:: wikimiki.org ::

Sexual Attraction

Sexual attraction

Sexual attraction, in species which reproduce sexually, is attraction to other members of the same species for reproduction. This type of attraction is important for the survival of sexually reproducing species.

Sexual attraction in animals

Sexual attractiveness in non-human animals depends on a wide variety of factors. Often, there is some element of the animal's body which exists for sexual attraction, like the bright plumage and crests of some species of birds. In many species, there are behaviours which appear to be sexual display. Some of these attributes seem to there to demonstrate fitness and health, for example by demonstrating the ability to sustain an "expensive" feature with no other apparent survival function. Conversely, the receiving sex may be predisposed to perceive these features as sexual attraction. It is possible that these features by the giving or the receiving ends cause major survival problems (see game theory), especially where, as in moose, a direct competitive element is involved. Frequently (especially in insects) chemical signals are used to generate sexual interest and to locate potential mates. These signals, known as pheromones, can produce a profound effect upon an animal's behaviour even when present in very minute quantities.

Common elements of sexual attraction in humans

Typically, sexual attraction refers to a person being drawn to another in order to have a sexual relationship. The concrete meaning of a sexual relationship differs across cultures and history. Because human social behavior is often highly complex, a sexual relationship may entail one which, at its beginning, has little or no sexual behavior, and only after a period of time, which can be a courtship period, or a threshold such as marriage, does sexual activity enter the interaction patterns. Certain aspects of what is sexually attractive is universally agreed upon across the human species, or nearly universal among particular cultures or regions, while other factors are determined more locally, among sub-cultures, or simply to the preferences of the individual, which may come about as a result of a variety of genetic and psychological factors. Sexual attractiveness of a person to another person depends on both persons; Much of human sexual attractiveness is governed by physical attractiveness. This involves the senses, in the beginning especially:
- visual perception (how the other looks)
- audition (how the other sounds (in their voice and movements))
- olfaction (how the other smells, naturally or artificially; the wrong smell may be repulsive). Some studies suggest that one source of physical attraction of a human male to a human female is dependent upon a proportion between the width of the hips and the width of the waist (aka waist-hip ratio) (see Golden ratio). As with other animals, pheromones may also enter into the picture, though less significantly than in the case of other animals. Theoretically, the "wrong" pheromone smell may cause someone to be disliked, even when they would otherwise appear attractive. Frequently a pleasant smelling perfume is used to encourage the member of the opposite sex to more deeply inhale the air surrounding its wearer, increasing the probability that the pheromones from the individual will also be inhaled. The importance of pheromones in human relationships is probably limited and widely [http://www.straightdope.com/classics/a2_206.html disputed], although it appears to have some scientific [http://www.sciam.com/article.cfm?articleID=0007F9B4-B6D4-1C60-B882809EC588ED9F&sc=I100322 basis]. A sexually attractive visual appearance in humans generally involves:
- a general body shape and appearance sanctioned by the local culture.
- a lack of visible disease or deformity.
- a high degree of mirror symmetry between the left and right sides of the body, particularly of the face.
- pleasing bodily posture.
- facial similarities to parents (see David Perrett study) However, these factors are complicated by many other factors. There may sometimes be a focus on particular features of the body, such as breasts, legs, hair, or musculature.

Factors determining sexual attraction to human females

A youthful, or neotenic, appearance is a notable factor governing the degree to which a female individual is regarded as sexually attractive. In Western societies, various cultural features may reflect the preference for neotenic female partners; many are dated to antiquity. These include depilatory practices (acomoclitism: intentional hair removal for visual and other effects) and a preference for light or blonde hair [http://www.humanevolution.net/a/marriage.html]. A strong aspect to sexual attraction is proportion. It is typical for a plastic surgeon to correct an error of proportion, such as making a nose that is too big smaller via rhinoplasty, or making breasts larger via breast implants. One idea of physical beauty regarding the breasts of women is that the best shape approaches the shape of a three dimensional parabola (which is called a Paraboloid of revolution) as opposed to a hyperbola, or a sphere. Conversely, the shape of the buttocks of an attractive person (male or female) tends to resemble the shape of a cardioid, which is the inverse transform of a parabola. In regard to the female genitalia, the aesthetic concensus stresses the roundness and largeness of the labia majora, and the symmetry of the labia minora. Vulval aesthetics are relatively new in being observed, as previously the female genitalia was regarded as either repulsive, uninteresting, nonexistent, or taboo in Western culture. The realization to the contrary following the feminist movement and sexual revolution has brought about a new realm of plastic surgery and so-called designer vaginas. The appearance of health also plays a part in physical attraction. Often, women with long hair are thought to appear more beautiful, as the ability to grow long, healthy looking hair is an indication of continuous health of an individual. Another indication of health of an individual is the ability to grow long, strong, healthy-looking fingernails. The preference for this effect has resulted in the fact that artificial nails and manicures have grown extensively popular for women beginning in the 20^th century. Toenails also feature as a component of sexual attractiveness to some degree. Weight, whether tending toward thinner or heavier, has sometimes been considered a physical factor governing attractiveness of both genders (typically women), but there is some debate suggesting that this is actually a social factor indicating desirability. In some cultures, both historically and in the present day, a female with greater than average weight has been seen as sexually attractive. However, this cannot be solely because fat deposits provide the energy needed for developing a healthy fetus, as in other cultures, women so thin as to stand a high risk of miscarriage are considered attractive. Rather, weight is a visible indicator of social status and wealth; in some societies, only the rich can afford to be fat, while in others, only the rich can afford liposuction and personal trainers, or have meaningful employment that promotes healthy diet and exercise habits. Therefore weight is at least partially an indicator of social status, which is itself sexually desirable to many.

Factors determining sexual attraction to human males

Sexual attraction to a man by a woman is determined largely by the height of the man. For the woman, the man should be at least a few percent taller than her in order to be perceived as handsome. In European populations the average height of males is about 175 cm whereas the average height of females is about 165 cm - a 6% difference. It would be preferable if the man is at least a little above the average in height in the given population of males. Males sometimes demonstrate attractiveness by demonstrating their levels of the hormone testosterone by growing larger and well-defined muscles through exercise. At various times in history and throughout various cultures and sub-cultures the growth, maintenance and display of facial or body hair produced as a by-product of testosterone activity within male bodies has been considered a primary characteristic of sexual attractiveness, and of a display of masculinity in general. Cultural development seems to oscillate through multi-generational cycles from one pole to another: extreme hair growth, especially of facial hair accompanied by elaborate grooming rituals is often followed within a couple of generations by a widespread antipathy to body hair and the widespread adoption of depilatory practices. The causal mechanism for this oscillation has not been established but differences in the simultaneous characterisation of body hair attractiveness within a culture between different social classes may indicate that the dynamic force driving the diffusion of differing male body hair social practices is in fact mate selection by females.

Personality and sexual attractiveness

Provided that all of the above listed aspects are reasonably normal, there is no requirement for great physical beauty for a person to be sexually attractive, and personality and good manners can come to the fore. In many cases, people with good personality can be strikingly sexually attractive, even if they are sexually unattractive in appearance. The personality characteristics of a dominant male may override any other logical or superficial flaws.

Other aspects

Many people exhibit high levels of sexual fetishism, and are sexually aroused by other stimuli not normally associated with sexual arousal. The degree to which such fetishism exists or has existed in different cultures is controversial. Often the result of a sexual attraction is sexual arousal.

External links

- [http://www.umkc.edu/sites/hsw/other/evolution.html Evolutionary Theory of sexual attraction]
- [http://serendip.brynmawr.edu/biology/b103/f02/web1/dfernandez.html Sexual Attraction Among Humans]
- [http://www.faceresearch.org/ FaceResearch] – Scientific research and online studies on the role of faces in sexual attraction
- [http://kspope.com/sexiss/research5.php Instances of Sexual Attraction between psychotherapists and clients]
- [http://flatrock.org.nz/topics/relationships/reunions_set_off_sex_urges.htm Reunions Set Off Sex Urges], Article on sexual attraction among birth relatives sparked by reunion.
- [http://www.guardian.co.uk/weekend/story/0,3605,956454,00.html Genetic sexual attraction], News report in the Guardian on persons who have had sexual relationships with or sexually attracted to relatives after reunion. Category:Human appearance Category:Intimate relationships

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

Reproduction

For other uses, see Reproduction (disambiguation) Reproduction is the biological process by which new individual organisms are produced. Reproduction is a fundamental feature of all known life; each individual organism exists as the result of reproduction. The known methods of reproduction are grouped into two main divisions, sexual and asexual reproduction. In asexual reproduction, an individual can reproduce without the involvement of another individual of that species. The division of a bacterial cell into two daughter cells is a common example of asexual reproduction. It is not, however, limited to single-cell organisms. Sexual reproduction, however, requires the involvement of two individuals of a species, typically one of each sex. Normal human reproduction is a common example of sexual reproduction. In general, more-complex organisms reproduce sexually while simpler, usually unicellular, organisms reproduce asexually.

Asexual reproduction

Asexual reproduction is the biological process by which an organism creates a genetically-similar copy of itself without a contribution of genetic material from another individual. Bacteria divide asexually via binary fission; viruses take control of host cells to produce more viruses; Hydras (invertebrates of the order Hydroidea) and yeasts are able to reproduce by budding. These organisms do not have different sexes, and they are capable of "splitting" themselves into two or more parts and are even able to regenerate their body parts. Some 'asexual' species, like hydra and jellyfish, may also sexually reproduce. For instance, most plants are capable of vegetative reproduction, reproduction without seeds or spores, but, also, they can reproduce sexually, by way of pollination. Likewise, bacteria may exchange genetic information by conjugation. Other ways of asexual reproduction include fragmentation and spore formation.

Sexual reproduction

Sexual reproduction is a biological process by which organisms create descendants through the combination of genetic material taken randomly and independently from two different members of the species. These organisms have two different adult sexes, male and female. One of each provides half of the new organism's DNA, creating haploid gametes. These two gametes fuse to form a diploid zygote. Humans, most animals, and flowering plants reproduce sexually. In mammals, the offspring resemble the adult form, but in other animals, such as butterflies, the offspring can look considerably different. Sexually-reproducing organisms have two sets of genes for every trait (called alleles). Offspring inherit one allele for each trait from each parent, thereby ensuring that offspring have a combination of the parents' genes. This recombination of genes during every generation leads to strong selective pressure for good genes needed for survival in the organism's environment. Further, by having two copies of every gene, only one of which is expressed, deleterious alleles can be masked, an advantage believed to have led to the evolutionary development of diploidy (Otto and Goldstein).

Mitosis and Meiosis

Mitosis and meiosis are an integral part of cell division. Mitosis occurs in somatic cells, while meiosis occurs in gametes.

Mitosis

The resultant number of cell in mitosis is twice the number of original cells. The number of chromosomes in the daughter cells is the same as that of the parent cell.
center

Meiosis

The resultant number of cells is four times the number of original cells. This results in cells with half the number of chromosomes present in the parent cell. A diploid cell forms two haploid cells. This process occurs in two phases, meiosis I and meiosis II.
center

Reproductive strategies

There is a wide range of reproductive strategies employed by different species. Some animals, such as the human and Northern Gannet, do not reach sexual maturity for many years after birth and even then produce few offspring. Others reproduce quickly; but, under normal circumstances, most offspring do not survive to adulthood. For example, a rabbit (mature after 8 months) can produce 10–30 offspring per year, and a fruit fly (10–14 days) can produce up to 900 offspring per year. These two main strategies are known as K-selection (few offspring) and r-selection (many offspring). Which strategy is favoured by evolution depends on a variety of circumstances. Animals with few offspring can devote more resources to the nurturing and protection of each individual offspring, thus reducing the need for a large number of offspring. On the other hand, animals with many offspring may devote less resources to each individual offspring; for these types of animals it is common for a large number of offspring to die soon after birth, but normally enough individuals survive to maintain the population.

Asexual vs. sexual reproduction

Organisms that reproduce through asexual reproduction tend to grow in number exponentially. However, because they rely on mutation for variations in their DNA, all members of the species have similar vulnerabilities. Organisms that reproduce sexually yield a smaller amount of offspring, but the large amount of variation in their genes makes them less susceptible to disease. Many organisms can reproduce sexually as well as asexually. Aphids, slime molds, sea anemones and many plants are examples. When environmental factors are favorable, asexual reproduction is employed to exploit suitable conditions for survival such as an abundant food supply, adequate shelter, favorable climate, disease, optimum pH or a proper mix of other lifestyle requirements. Populations of these organisms increase exponentially via asexual reproductive strategies to take full advantage of the rich supply resources. When food sources have been depleted, the climate becomes hostile, or individual survival is jeopardized by some other adverse change in living conditions, these organisms switch to sexual forms of reproduction. Sexual reproduction ensures a mixing of the gene pool of the species. The variations found in offspring of sexual reproduction allow some individuals to be better suited for survival and provide a mechanism for selective adaptation to occur. In addition, sexual reproduction usually results in the formation of a life stage that is able to endure the conditions that threaten the offspring of an asexual parent. Thus, seeds, spores, eggs, pupae, cysts or other "over-wintering" stages of sexual reproduction ensure the survival during unfavorable times and the organism can "wait out" adverse situations until a swing back to suitability occurs.

The Red Queen hypothesis

Sexual reproduction is best known for providing means of adaptation to an ever-changing environment by adding phenotypic variance to the population. The variation produced by sexual reproduction is not only outward (such as the advent of horns or fusion of digits) but also physiological. One hypothesis termed the Red Queen Hypothesis states that sexual reproduction leading to variation is as much an evolutionary arms race against parasites as it is ammunition against environmental factors. Variation that arises from sexual reproduction makes it harder for parasites to "get accustomed" to their hosts, ultimately making them less efficient. However, the idea of the hypothesis is that parasites continue to evolve as well, making both species continuously change but stay in the same place relative to each other. Organisms that facultatively reproduce asexually or sexually usually enter their sexual reproduction phase when pathogens become prevalent in the population.³

Life without reproduction

The existence of life without reproduction is the subject of some speculation. The biological study of how the origin of life led from non-reproducing elements to reproducing organisms is called abiogenesis. Whether or not there were several independent abiogenetic events, biologists believe that the last common ancestor to all present life on earth lived about 3.5 billion years ago. Today, some scientists have speculated about the possibility of creating life non-reproductively in the laboratory. One group of scientists has succeeded in producing a simple virus from entirely non-living materials. The production of a truly living organism, such as a simple bacterium, with no ancestors would be a much more complex task, but may well be possible according to current understanding of biology. Finally, life without reproduction is a feature of many religious Creation myths. The biblical Adam, for example, was created by God and had no ancestors.

Mechanical reproduction

A major goal in the field of robots is self-replicating machines. Since all robots (at least in modern times) have a fair number of the same features, a self-replicating robot (or possibly a hive of robots) would need to do the following:
- Obtain construction materials
- Manufacture new parts
- Provide a consistent power source
- Program the new members To date, this has not been done. On a nanotechnical scale, nanomachines might also be designed to reproduce under their own power. This, in turn, has given rise to the "gray goo" theory of Armaggedon, as featured in such science fiction novels as Bloom and Prey. For a detailed article on mechanical reproduction as it relates to the industrial age see mass production.

References

#S. P. Otto and D. B. Goldstein. "Recombination and the Evolution of Diploidy". Genetics. Vol 131 (1992): 745-751. #Pang, K. "Certificate Biology: New Mastering Basic Concepts", Hong Kong, 2003. #Zimmer, Carl. "Parasite Rex: Inside the Bizarre World of Nature's Most Dangerous Creatures", New York: Touchstone, 2001.

External links

- [http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/A/AsexualReproduction.html Asexual Reproduction]
- [http://a-history.net Human reproductive system webpage]
- [http://www.biolreprod.org/ Journal of Biology of Reproduction]
- [http://www.andrologyjournal.org/ Journal of Andrology] Category:Developmental biology ja:生殖

Sexual reproduction

Sexual reproduction is a process of reproduction involving the merging of two gametes from the same (usually) species to produce a new organism. One advantage of this form of reproduction over asexual reproduction is that the DNA of the offspring is significantly different from that of the two gametes; this allows species to change more rapidly than through mutation alone. The DNA is different because each contributing organism randomly and independently donates half of their DNA to the sex cells in a process called meiosis. These cells then, through a variety of processes, depending on the particular species, meet and merge together to produce a new organism with different DNA. Sexual reproduction is the primary method of reproduction for the vast majority of visible organisms, including almost all animals and plants, though the process is often significantly different, especially for plants and trees. Bacterial conjugation, the transfer of DNA between two bacteria, is often mistakenly confused with sexual reproduction, because the mechanics are similar. The first fossilized evidence of sexually reproducing organisms is from eukaryotes of the Stenian period, about 1.2 to 1 billion years before the present time.

Sexual reproduction of protists and fungi

Many protists and fungi reproduce sexually. Although they are unicellular, at times of reproduction the "father" cell and the "mother" cell combines together. Next, their genetic information combines together into a new formation, and by cell division the offspring is born.

Reproduction in flowering plants

:Main articles: Plant sexuality, Flowering plants, Flowers In flowering plants, a stamen produces gametes called pollen grains, which attach to a pistil, in which the female gametes (ovules) are located. Here, the female gamete is fertilized and develops into a seed. The ovary, which produced the gamete then grows into a fruit, which surrounds the seed(s). Plants may either self-pollinate or cross-pollinate.

Reproduction in reptiles

Female reptiles lay eggs, fertilized by the male, in which the young gestate.

Reproduction in birds

Male and female birds both have cloacas. The female lays eggs, fertilized by the male, in which the young gestate.

Reproduction in mammals

In placental mammals, the offspring are born as young, complete animals with the entire sets of sex organs, although not functioning. After several months or years, the sex organs in the mammals start to grow and the animal becomes sexually mature. Most mammals are only fertile during certain times of the year; during those times, they are sometimes said to be “in heat.” At this point, the animal is ready to mate. Individual male and female mammals meet and carry out copulation, the beginning stage of sexual reproduction. In primates, the sexual partner for each primate is monogamously specific. For most other mammals, males and females occasionally exchange sexual partners.

The mammalian male

The male reproductive system contains two main divisions: the penis, which is inserted into the female and carries the sperm inside it, and the testes, which produce the sperm. In humans, both of these organs are outside the abdominal cavity, but they can be primarily housed within the abdomen in other animals (for instance, in dogs, the penis is internal except when mating). Having the testes outside the abdomen best facilitates temperature regulation of the sperm, which require specific temperatures to survive. Sperm are the smaller of the two gametes and are generally very short-lived, requiring males to produce them continuously from the time of sexual maturity until death. They are motile and swim by chemotaxis.

The mammalian female

The female reproductive system likewise contains two main divisions: the vagina and uterus, which act as the receptacle for the male's sperm, and the ovaries, which produce the female's ova. All of these parts are always internal. The vagina is attached to the uterus through the cervix, while the uterus is attached to the ovaries via the Fallopian tubes. At certain intervals, the ovaries release an ovum (the singular of ova), which passes through the fallopian tube into the uterus. If, in this transit, it meets with sperm, the sperm penetrate and merge with the egg, fertilizing it. The fertilization usually occurs in the oviducts, but can happen in the uterus itself. The zygote then implants itself in the wall of the uterus, where it begins the processes of embryogenesis and morphogenesis. When developed enough to survive outside the womb, the cervix dilates and contractions of the uterus propel the fetus through the birth canal, which is the vagina. The ova are larger than sperm and are generally all created by birth. They are for the most part stationary, aside from their transit to the uterus, and contain nutrients for the later zygote and embryo. Over a regular interval, a process of oogenesis matures one ovum to be sent down the Fallopian tube attached to its ovary in anticipation of fertilization. If not fertilized, this egg is flushed out of the system through menstruation in humans and great apes and reabsorbed in all other mammals in the estrus cycle.

Gestation

:Main articles: Mammalian gestation, Pregnancy Gestation, called pregnancy in humans, is the period of time during which the fetus develops, dividing via mitosis inside the female. During this time, the fetus receives all of its nutrition and oxygenated blood from the female, filtered through the placenta, which is attached to the fetus' abdomen via an umbilical cord. This drain of nutrients can be quite taxing on the female, who is required to ingest significantly higher levels of calories. In addition, certain vitamins and other nutrients are required in greater quantities than normal, often creating abnormal eating habits. The length of gestation, called the gestation period, varies greatly from species to species; it is 38 weeks in humans, 56-60 in giraffes and 16 days in hamsters.

Birth

Once the fetus is sufficiently developed, chemical signals start the process of birth, which begins with contractions of the uterus and the dilation of the cervix. The fetus then descends to the cervix, where it is pushed out into the vagina, and eventually out of the female. The newborn, which is called an infant in humans, should typically begin respiration on its own shortly after birth. Not long after, the placenta is passed as well. Most mammals eat this, as it is a good source of protein and other vital nutrients needed for caring for the young. The end of the umbilical cord attached to the young’s abdomen eventually falls off on its own.

Monotremes

Monotremes, only five species of which exist, all from Australia and New Guinea, lay eggs. They have one opening for excretion and reproduction called the cloaca. They hold the eggs internally for several weeks, providing nutrients, and then lay them and cover them like birds. After less than two weeks the young hatches and crawls into its mother’s pouch, much like marsupials, where it nurses for several weeks as it grows.

Marsupials

Marsupials reproduce in essentially the same manner, though their young are born at a far earlier stage of development than other mammals. After birth, marsupial joeys crawl into their mother’s pouch and attach to a teat, where they receive nourishment and finish developing into self-sufficient animals.

References

# Pang, K. "Certificate Biology: New Mastering Basic Concepts", Hong Kong, 2003. # [http://www.biolreprod.org/ Journal of Biology of Reproduction], accessed in August 2005. Category:Developmental biology Category:Biological reproduction Category:Sexuality

Game theory

:You may also be interested in: Games in general, a band named Game Theory, or Combinatorial game theory (used to study games like Nim, Chess, and Go). Game theory is a branch of applied mathematics that studies strategic situations where players choose different actions in an attempt to maximize their returns. First developed as a tool for understanding economic behavior, game theory is now used in many diverse academic fields, ranging from biology to philosophy. Game theory saw substantial growth during the Cold War because of its application to military strategy, most notably to the concept of mutually assured destruction. Beginning in the 1970s, game theory has been applied to animal behavior, including species' development by natural selection. Because of interesting games like the Prisoner's dilemma, where mutual self-interest hurts everyone, game theory has been used in ethics and philosophy. Finally, game theory has recently drawn attention from computer scientists because of its use in artificial intelligence and cybernetics. In addition to its academic interest, game theory has received attention in popular culture. An important figure in game theory, John Nash was the subject of the 2001 film A Beautiful Mind. Several game shows have adopted game theoretic situations, including Friend or Foe and Deal or No Deal. Although similar to decision theory, game theory studies decisions that are made in an environment where various players interact. In other words, game theory studies choice of optimal behavior when costs and benefits of each option are not fixed, but depend upon the choices of other individuals.

Representation of games

The games studied by game theory are well-defined mathematical objects. A game consists of a set of players, a set of moves (or strategies) available to those players, and a specification of payoffs for each combination of strategies. There are two ways of representing games that are common in the literature.

Normal form

The normal (or strategic form) game is a matrix which shows the players, strategies, and payoffs (see the example to the right). Here there are two players; one chooses the row and the other chooses the column. Each player has two strategies, which are specified by the number of rows and the number of columns. The payoffs are provided in the interior. The first number is the payoff received by the row player (Player 1 in our example); the second is the payoff for the column player (Player 2 in our example). Suppose that Player 1 plays top and that Player 2 plays left. Then Player 1 gets 4, and Player 2 gets 3. When a game is presented in normal form, it is presumed that each player acts simultaneously or, at least, without knowing the actions of the other. If players have some information about the choices of other players, the game is usually presented in extensive form.

Extensive form

matrix Extensive form games attempt to capture games with some important order. Games here are presented as trees (as pictured to the left). Here each vertex (or node) represents a point of choice for a player. The player is specified by a number listed by the vertex. The lines out of the vertex represent a possible action for that player. The payoffs are specified at the bottom of the tree. In the game pictured here, there are two players. Player 1 moves first and chooses either F or U. Player 2 sees Player 1s move and then chooses A or R. Suppose that Player 1 chooses U and then Player 2 chooses A, then Player 1 gets 8 and Player 2 gets 2. Extensive form games can also capture simultaneous-move games as well. Either a dotted line or circle is drawn around two different vertices to represent them as being part of the same information set (i.e., the players do not know at which point they are).
Types of games

Symmetric and asymmetric
A symmetric game is a game where the payoffs for playing a particular strategy depend only on the other strategies employed, not on who is playing them. If the identities of the players can be changed without changing the payoff to the strategies, then a game is symmetric. Many of the commonly studied 2x2 games are symmetric. The standard representations of Chicken, the Prisoner's Dilemma, and the Stag hunt are all symmetric games. Most commonly studied asymmetric games are games where there are not identical strategy sets for both players. For instance, the Ultimatum game and similar the Dictator game have different strategies for each player. It is possible, however, for a game to have identical strategies for both players, yet be asymmetric. For example, the game pictured to the right is asymmetric despite having identical strategy sets for both players.
Zero sum and non-zero sum
In zero-sum games the total benefit to all players in the game, for every combination of strategies, always adds to zero (or more informally put, a player benefits only at the expense of others). Poker exemplifies a zero-sum game, because one wins exactly the amount one's opponents lose. Other zero sum games include Matching pennies and most classical board games including Go and Chess. Many games studied by game theorists (including the famous Prisoner's Dilemma) are non-zero-sum games, because some outcomes have net results greater or less than zero. Informally, in non-zero-sum games, a gain by one player does not necessarily correspond with a loss by another. It is possible to transform any game into a zero-sum game by adding an additional dummy player (often called "the board"), whose losses compensate the players' net winnings.
Simultaneous and sequential
Simultaneous games are games where both players move simultaneously, or if they do not move simultaneously, the later players are unaware of the earlier players' actions (making them effectively simultaneous). Sequential games (or dynamic games) are games where later players have some knowledge about earlier actions. This need not be perfect knowledge about every action of earlier players; it might be very little information. For instance, a player may know that an earlier player did not perform one particular action, while she does not know which of the other available actions the first player actually performed. The difference between simultaneous and sequential games is captured in the different representations discussed above. Normal form is used to represent simultaneous games, and extensive form is used to represent sequential ones.
Perfect information and imperfect information
extensive form An important subset of sequential games consists of games of perfect information. A game is one of perfect information if all players know the moves previously made by all other players. Thus, only sequential games can be games of perfect information, since in simultaneous games not every player knows the actions of the others. Most games studied in game theory are imperfect information games, although some interesting games are games of perfect information, including the Ultimatum Game and Centipede Game. Many popular games are games of perfect information including Chess, Go, and Mancala. Perfect information is often confused with complete information, which is a similar concept. Complete information requires that every player know the strategies and payoffs of the other players but not necessarily the actions.
Infinitely long games
For obvious reasons, games as studied by economists and real-world game players are generally finished in a finite number of moves. Pure mathematicians are not so constrained, and set theorists in particular study games that last for infinitely many moves, with the winner (or other payoff) not known until after all those moves are completed. The focus of attention is usually not so much on what is the best way to play such a game, but simply on whether one or the other player has a winning strategy. (It can be proved, using the axiom of choice, that there are games—even with perfect information, and where the only outcomes are "win" or "lose"—for which neither player has a winning strategy.) The existence of such strategies, for cleverly designed games, has important consequences in descriptive set theory.
Uses of game theory
Games in one form or another are widely used in many different academic disciplines.
Economics and business
Economists have used game theory to analyze a wide array of economic phenomena, including auctions, bargaining, duopolies and oligopolies, social network formation, and voting systems. This research usually focuses on particular sets of strategies known as equilibria in games. These "solution concepts" are usually based on what is required by norms of rationality. The most famous of these is the Nash equilibrium. A set of strategies is a Nash equilibrium if each represents a best response to the other strategies. So, if all the players are playing the strategies in a Nash equilibrium, they have no incentive to deviate, since their strategy is the best they can do given what others are doing. The payoffs of the game are generally taken to represent the utility of individual players. Often in modeling situations the payoffs represent money, which presumably corresponds to an individual's utility. This assumption, however, can be faulty. A prototypical paper on game theory in economics begins by presenting a game that is an abstraction of some particular economic situation. One or more solution concepts are chosen, and the author demonstrates which strategy sets in the presented game are equilibria of the appropriate type. Naturally one might wonder to what use should this information be put. Economists and business professors suggest two primary uses.
Descriptive
utility The first use is to inform us about how actual human populations behave. Some scholars believe that by finding the equilibria of games they can predict how actual human populations will behave when confronted with situations analogous to the game being studied. This particular view of game theory has come under recent criticism. First, it is criticized because the assumptions made by game theorists are often violated. Game theorists may assume players always act rationally to maximize their wins (the Homo economicus model), but real humans often act either irrationally, or act rationally to maximize the wins of some larger group of people (altruism). Game theorists respond by comparing their assumptions to those used in physics. Thus while their assumptions do not always hold, they can treat game theory as a reasonable scientific ideal akin to the models used by physicists. However, additional criticism of this use of game theory has been levied because some experiments have demonstrated that individuals do not play equilibrium strategies. For instance, in the Centipede game, Guess 2/3 of the average game, and the Dictator game, people regularly do not play Nash equilibria. There is an ongoing debate regarding the importance of these experiments. Alternatively, some authors claim that Nash equilibria do not provide predictions for human populations, but rather provide an explanation for why populations that play Nash equilibria remain in that state. However, the question of how populations reach those points remains open. Some game theorists have turned to evolutionary game theory in order to resolve these worries. These models presume either no rationality or bounded rationality on the part of players. Despite the name, evolutionary game theory does not necessarily presume natural selection in the biological sense. Evolutionary game theory includes both biological as well as cultural evolution and also models of individual learning (for example, fictitious play dynamics).
Normative
On the other hand, some scholars see game theory not as a predictive tool for the behavior of human beings, but as a suggestion for how people ought to behave. Since a Nash equilibrium of a game constitutes one's best response to the actions of the other players, playing a strategy that is part of a Nash equilibrium seems appropriate. However, this use for game theory has also come under criticism. First, in some cases it is appropriate to play a non-equilibrium strategy if one expects others to play non-equilibrium strategies as well. For an example, see Guess 2/3 of the average. Second, the Prisoner's Dilemma presents another potential counterexample. In the Prisoner's Dilemma, each player pursuing his own self-interest leads both players to be worse off than had they not pursued their own self-interests. Some scholars believe that this demonstrates the failure of game theory as a recommendation for behavior.
Biology
Unlike economics, the payoffs for games in biology are often interpreted as corresponding to fitness. In addition, the focus has been less on equilibria that correspond to a notion of rationality, but rather on ones that would be maintained by evolutionary forces. The most well-known equilibrium in biology is know as the Evolutionary stable strategy or (ESS), and was first introduced by John Maynard Smith (described in his 1982 book). Although its initial motivation did not involve any of the mental requirements of the Nash equilibrium, every ESS is a Nash equilibrium. In biology, game theory has been used to understand many different phenomena. It was first used to explain the evolution (and stability) of the approximate 1:1 sex ratios. Ronald Fisher (1930) suggested that the 1:1 sex ratios are a result of evolutionary forces acting on individuals who could be seen as trying to maximize their number of grandchildren. Additionally, biologists have used evolutionary game theory and the ESS to explain the emergence of animal communication (Maynard Smith & Harper, 2003). The analysis of signaling games and other communication games has provided some insight into the evolution of communication among animals. Finally, biologists have used the Hawk-Dove game (also known as Chicken) to analyze fighting behavior and territoriality.
Computer science and logic
Game theory has come to play an increasingly important role in logic and in computer science. Several logical theories have a basis in game semantics. In addition, computer scientists have used games to model interactive computations. Computability logic attempts to develop a comprehensive formal theory (logic) of interactive computational tasks and resources, formalizing these entities as games between a computing agent and its environment.
Philosophy
Game theory has been put to several uses in philosophy. Responding to two papers by W.V.O. Quine (1960, 1967), David Lewis (1969) used game theory to develop a philosophical account of convention. In so doing, he provided the first analysis of common knowledge and employed it in analyzing play in coordination games. In addition, he first suggested that one can understand meaning in terms of signaling games. This later suggestion has been pursued by several philosophers since Lewis (Skyrms 1996, Grim et al. 2004). In ethics, some authors have attempted to pursue the project, begun by Thomas Hobbes, of deriving morality from self-interest. Since games like the Prisoner's Dilemma present an apparent conflict between morality and self-interest, explaining why cooperation is required by self-interest is an important component of this project. This general strategy is a component of the general social contract view in political philosophy (for examples, see Gauthier 1987 and Kavka 1986). Finally, other authors have attempted to use evolutionary game theory in order to explain the emergence of human attitudes about morality and corresponding animal behaviors. These authors look at several games including the Prisoner's Dilemma, Stag hunt, and the Nash bargaining game as providing an explanation for the emergence of attitudes about morality (see, e.g., Skyrms 1996, 2004; Sober and Wilson 1999).
History of game theory
The first known discussion of game theory occurred in a letter written by James Waldegrave in 1713. In this letter, Waldegrave provides a minimax mixed strategy solution to a two-person version of the card game le Her. It was not until the publication of Antoine Augustin Cournot's Researches into the Mathematical Principles of the Theory of Wealth in 1838 that a general game theoretic analysis was pursued. In this work Cournot considers a duopoly and presents a solution that is a restricted version of the Nash equilibrium. Although Cournot's analysis is more general than Waldegrave's, game theory did not really exist as a unique field until John von Neumann published a series of papers in 1928. These results were later expanded in the 1944 book The Theory of Games and Economic Behavior by von Neumann and Oskar Morgenstern. This profound work contains the method for finding optimal solutions for two-person zero-sum games. During this time period, work on game theory was primarily focused on cooperative game theory, which analyzes optimal strategies for groups of individuals, presuming that they can enforce agreements between them about proper strategies. In 1950, the first discussion of the Prisoner's dilemma appeared, and an experiment was undertaken on this game at the RAND corporation. Around this same time, John Nash developed a definition of an "optimum" strategy for multiplayer games where no such optimum was previously defined, known as Nash equilibrium. This equilibrium is sufficiently general, allowing for the analysis of non-cooperative games in addition to cooperative ones. Game theory experienced a flurry of activity in the 1950s, during which time the concepts of the core, the extensive form game, fictitious play, repeated games, and the Shapley value were developed. In addition, the first applications of Game theory to philosophy and political science occurred during this time. In 1965, Reinhard Selten introduced his solution concept of subgame perfect equilibria, which further refined the Nash equilibrium (later he would introduce trembling hand perfection as well). In 1967, John Harsanyi developed the concepts of complete information and Bayesian games. He, along with John Nash and Reinhard Selten, won The Bank of Sweden Prize in Economic Sciences in Memory of Alfred Nobel (also known as The Nobel Prize in Economics) in 1994. In the 1970s, game theory was extensively applied in biology, largely as a result of the work of John Maynard Smith and his evolutionary stable strategy. In addition, the concepts of correlated equilibrium, trembling hand perfection, and common knowledge were introduced and analyzed. In 2005, the game theorists Thomas Schelling and Robert Aumann won the Nobel Prize in Economics. Schelling worked on dynamic models, early examples of evolutionary game theory. Aumann contributed more to the equilibrium school, developing an equilibrium coarsening correlated equilibrium and developing extensive analysis of the assumption of common knowledge.
Notes
# [http://www.gametheory.net GameTheory.net] has an extensive list of [http://www.gametheory.net/popular/ references to game theory in popular culture]. # Some scholars would consider certain asymmetric games as examples of these games as well. However, the most common payoffs for each of these games are symmetric. # Experimental work in game theory goes by many names, experimental economics, behavioral economics, and behavioral game theory are several. For a recent discussion on this field see Camerer 2003. # For a more detailed discussion of the use of Game Theory in ethics see the Stanford Encyclopedia of Philosophy's entry [http://plato.stanford.edu/entries/game-ethics/ game theory and ethics]. # Although common knowledge was first discussed by the philosopher David Lewis in his disertation (and later book) Convention in the late 1960s, it was not widely considered by economists until Robert Aumann's work in the 1970s.
References
;Textbooks and general reference texts
- Gibbons, Robert (1992) Game Theory for Applied Economists, Princeton University Press ISBN 0691003955 (readable; suitable for advanced undergraduates. Published in Europe by Harvester Wheatsheaf (London) with the title A primer in game theory)
- Ginits, Herbert (2000) Game Theory Evolving Princeton University Press ISBN 0691009430
- Osborne, Martin and Ariel Rubinstein: A Course in Game Theory, MIT Press, 1994, ISBN 0-262-65040-1 (modern introduction at the introductory graduate level)
- Fudenberg, Drew and Jean Tirole: Game Theory, MIT Press, 1991, ISBN 0262061414 (the definitive reference text) ;Historically important texts
- Fisher, Ronald (1930) The Genetical Theory of Natural Selection Clarendon Press, Oxford.
- Luce, Duncan and Howard Raiffa Games and Decisions: Introduction and Critical Survey Dover ISBN 0486659437
- Maynard Smith, John Evolution and the Theory of Games, Cambridge University Press 1982
- Morgenstern, Oskar and John von Neumann (1947) The Theory of Games and Economic Behavior Princeton University Press
- Nash, John (1950) "Equilibrium points in n-person games" Proceedings of the National Academy of the USA 36(1):48-49.
- Poundstone, William Prisoner's Dilemma: John Von Neumann, Game Theory and the Puzzle of the Bomb, ISBN 038541580X ;Other print references
- Camerer, Colin (2003) Behavioral Game Theory Princeton University Press ISBN 0691090394
- Gauthier, David (1987) Morals by Agreement Oxford University Press ISBN 0198249926
- Grim, Patrick, Trina Kokalis, Ali Alai-Tafti, Nicholas Kilb, and Paul St Denis (2004) "Making meaning happen." Journal of Experimental & Theoretical Artificial Intelligence 16(4): 209-243.
- Kavka, Gregory (1986) Hobbesian Moral and Political Theory Princeton University Press. ISBN 069102765X
- Lewis, David (1969) Convention: A Philosophical Study
- Maynard Smith, J. and Harper, D. (2003) Animal Signals. Oxford University Press. ISBN 0198526857
- Quine, W.v.O (1967) "Truth by Convention" in Philosophica Essays for A.N. Whitehead Russel and Russel Publishers. ISBN 0846209705
- Quine, W.v.O (1960) "Carnap and Logical Truth" Synthese 12(4):350-374.
- Skyrms, Brian (1996) Evolution of the Social Contract Cambridge University Press. ISBN 0521555833
- Skyrms, Brian (2004) The Stag Hunt and the Evolution of Social Structure Cambridge University Press. ISBN 0521533929.
- Sober, Elliot and David Sloan Wilson (1999) Unto Others: The Evolution and Psychology of Unselfish Behavior Harvard University Press. ISBN 0674930479 ;Websites
- Paul Walker, [http://william-king.www.drexel.edu/top/class/histf.html An Outline of the History of Game Theory].
- Alvin Roth: [http://www.economics.harvard.edu/~aroth/alroth.html Game Theory and Experimental Economics page] - Comprehensive list of links to game theory information on the Web
- Mike Shor: [http://www.gametheory.net Game Theory .net] - Lecture notes, interactive illustrations and other information.
- Jim Ratliff's [http://virtualperfection.com/gametheory/ Graduate Course in Game Theory] (lecture notes).
- [http://homepages.cwi.nl/~robu/ Valentin Robu]'s [http://homepages.cwi.nl/~robu/aamas/aamas_demo.html software tool] for simulation of bilateral negotiation (bargaining)
- [http://www.csc.villanova.edu/~japaridz Giorgi Japaridze]: [http://www.csc.villanova.edu/~japaridz/CL/gsoll.html Game Semantics or Linear Logic?] - Discussion of games in logic, and links.
- Don Ross: [http://plato.stanford.edu/entries/game-theory/ Review Of Game Theory].
- Bruno Verbeek and Christopher Morris: [http://plato.stanford.edu/entries/game-ethics/ Game Theory and Ethics]
- Chris Yiu's [http://www.yiu.co.uk/gametheory/ Game Theory Lounge]
- tutor2u [http://www.tutor2u.net/newsmanager/templates/?a=840&z=1 Student notes and a presentation on Game Theory]
- Elmer G. Wiens: [http://www.egwald.com/operationsresearch/gameintroduction.php Game Theory] - Introduction, worked examples, play online two-person zero-sum games.
- [http://www.socialcapitalgateway.org/eng-gametheory.htm Web sites on game theory and social interactions]
- Category:Artificial intelligence ja:ゲーム理論

Pheromones

gland (white-at tip of abdomen) releasing pheromone to entice swarm into an empty hive]] A pheromone is any chemical produced by a living organism that transmits a message to other members of the same species. There are alarm pheromones, food trail pheromones, sex pheromones, and many others. Their use among insects has been particularly well documented, although many vertebrates also communicate using pheromones. Their use by humans is controversial. Insect pheromones of pest species, such as the Japanese beetle and the gypsy moth, can be used to trap them or to create confusion so that the pests do not lay eggs on crops. Bombykol is a pheromone released by the female silkworm to attract mates. In mammals and reptiles, pheromones may be detected by the vomeronasal organ, or Jacobson's organ, which lies between the nose and mouth, although some are detected by regular olfactory membranes.

Human pheromones

Pheromones are a popular device in fiction, including the novel Jitterbug Perfume by Tom Robbins and the film Love Potion No. 9. They were also mentioned in an episode of Wolfgang Petersen's The Agency. Some commercially-available substances are advertised using claims that the products contain sex pheromones and can act as an aphrodisiac. These claims often lack credence due to an excessive marketing of pheromones by unsolicited e-mail, and their effectiveness has not been demonstrated scientifically. Nevertheless, a few well-controlled scientific studies have been published demonstrating that humans may use pheromones in some circumstances. The best-studied case involves the synchronization of menstrual cycles among women based on odor cues (by [http://pondside.uchicago.edu/ceb/faculty/mcclinto.html Martha McClintock], Professor of Psychology at the University of Chicago). This study states that there are two types of pheromone involved: "One, produced prior to ovulation, shortens the ovarian cycle, and the second, produced just at ovulation, lengthens the cycle". Other studies have suggested that people can use odor cues to select mates who are not closely related to themselves. Pheromones in humans are believed to be produced by the apocrine glands. These glands become functional after reaching puberty, which could explain why most people develop an attraction for others at that time. Pheromones could also be the reason why a person can sense "chemistry", or feel an instant attraction or dislike when first meeting someone. "Using a brain imaging technique, Swedish researchers have shown that homosexual and heterosexual men respond differently to two odors that may be involved in sexual arousal, and that the gay men respond in the same way as women. The new research may open the way to studying human pheromones, as well as the biological basis of sexual preference. Pheromones, chemicals emitted by one individual to evoke some behavior in another of the same species, are known to govern sexual activity in animals, but experts differ as to what role, if any, they play in making humans sexually attractive to one another." [http://www.nytimes.com/2005/05/09/science/09cnd-smell.html?ei=5065&en=bf437458d36709cf&ex=1116302400&partner=MYWAY&pagewanted=print New York Times]

External links

- [http://www.pherobase.com/ Pherobase], the database of insect pheromones
- [http://www.sfsu.edu/~news/prsrelea/fy01/091.htm SFSU study shows that synthetic pheromones in women's perfume increase intimate contact with men]
- [http://www.hot-pheromones.com Human Pheromones, do they work ?]
- [http://www.emotion.caltech.edu/courses/SS140/April4-2.pdf Human pheromones: have they been demonstrated?]
- [http://www.jambell.com/Xcite/ A tale of pheromone marketing in the early days] Category:Biochemistry Category:Olfaction ja:フェロモン

Courtship

Courtship or dating is the process of selecting and attracting a mate for marriage, sexual intercourse, or other intimate activities. intimate, Washington, DC.]] In many traditional societies, courtship is a highly structured activity, with well-known rules. In many cultures, courtship is made redundant, or eliminated altogether, by the practice of arranged marriages, where partners are chosen for young people, typically by their parents. In some societies, the parents or community choose potential partners, and then allow limited dating to determine whether the parties are suited. In Japan, there is a type of courtship called Omiai. It is a formal date with the intention of finding someone to marry.

Dating and alternative courtship customs

In Western societies, a date is an occasion when one socializes with a potential lover or spouse. In this sense, the purpose of a date is for the people dating to get to know each other and decide whether they want to have a relationship. However, the term is also used to mean a social evening between people who already have a long-term, established relationship or marriage. In such cases the goal of dating is no longer courtship, but instead an opportunity to