flower form, though diverse, has a basic structural blueprint that serves to identify this genus. The center of diversity is in Papua New Guinea which seems to be the evolutionary homeland, though the genus is widespread occurring in Australia, Southeast Asia, Africa and in tropical central and South America. Some species are known for their extremes of vegetive and floral form:
- Bulbophyllum beccarii
- Bulbophyllum barbigerum
- Bulbophyllum falcatum
- Bulbophyllum globuliforme
- Bulbophyllum macphersonii
- Bulbophyllum medusae New species are being discovered every year, such as
- Bulbophyllum ciluliae Bianch. & J.A.N.Bat., Sitientibus 2004
- Bulbophyllum orezii Sath.Kumar 2004

Gallery

Image:Bulbophyllum-falcatum.jpg|Bulbophyllum falcatum Image:Bulbophyllum-lepidum.jpg|Bulbophyllum lepidum Image:Bulbophyllum-putidum.jpg|Bulbophyllum putidum Image:Bulbophyllum-pectenveneris.jpg|Bulbophyllum pectenveneris Category:Australian orchids Category:Orchid genera

United States

:For alternative meanings, see the disambiguation page for US, USA, United States, or American. The United States of America is a federal democratic republic situated primarily in central North America. It comprises 50 states and one federal district, and has several territories. It is also referred to, with varying formality, as the United States, the U.S., the U.S.A., the States, or simply and most commonly, America. The official founding date of the United States is July 4, 1776, when the Second Continental Congress—representing thirteen British colonies—adopted the Declaration of Independence. However, the structure of the government was profoundly changed in 1788, when the states replaced the Articles of Confederation with the United States Constitution. The date on which each of the fifty states adopted the Constitution is typically regarded as the date that state "entered the Union" (became part of the United States). Since the mid-20th century, following World War II, the United States has emerged as a dominant global influence in economic, political, military, scientific, technological, and cultural affairs.

Geography and climate

The United States shares land borders with Canada (to the north) and Mexico (to the south), and territorial water boundaries with Canada, Russia, the Bahamas, and numerous smaller nations. It is otherwise bounded by the Pacific Ocean and the Bering Sea, in the west; the Arctic Ocean, in the northernmost areas; and the Atlantic Ocean, the Gulf of Mexico, and the Caribbean Sea, in the eastern and southeastern areas. Forty-eight of the states are in the single region between Canada and Mexico; this group is referred to, with varying precision and formality, as the continental or contiguous United States, sometimes abbreviated CONUS, and as the Lower 48. Alaska, which is not included in the term contiguous United States, is at the northwestern end of North America, separated from the Lower 48 by Canada. The archipelago of Hawaii is in the Pacific Ocean. The capital city, Washington, District of Columbia is a federal district located on land donated by the state of Maryland. (Virginia also donated land, but it was returned in 1847.) The United States also has overseas territories with varying levels of independence and organization. When inland water is included in the total area, only Russia and Canada are larger than the United States; if inland water is excluded, China ranks third and the U.S. ranks fourth. The United States' total area is 3,718,711 square miles (9,631,418 km²), of which land makes up 3,537,438 square miles (9,161,923 km²) and water makes up 181,273 square miles (469,495 km²). The United States' landscape is one of the most varied among those of the world's nations: among its many features are temperate forestland and rolling hills, on the east coast; mangrove, in Florida; the Great Plains, in the center of the country; the Mississippi–Missouri river system; the Great Lakes, four of the five of which are shared with Canada; the Rocky Mountains, west of the Great Plains; deserts and temperate coastal zones, west of the Rocky Mountains; and temperate rain forests, in the Pacific northwest. Alaska's tundra, and the volcanic, tropical islands of Hawaii add to the geographic diversity. Hawaii The climate varies along with the landscape, from tropical in Hawaii and southern Florida to tundra in Alaska and atop some of the highest mountains. Most of the North and East experience a temperate continental climate, with warm summers and cold winters. Most of the South experiences a subtropical humid climate with mild winters and long, hot, humid summers. Rainfall decreases markedly from the humid forests of the Eastern Great Plains to the semi-arid shortgrass prairies on the high plains abutting the Rocky Mountains. Arid deserts, including the Mojave, extend through the lowlands and valleys of the southwest, from westernmost Texas to California and northward throughout much of Nevada. Some parts of California have a Mediterranean climate. Rainforests line the windward mountains of the Pacific Northwest from Oregon to Alaska.

History

American history started with the migration of people from Asia across the Bering land bridge approximately 12,000 years ago following large animals that they hunted into the Americas. These Native Americans left evidence of their presence in petroglyphs, burial mounds, and other artifacts. It is estimated that 2-9 million people lived in the territory now occupied by the U.S. before European contact, and the subsequent introduction of foreign diseases such as small pox that greatly diminished the native populations. Some advanced societies were the Anasazi of the southwest, who inhabited Chaco Canyon, and the Woodland Indians, who built Cahokia, located near present-day St Louis, a city with a population of 40,000 at its peak in AD 1200. Vikings first visited North America around 1000, but did not settle permanently. Following the discovery voyages of Christopher Columbus around 1492, other Europeans began to explore and settle there. During the 1500s and 1600s, the Spanish settled parts of the present-day Southwest and Florida, founding St. Augustine, Florida in 1565 and Santa Fe (in what is now New Mexico) in 1607. The first successful English settlement was at Jamestown, Virginia, also in 1607. Within the next two decades, several Dutch settlements, including New Amsterdam (the predecessor to New York City), were established in what are now the states of New York and New Jersey. In 1637, Sweden established a colony at Fort Christina (in what is now Delaware), but lost the settlement to the Dutch in 1655. This was followed by extensive British settlement of the east coast. The British colonists remained relatively undisturbed by their home country until after the French and Indian War, when France ceded Canada and the Great Lakes region to Britain. Britain then imposed taxes on the 13 colonies, widely regarded by the colonists as unfair because they were denied representation in the British Parliament. Tensions between Britain and the colonists increased, and the thirteen colonies eventually rebelled against British rule. British Parliament, George Washington (1789-1797).]] In 1776, the 13 colonies split from Great Britain and formed the United States, the world's first constitutional and democratic federal republic, after their Declaration of Independence of that year, and the Revolutionary War (1775 to 1783). The original political structure was a confederation in 1777, ratified in 1781 as the Articles of Confederation. After long debate, this was supplanted by the Constitution in 1789, forming a more centralized federal government. Prior to all these was the Albany Congress in 1754, in which a union was first seriously proposed. From early colonial times, there was a shortage of labor, which encouraged unfree labor, particularly indentured servitude and slavery. In the mid-19th century, a major division occurred in the United States over the issue of states' rights and the expansion of slavery. The northern states had become opposed to slavery, while the southern states saw it as necessary for the continued success of southern agriculture and wanted it expanded to the territories. Several federal laws were passed in an attempt to settle the dispute, including the Missouri Compromise and the Compromise of 1850. The dispute reached a crisis in 1861, when seven southern states seceded¹ from the Union and formed the Confederate States of America, leading to the Civil War. Soon after the war began, four more southern states seceded. During the war, Abraham Lincoln issued the Emancipation Proclamation, mandating the freedom of all slaves in states in rebellion, though full emancipation did not take place until after the end of the war in 1865, the dissolution of the Confederacy, and the Thirteenth Amendment took effect. The Civil War effectively ended the question of a state's right to secede, and is widely accepted as a major turning point after which the federal government became more powerful than state governments. Thirteenth Amendment). The title of the painting, from a 1726 poem by Bishop Berkeley, was a phrase often quoted in the era of Manifest Destiny, expressing a widely held belief that civilization had steadily moved westward throughout history. [http://americanart.si.edu/t2go/1lw/1931.6.1.html (more)] ]] During the 19th century, many new states were added to the original 13 as the nation expanded across the continent. Manifest Destiny was a philosophy that encouraged westward expansion in the United States. As the population of the Eastern states grew and as a steady increase of immigrants entered the country, settlers moved steadily westward across North America. In the process, the U.S. displaced most American Indian nations. This displacement of American Indians continues to be a matter of contention in the U.S. with many tribes attempting to assert their original claims to various lands. In some areas American Indian populations were reduced by foreign diseases contracted through contact with European settlers, and US settlers acquired those emptied lands. In other instances American Indians were removed from their traditional lands by force. Though some would say the U.S. was not a colonial power until the Spanish-American War when it acquired Puerto Rico, Guam and the Philippines, the dominion exercised over land in North America the United States claimed is essentially colonial. The Philippines became independent in 1946. During this period, the nation also became an industrial power. This continued into the 20th century, which has been termed "the American Century" because of the nation's overriding influence on the world. The US became a center for innovation and technological development; major technologies that America either developed or was greatly involved in improving include the telephone, television, computer, the Internet, nuclear weapons, nuclear power, aviation, and aeronautics. In addition to the Civil War, another major traumatic experience for the nation was the Great Depression (1929 to 1939). The nation has also taken part in several major foreign wars, including World War I and World War II (in both of which the US later joined the Allies). During the Cold War, the US was a major player in the Korean War and Vietnam War, and, along with the Soviet Union, was considered one of the world's two "superpowers". With the collapse of the Soviet Union, the US emerged as the world's leading economic and military power. Beginning in the 1990s, the United States became very involved in police actions and peacekeeping, including actions in Kosovo, Haiti, Somalia and Liberia, and the first Persian Gulf War driving Iraq out of Kuwait. After attacks on the World Trade Center and the Pentagon on September 11, 2001, the United States and other allied nations found themselves involved in what has come to be called the "War on Terrorism," which has primarily encompassed military actions in both Afghanistan and Iraq.

Government

Iraq of the United States.]]

Republic and suffrage

The United States is an example of a constitutional republic, with a government composed of and operating through a set of limited powers imposed by its design and enumerated in the United States Constitution. Specifically, the nation operates as a presidential democracy. There are three levels of government: federal, state, and local. Officials of each of these levels are either elected by eligible voters via secret ballot or appointed by other elected officials. Americans enjoy almost universal suffrage from the age of 18 regardless of race, sex, or wealth. There are some limits, however: felons are disenfranchised and in some states former felons are likewise. Furthermore, the national representation of territories and the federal district of Washington, DC in Congress is limited: residents of the District of Columbia are subject to federal laws and federal taxes but their only Congressional representative is a non-voting delegate.

Federal government

The federal government is the national government, comprising the Legislative Branch (led by Congress), the Executive Branch (led by the President), and the Judicial Branch (led by the Supreme Court). These three branches were designed to apply checks and balances on each other. The Constitution limits the powers of the federal government to defense, foreign affairs, the issuing and management of currency, the management of trade and relations between the states, and the protection of human rights. In addition to these explicitly stated powers, the federal government—with the assistance of the Supreme Court—has gradually extended these powers into such areas as welfare and education, on the basis of the "necessary and proper" clause of the Constitution.

The Congress

necessary and proper The Congress of the United States is the legislative branch of the federal government of the United States. It is bicameral, comprising the House of Representatives and the Senate. The House of Representatives consists of 435 members, each of whom represents a congressional district and serves for a two-year term. House seats are apportioned among the states by population; in contrast, each state has two Senators, regardless of population. There are a total of 100 senators, who serve six-year terms. The powers of Congress are limited to those enumerated in the Constitution; all other powers are reserved to the states and the people. The Constitution also includes the necessary-and-proper clause, which grants Congress the power to "make all laws which shall be necessary and proper for carrying into execution the foregoing powers."

The President

necessary-and-proper clause At the top level of the executive branch is the President of the United States. The President and Vice-President are elected as 'running mates' for four-year terms by the Electoral College, for which each state, as well as the District of Columbia, is allocated a number of seats based on its representation (or ostensible representation, in the case of D. C.) in both houses of Congress (see U.S. Electoral College). The relationship between the President and the Congress reflects that between the English monarchy and parliament at the time of the framing of the United States Constitution. Congress can legislate to constrain the President's executive power, even with respect to his or her command of the armed forces; however, this power is used only very rarely—a notable example was the constraint placed on President Richard Nixon's strategy of bombing Cambodia during the Vietnam War. The President cannot directly propose legislation, and must rely on supporters in Congress to promote his or her legislative agenda. The President's signature is required to turn congressional bills into law; in this respect, the President has the power—only occasionally used—to veto congressional legislation. Congress can override a presidential veto with a two-thirds majority vote in both houses. The ultimate power of Congress over the President is that of impeachment or removal of the elected President through a House vote, a Senate trial, and a Senate vote. The threat of using this power has had major political ramifications in the cases of Presidents Andrew Johnson, Richard Nixon, and Bill Clinton. The President makes around 2,000 executive appointments, including members of the Cabinet and ambassadors, which must be approved by the Senate; the President can also issue executive orders and pardons, and has other Constitutional duties, among them the requirement to give a State of the Union address to Congress once a year. Although the President's constitutional role may appear to be constrained, in practice, the office carries enormous prestige that typically eclipses the power of Congress: the Presidency has justifiably been referred to as 'the most powerful office in the world'. The Vice President is first in the line of succession, and is the President of the Senate ex officio, with the ability to cast a tie-breaking vote. The members of the President's Cabinet are responsible for administering the various departments of state, including the Department of Defense, the Justice Department, and the State Department. These departments and department heads have considerable regulatory and political power, and it is they who are responsible for executing federal laws and regulations. George W. Bush is the 43rd President, currently serving his second term.

The Courts

George W. Bush The highest court is the Supreme Court, which consists of nine justices. The court deals with federal and constitutional matters, and can declare legislation made at any level of the government as unconstitutional, nullifying the law and creating precedent for future law and decisions. Below the Supreme Court are the courts of appeals, and below them in turn are the district courts, which are the general trial courts for federal law. Separate from, but not entirely independent of, this federal court system are the individual court systems of each state, each dealing with its own laws and having its own judicial rules and procedures. A case may be appealed from a state court to a federal court only if there is a federal question; the supreme court of each state is the final authority on the interpretation of that state's laws and constitution.

State and local governments

supreme court of each state. Note that Alaska and Hawaii are shown at different scales, and that the Aleutian Islands and the uninhabited Northwestern Hawaiian Islands are omitted from this map.]] The state governments have the greatest influence over people's daily lives. Each state has its own written constitution and has different laws. There are sometimes great differences in law and procedure between the different states, concerning issues such as property, crime, health, and education. The highest elected official of each state is the Governor. Each state also has an elected legislature (bicameral in every state except Nebraska), whose members represent the different parts of the state. Of note is the New Hampshire legislature, which is the third-largest legislative body in the English-speaking world, and has one representative for every 3,000 people. Each state maintains its own judiciary, with the lowest level typically being county courts, and culminating in each state supreme court, though sometimes named differently. In some states, supreme and lower court justices are elected by the people; in others, they are appointed, as they are in the federal system. The institutions that are responsible for local government are typically town, city, or county boards, making laws that affect their particular area. These laws concern issues such as traffic, the sale of alcohol, and keeping animals. The highest elected official of a town or city is usually the mayor. In New England, towns operate directly democratically, and in some states, such as Rhode Island and Connecticut, counties have little or no power, existing only as geographic distinctions. In other areas, county governments have more power, such as to collect taxes and maintain law enforcement agencies.

Political divisions

With the Declaration of Independence, the thirteen colonies proclaimed themselves to be nation states modeled after the European states of the time. Although considered as sovereigns initially, under the Articles of Confederation of 1781 they entered into a "Perpetual Union" and created a fully sovereign federal state, delegating certain powers to the national Congress, including the right to engage in diplomatic relations and to levy war, while each retaining their individual sovereignty, freedom and independence. But the national government proved too ineffective, so the administrative structure of the government was vastly reorganized with the United States Constitution of 1789. Under this new union, the continued status of the individual states as sovereign nation states fell into dispute in 1861, as several states attempted to secede from the union; in response, then-President Abraham Lincoln claimed that such secession was illegal, and the result was the American Civil War. Since the Union victory in 1865, the independent status of the individual states has not been broached again by any state, and the status of each state within the union has been deemed by mainstream officials and academics to be settled as being subordinate to the union as a whole. In subsequent years, the number of states grew steadily due to western expansion, the purchase of lands by the national government from other nation states, and the subdivision of existing states, resulting in the current total of 50. The states are generally divided into smaller administrative regions, including counties, cities and townships. The United States–Canadian border is the longest undefended political boundary in the world. The U.S. is divided into three distinct sections:
- the "continental United States," also known as "the Lower 48" and more accurately termed the conterminous, coterminous or contiguous United States
- Alaska, which is physically connected only to Canada
- the archipelago of Hawaii, in the central Pacific Ocean. The United States also holds several other territories, districts, and possessions, notably the federal district of the District of Columbia, which is the nation's capital, and several overseas insular areas, the most significant of which are American Samoa, Guam, the Northern Mariana Islands, Puerto Rico, and the United States Virgin Islands. The Palmyra Atoll is the United States' only incorporated territory; it is unorganized and uninhabited. The United States Navy has held a base at a portion of Guantanamo Bay, Cuba, since 1898. The United States government possesses a lease to this land, which only mutual agreement or United States abandonment of the area can terminate. The present Cuban government of Fidel Castro disputes this arrangement, claiming Cuba was not truly sovereign at the time of the signing. The United States argues this point moot because Cuba apparently ratified the lease post-revolution, and with full sovereignty, when it cashed one rent check in accordance with the disputed treaty.

Foreign relations and military

sovereign] The immense military and economic dominance of the United States has made foreign relations an especially important topic in its politics, with considerable concern about the image of the United States throughout the world. Reactions towards the United States by other nationalities are often strong, ranging from uninhibited admiration and mimicking of all things American to anti-Americanism. US foreign policy has swung about several times over the course of its history between the poles of strict isolationism and imperialism and everywhere in between. Three of the nation's four military branches are administered by the Department of Defense: the Army, the Navy (including the Marine Corps), and the Air Force. The Coast Guard falls under the jurisdiction of the Department of Homeland Security in peacetime, but is placed under the Department of the Navy in time of war. The combined United States armed forces consist of 1.4 million active duty personnel, along with several hundred thousand each in the Reserves and the National Guard. Military conscription ended in 1973. The United States Armed forces are considered to be the most powerful military (of any sort) on Earth and their force projection capabilities are unrivaled by any other nation. The 2005 defense budget amounted to $401.7 billion, which is an increase of 4% over 2004 and of 35% since 2001. Over 50% of that number is spent in research & development. (For comparison, in 2004 the European Union (considered as the second-largest military force) had a combined total of 1.6 million troops, and a defense budget of €160 billion, with less than 10% of that being spent on R&D.)

Largest cities

The United States has dozens of major cities, including 11 of the 55 global cities of all types — with three "alpha" global cities: New York City, Los Angeles, and Chicago. The figures expressed below are for populations within city limits. A different ranking is evident when considering U.S. metro area populations, although the top three would be unchanged. Note that some cities not listed (such as Atlanta, Boston, Las Vegas, Miami, Nashville, New Orleans, Seattle, and Washington, D.C.) are still considered important on the basis of other factors and issues, including culture, economics, heritage, and politics. The twenty largest cities, based on the United States Census Bureau's 2004 estimates, are as follows:

Economy

The United States has the largest single-country economy in the world, with a per-capita gross domestic product of $40,100. In this market-oriented economy, private individuals and business firms make most of the decisions, and the federal and state governments buy needed goods and services predominantly in the private marketplace. gross domestic product The largest industry of the U.S. is now service, which employs roughly three quarters of the U.S. work force. The United States has many natural resources, including oil and gas, metals, and such minerals as gold, soda ash, and zinc. In agriculture, the U.S. is a top producer of, among other crops, corn, soy beans, and wheat; the United States is a net exporter of food. The U.S. manufacturing sector produces goods such as, cars, airplanes, steel, and electronics, among many others. Economic activity varies greatly from one part of the country to another, with many industries being largely dependent on a certain city or region; New York City is the center of the American financial, publishing, broadcasting, and advertising industries; Silicon Valley is the country’s primary location for high-technology companies, while Los Angeles is the most important center for film production. The Midwest is known for its reliance on manufacturing and heavy industry, with Detroit, Michigan, serving as the center of the American automotive industry; the Great Plains are known as the "breadbasket" of America for their tremendous agricultural output; the intermountain region serves as a mining hub and natural gas resource; the Pacific Northwest for fish and timber, while Texas is largely associated with the oil industry; the Southeast is a major hub for both medical research and the textiles industry. Several countries continue to link their currency to the dollar or even use it as a currency (such as Ecuador), although this practice has subsided since the collapse of the Bretton Woods system. Many markets are also quoted in dollars, such as those of oil and gold. The dollar is also the predominant reserve currency in the world, and more than half of global reserves are in dollars. The largest trading partner of the United States is Canada (19%), followed by China (12%), Mexico (11%), and Japan (8%). More than 50% of total trade is with these four countries. In 2003, the United States was ranked as the third most visited tourist destination in the world; its 40,400,000 visitors ranked behind France's 75,000,000 and Spain's 52,500,000. Labor unions have existed since the 19th century, and grew large and powerful from the 1930s to the 1950s. See Labor history of the United States. Since 1970 they have shrunk in the private sector and now cover fewer than 8% of the workers. However union membership has grown rapidly in the public sector, especially among teachers, nurses, police, postal workers, and municipal clerks. There have been few strikes in recent years. The United States' imports exceed exports by 80%, leading to an annual trade deficit of $700,000,000,000, or 6% of gross domestic product. It is the largest debtor nation in the world, with total gross foreign debt of over $13,000,000,000,000 (2005 estimate); and it absorbs more than 50% of global savings annually. Since the 1980s, the U.S. has increased the use of neoliberal economic policies that reduce government intervention and reduce the size of the welfare state, backing away from the more interventionist Keynsian economic policies that had been in favor since the Great Depression. As a result, the United States provides fewer government-delivered social welfare services than most industrialized nations, choosing instead to keep its tax burden lower and relying more heavily on the free market and private charities. Sixteen states and the District of Columbia have minimum wages higher than the national level ($5.15 per-hour), including the highest, Washington State at $7.35. Twenty-six states are the same as the federal level; two--Ohio and Kansas--are below; and six do not have state laws. America's wealth is relatively highly concentrated. The average C.E.O. earns 500 times the typical amount a worker grosses, this is up from 25 times in the late 1970s. In terms of wealth the top 1% of Americans own 40% of all assets and 50.1% of the country's income goes to the top twenty percent of households. Average wages for the majority of employees have been largely stagnating since the 1970s. America's poverty line defined as a family of four earning less than $19,157 is at 12.7% of the general population. Approximately one out of every five children in the United States grows up below the official poverty line. Among racial groups; African Americans have the lowest median income while Asians had the highest. Regionally, the southern states had the lowest median incomes while the West Coast and New England had the highest. The current Federal Reserve Chairman Alan Greenspan remarked that the U.S.’s growing income inequality since the 1970s is, "not the type of thing which a democratic society - a capitalist democratic society - can really accept without addressing."[http://www.csmonitor.com/2005/0614/p01s03-usec.html?s=itm] However, Greenspan also noted, "...you can look at the system and say it's got a lot of problems to it, and sure it does. It always has. But you can't get around the fact that this is the most extraordinarily successful economy in history."

Transportation

Alan Greenspan ]] Because the United States is a relatively young nation, most of the development of U.S. cities has taken place since the invention of the automobile. To link its vast territory, the United States built a network of high-capacity, high-speed highways, of which the most important element is the Interstate Highway system, commissioned in the 1950s by President Dwight D. Eisenhower and modeled after the German Autobahn. The United States also has a transcontinental rail system, which is used for moving freight across the lower forty-eight states. Passenger rail service is provided by Amtrak, which serves forty-six of the lower forty-eight states. Many cities in the United States have extensive mass-transit systems. New York City operates one of the world's largest and most heavily used subway systems. The regional rail and bus networks that extend into Long Island, New Jersey, Upstate New York, and Connecticut are among the most heavily used in the world. Air travel is often preferred for destinations over 300 miles (500 kilometers) away. In terms of passengers, seventeen of the world's thirty busiest airports in 2004 were in the U.S., including the world's busiest, Hartsfield-Jackson Atlanta International Airport; in terms of cargo, in the same year, twelve of the world's thirty busiest airports were in the U.S., including the world's busiest, Memphis International Airport. There are several major seaports in the United States; the three busiest are the Port of Los Angeles, California; the Port of Long Beach, California; and the Port of New York and New Jersey. Others include Houston, Texas; Charleston, South Carolina; Savannah, Georgia; Miami, Florida; Portland, Oregon; San Francisco, California; Boston, Massachusetts; Philadelphia, Pennsylvania; and Seattle, Washington; plus, outside the contiguous forty-eight states, Anchorage, Alaska, and Honolulu, Hawaii.

Society

Demographics

Hawaii The mean center of the U.S. population continues to drift farther west and south. The fastest growing region is the western United States followed by the southern portion. According to Census 2000, the states that saw the greatest increases from 1990 were: Nevada (66.3%), Arizona (40%), Colorado (30.6%), Utah (29.6%), Idaho (28.5%), Georgia (26.4%), Florida (23.5%), Texas (22.8%), North Carolina (21.4%), and Washington (21.1%). [http://www.census.gov/population/cen2000/phc-t2/tab03.pdf]

Ethnicity and race

:Main article: Racial demographics of the United States The United States is a very racially diverse country. According to the 2000 census, it has 31 ethnic groups with at least one million members each, and numerous others represented in smaller amounts. The majority of Americans descend from white European immigrants who arrived at the establishment of the first colonies (most after Reconstruction). This majority--69.1% in 2000--decreases each year, and is expected to become a plurality within a few decades. The most frequently stated European ancestries are German (15.2%), Irish (10.8%), English (8.7%), Italian (5.6%) and Scandinavian (3.7%). Many immigrants also hail from Slavic countries such as Poland and Russia. Other significant immigrant populations came from eastern and southern Europe and French Canada. Russia Hispanics from Mexico and South and Central America are the largest minority group in the country, comprising 12.5% of the population (2000 census). People of Mexican descent made up 7.3% of the population in the 2000 census, and this proportion is expected to increase significantly in the coming decades. About 12.3% (2000 census) of the American people are African Americans (Blacks). African Americans are spread throughout the country, but their presence is largest in the South. Asian Americans--including Native Hawaiians and Pacific Islanders--are a third significant minority (3.7% of the population in 2000). Most Asian Americans are concentrated on the West Coast and Hawaii. The largest groups are immigrants or descendants of emigrants from the Philippines, China, India, Vietnam, South Korea, and Japan. Indigenous peoples in the United States, such as American Indians and Inuit, make up 0.9% of the population (2000 census). About 35% live on Indian reservations.

Religion

Polls estimate that just under 80 percent of Americans are Christians of various denominations. The other 20 percent comprises other religions such as Hinduism, Judaism, Islam, and Buddhism, other various faiths, and those without a specific religion. The United States is noteworthy among developed nations for its relatively high level of religiosity. According to a 2004 Gallup poll, about 44% of Americans attend a religious service at least once a week. However, this rate is not uniform across the country; attendance is more common in the Bible Belt—composed largely of Southern and Midwestern states—than in the Northeast and West Coast. In the Southern states, Baptists are the largest group, followed by Methodists; Roman Catholics are dominant in the Northeast and in large parts of the Midwest due to their being settled by descendants of Catholic immigrants from Europe (such as Germany, Ireland, Italy, and Poland) or other parts of North America (mainly Quebec and Puerto Rico). The rest of the country for the most part has a complex mixture of various Christian groups.

Education

West Coast's home at Monticello and the University of Virginia (library building shown above, and designed by Jefferson), the only collegiate campus on the list. Both sites are located in Charlottesville, Virginia.]] In the United States, education is a state, not federal, responsibility, and the laws and standards vary considerably. However, the federal government, through the Department of Education, is involved with funding of some programs and exerts some influence through its ability to control funding. In most states, all students must attend mandatory schooling starting with kindergarten, which children normally enter at age 5, and following through 12th grade, which is normally completed at age 18

Tropics

The tropics are the geographic region of the Earth centered on the equator and limited in latitude by the two tropics: the Tropic of Cancer in the northern hemisphere and the Tropic of Capricorn in the southern hemisphere. This area lies approximately between 23°30'/23.5° N latitude and 23°30'/23.5° S latitude, and includes all the parts of the Earth where the sun reaches a point directly overhead at least once during the solar year. (In the temperate zones, north of the Tropic of Cancer and south of the Tropic of Capricorn, the sun never reaches an altitude of 90° or directly overhead.) The word "tropics" comes from Greek tropos meaning "turn", because the apparent position of the Sun oscillates between the two tropics with a period that defines the average length of a year. Tropical plants and animals are those species native to the tropics. Tropical is also sometimes used in a general sense of a place that is warm and moist year-round, often with the sense of lush vegetation. However, there are places in the tropics that are anything but "tropical" in this sense, with even alpine tundra and snow-capped peaks, including Mauna Kea, Mt. Kilimanjaro, and the Andes as far south as the northernmost parts of Chile and Argentina. In Köppen's scheme of climate classification, a tropical climate is defined as a non-arid climate in which all twelve months have mean temperatures above 18 °C (64.4 °F).

Examples of tropical cities

- Bombay, India (19.1º N)
- Hong Kong, Hong Kong SAR, People's Republic of China (22.3º N)
- Rio de Janeiro, Brazil (22.9º S)
- Singapore (1.4º N)
- Manila, Philippines (14.6º N)
- Cairns, Queensland, Australia (16.7º S)