Mycogen Seeds

Mycogen Seeds, headquartered in Indianapolis, Indiana, United States, provides seeds for agriculture. Mycogen is one of the largest sunflower seed producers.[citation needed] Mycogen produces, markets and sells hybrid seed corn. The company also markets and sells sorghum, sunflower, soybean, alfalfa, and canola.

The Mycogen Corporation was formed in 1982 by members of the San Diego business and scientific communities, including David H. Rammler, a partner in the venture capital firm of Vanguard Associates, who served as the first chairman of the company, and Andrew C. Barnes, a biochemist with an MBA from the Stanford School of Business. The original concept was to develop environmentally safe herbicides from fungi using genetic engineering, thus the name Mycogen, coined from the Greek words for fungus and genetics.[citation needed]

The development of MAs

1st generation

The first generation of MA refers to hybrid algorithms, a marriage between a population-based global search (often in the form of an evolutionary algorithm) coupled with a cultural evolutionary stage. This first generation of MA although encompasses characteristics of cultural evolution (in the form of local refinement) in the search cycle, it may not qualify as a true evolving system according to Universal Darwinism, since all the core principles of inheritance/ memetic transmission, variation and selection are missing. This suggests why the term MA stirred up criticisms and controversies among researchers when first introduced in The Selfish Gene.


2nd generation

Multi-meme , Hyper-heuristic and Meta-Lamarckian MA are referred to as second generation MA exhibiting the principles of memetic transmission and selection in their design. In Multi-meme MA, the memetic material is encoded as part of the genotype. Subsequently, the decoded meme of each respective individual / chromosome is then used to perform a local refinement. The memetic material is then transmitted through a simple inheritance mechanism from parent to offspring(s). On the other hand, in hyper-heuristic and meta-Lamarckian MA, the pool of candidate memes considered will compete, based on their past merits in generating local improvements through a reward mechanism, deciding on which meme to be selected to proceed for future local refinements. Memes with a higher reward have a greater chance of being replicated or copied. For a review on second generation MA, i.e., MA considering multiple individual learning methods within an evolutionary system, the reader is referred to .

3rd generation

Co-evolution and self-generation MAs may be regarded as 3rd generation MA where all three principles satisfying the definitions of a basic evolving system has been considered. In contrast to 2nd generation MA which assumes the pool of memes to be used being known a priori, a rule-based representation of local search is co-adapted alongside candidate solutions within the evolutionary system, thus capturing regular repeated features or patterns in the problem space.

Some design notes
The frequency and intensity of individual learning directly define the degree of evolution (exploration) against individual learning (exploitation) in the MA search, for a given fixed limited computational budget. Clearly, a more intense individual learning provides greater chance of convergence to the local optima but limits the amount of evolution that may be expended without incurring excessive computational resources. Therefore, care should be taken when setting these two parameters to balance the computational budget available in achieving maximum search performance. When only a portion of the population individuals undergo learning, the issue on which subset of individuals to improve need to be considered to maximize the utility of MA search. Last but not least, the individual learning procedure/meme used also favors a different neighborhood structure, hence the need to decide which meme or memes to use for a given optimization problem at hand would be required.

How often should individual learning be applied?
One of the first issues pertinent to memetic algorithm design is to consider how often the individual learning should be applied, i.e., individual learning frequency. In the effect of individual learning frequency on MA search performance was considered where various configurations of the individual learning frequency at different stages of the MA search were investigated. Conversely, it was shown in that it may be worthwhile to apply individual learning on every individual if the computational complexity of the individual learning is relatively low.

On which solutions should individual learning be used?
On the issue of selecting appropriate individuals among the EA population that should undergo individual learning, fitness-based and distribution-based strategies were studied for adapting the probability of applying individual learning on the population of chromosomes in continuous parametric search problems with Land extending the work to combinatorial optimization problems. Bambha et al. introduced a simulated heating technique for systematically integrating parameterized individual learning into evolutionary algorithms to achieve maximum solution quality.

Memetic algorithm

Memetic algorithms (MA) represent one of the recent growing areas of research in evolutionary computation. The term MA is now widely used as a synergy of evolutionary or any population-based approach with separate individual learning or local improvement procedures for problem search. Quite often, MA are also referred to in the literature as Baldwinian EAs, Lamarckian EAs, cultural algorithms or genetic local search.

Introduction
The theory of “Universal Darwinism” was coined by Richard Dawkins in 1983 to provide a unifying framework governing the evolution of any complex system. In particular, “Universal Darwinism” suggests that evolution is not exclusive to biological systems, i.e., it is not confined to the narrow context of the genes, but applicable to any complex system that exhibit the principles of inheritance, variation and selection, thus fulfilling the traits of an evolving system. For example, the new science of memetics represents the mind-universe analogue to genetics in culture evolution that stretches across the fields of biology, cognition and psychology, which has attracted significant attention in the last decades. The term “meme” was also introduced and defined by Dawkins [1] in 1976 as “the basic unit of cultural transmission, or imitation”, and in the English Oxford Dictionary as “an element of culture that may be considered to be passed on by non-genetic means”.

Inspired by both Darwinian principles of natural evolution and Dawkins’ notion of a meme, the term “Memetic Algorithm” (MA) was first introduced by Moscato in his technical report in 1989 where he viewed MA as being close to a form of population-based hybrid genetic algorithm (GA) coupled with an individual learning procedure capable of performing local refinements. The metaphorical parallels, on the one hand, to Darwinian evolution and, on the other hand, between memes and domain specific (local search) heuristics are captured within memetic algorithms thus rendering a methodology that balances well between generality and problem specificity. In a more diverse context, memetic algorithms are now used under various names including Hybrid Evolutionary Algorithms, Baldwinian Evolutionary Algorithms, Lamarckian Evolutionary Algorithms, Cultural Algorithms or Genetic Local Search. In the context of complex optimization, many different instantiations of memetic algorithms have been reported across a wide range of application domains, in general, converging to high quality solutions more efficiently than their conventional evolutionary counterparts.

In general, using the ideas of memetics within a computational framework is called "Memetic Computing" (MC). With MC, the traits of Universal Darwinism are more appropriately captured. Viewed in this perspective, MA is a more constrained notion of MC. More specifically, MA covers one area of MC, in particular dealing with areas of evolutionary algorithms that marry other deterministic refinement techniques for solving optimization problems. MC extends the notion of memes to cover conceptual entities of knowledge-enhanced procedures or representations.

Jasmati

Jasmati Rice is a genetically-engineered hybrid long grain of rice whose name is derived from Jasmine rice and Basmati. It is said to possess the traits of both grains - namely the softness (when cooked) of Basmati, and the nutty aroma of Jasmine - the latter in muted tones so as to be more subtle. Whereas Jasmine Rice is not as widely sold in the average American supermarket, Jasmati has become a more common find.

It appears to have been first created in the United States as a way of capitalizing upon the market successes of both Jasmine and Basmati rice, the patents of which were held by farmers in Thailand and India, respectively.[dubious – discuss] To what degree Jasmati is derived from either of its etymological parent grains is unknown and highly disputed. The patent for Jasmati, registered in 1993 by the Texas-based corporation, Ricetec, created had many legal implications for Thai and Indian farmers who rely heavily on the exports of the parent crops, and proceeded to cause considerable controversy.

Presently the debate seems to have simmered. As the degree to which Jasmati draws from Jasmine cannot be ascertained, the informed consumer should be aware that it is therefore a different grain and may or may not be a complete substitute for either Jasmine or Basmati. This also suggests that Jasmati may have its own unique merits as a cooking ingredient

Radiation reduced hybrid

Radiation reduced hybrid is procedure for discovering the location of genetic markers relative to one another. The relative location of these markers can be combined into a physical map or a genetic map. The radiation hybrid technique begins as another way to amplify and purify DNA, the first step in any sequencing project; radiation is used to break the DNA into pieces and these pieces are incorporated into the hybrid cell; the hybrid can be grown in large quantities. One can then check for the presence of various genetic markers using PCR and linkage analysis to resolve the distance between the markers. That is, if two genetic markers are near each other, they are less likely to be separated by the DNA-breaking radiation. The technique is similar to traditional linkage analysis, where we depend on genetic recombination to calculate the distance between two genetic markers.

Procedure
The procedure utilizes two cell lines, neither of which can survive in toxic media, but contain genes that that can resist the toxin when contained by the same cell. The cell line under study is irradiated, causing breaks in the DNA. These cells are fused with the other cell line, producing a hybrid. If the hybrid incorporates genes from both cells, it will be able to survive in the toxic media.

The cells that survive can be grown in large quantities, thus amplifying the DNA that was incorporated from the irradiated cell line. One can prepare a sample of DNA from the hybrid cell line and use PCR to amplify two specific genetic markers. By running the PCR products on a gel, one can determine if both markers are in the cell line. If both markers are usually in the hybrid, we can conclude that they are near each other.

Hybridism in plants

The earliest recorded observation of a hybrid plant is by JG Gmelin towards the end of the 17th century; the next is that of Thomas Fairchild, who in the second decade of the 18th century, produced a cross which is still grown in gardens under the name of "Fairchild's Sweet William." Linnaeus made many experiments in the cross-fertilization of plants and produced several hybrids. Joseph Gottlieb Kolreuter was a pioneer in plant hybridism, having discovered a great deal about the subject.

Hybridism in animals

The interest in animal hybridism was for a long time chiefly of a practical nature. An example of ancient hybridism is the case of the horse and donkey creating a mule, which is primarily used for transportation and heavy pulling. Animals in the panthera genus are well known for being able to procreate together, creating countless hybrids, like ligers and leguars. Hybrids are often created in dog breeding to create new cross-breeds. People often mix different dogs with poodles, because of the poodle's temperament and the fact that it does not shed its fur. Popular poodle hybrids are goldendoodles, schnoodles, cockapoos, and labradoodles. An example of a non-panthera cat hybrid is the savannah, which is a cross between an African wild cat called the serval and a domesticated cat.

Theory of hybridism

Charles Darwin was interested in hybridism as an experimental side of biology, but still more from the bearing of the facts on the theory of the origin of species. It is obvious that although hybridism is occasionally possible as an exception to the general infertility of species inter se, the exception is still more minimized when it is re-membered that the hybrid progeny usually display some degree of sterility. The main facts of hybridism appear to lend support to the old doctrine that there are placed between all species the barriers of mutual sterility. The argument for the fixity of species appears still stronger when the general infertility of species crossing is contrasted with the general fertility of the crossing of natural and artificial varieties. Darwin himself, and afterwards George Romanes, showed, however, that the theory of natural selection did not require the possibility of the commingling of specific types, and that there was no reason to suppose that the mutation of species should depend upon their mutual crossing. There existed more than enough evidence, and this has been added to since, to show that infertility with other species is no criterion of a species, and that there is no exact parallel between the degree of affinity between forms and their readiness to cross. The problem of hybridism is no more than the explanation of the generally reduced fertility of remoter crosses as compared with the generally increased fertility of crosses between organisms slightly different. Darwin considered and rejected the view that the inter-sterility of species could have been the result of natural selection.

"At one time it appeared to me probable, as it has to others, that the sterility of first crosses and of hybrids might have been slowly acquired through the natural selection of slightly lessened degrees of fertility, which, like any other variation, spontaneously appeared in certain individuals of one variety when crossed with those of another variety. For it would clearly be advantageous to two varieties or incipient species if they could be kept from blending, on the same principle that, when man is selecting at the same time two varieties, it is necessary that he should keep them separate. In the first place, it may be remarked that species inhabiting distinct regions are often sterile when crossed; now it could clearly have been of no advantage to such separated species to have been rendered mutually sterile and, consequently, this could not have been effected through natural selection; but it may perhaps be argued that, if a species were rendered sterile with some one compatriot, sterility with other species would follow as a necessary contingency. In the second place, it is almost as much opposed to the theory of natural selection as to that of special creation, that in reciprocal crosses the male element of one form should have been rendered utterly impotent on a second form, whilst at the same time the male element of this second form is enabled freely to fertilize the first form; for this peculiar state of the reproductive system could hardly have been advantageous to either species."

Darwin came to the conclusion that the sterility of crossed species must be due to some principle quite independent of natural selection. In his search for such a principle he brought together much evidence as to the instability of the reproductive system, pointing out in particular how frequently wild animals in captivity fail to breed, whereas some domesticated races have been so modified by confinement as to be fertile together although they are descended from species probably mutually infertile. He was disposed to regard the phenomena of differential sterility as, so to speak, by-products of the process of evolution.

George Romanes afterwards developed his theory of physiological selection, in which he supposed that the appearance of differential fertility within a species was the starting-point of new species; certain individuals by becoming fertile only inter se proceeded along lines of modification diverging from the lines followed by other members of the species. Physiological selection in fact would operate in the same fashion as geographical isolation; if a portion of a species separated on an island tends to become a new species, so also a portion separated by infertility with the others would tend to form a new species. According to Romanes, therefore, mutual infertility was the starting-point, not the result, of specific modification. Romanes, however, did not associate his interesting theory with a sufficient number of facts, and it has left little mark on the history of the subject. Alfred Russel Wallace, on the other hand, has argued that sterility between incipient species may have been increased by natural selection in the same fashion as other favourable variations are supposed to have been accumulated. He thought that "some slight degree of infertility was a not infrequent accompaniment of the external differences which always arise in a state of nature between varieties and incipient species."

August Weismann concluded, from an examination of a series of plant hybrids, that from the same cross hybrids of different character may be obtained, but that the characters are determined at the moment of fertilization; for he found that all the flowers on the same hybrid plant resembled one another in the minutest details of colour and pattern. Darwin already had pointed to the act of fertilization as the determining point, and it is in this direction that the theory of hybridism has made the greatest advance.

The starting-point of the modern views comes from the experiments and conclusions on plant hybrids made by Gregor Mendel and published in 1865. It is uncertain if Darwin had paid attention to this work; Romanes, writing in the 9th edition of the Encyclopædia Britannica, cited it without comment. First Hugo de Vries, then William Bateson and a series of observers returned to the work of Mendel, and made it the foundation of much experimental work and still more theory. It is still too soon to decide if the confident predictions of the Mendelians are justified, but it seems clear that a combination of Mendel's numerical results with Weismann's (see Heredity) conception of the particulate character of the germ-plasm, or hereditary material, is at the root of the phenomena of hybridism, and that Darwin was justified in supposing it to lie outside the sphere of natural selection and to be a fundamental fact of living matter.

Hybridism

Hybridism is the occurrence of offspring of genetically dissimilar parents or stock, especially the offspring produced by breeding plants or animals of different varieties, species, or races. The term hybridism is believed to be derived from the Latin word hybrida, hibrida or ibrida, translated to insult or outrage. As a general rule, animals and plants belonging to distinct species do not produce offspring when crossed with each other, and the term hybrid has been employed for the result of a fertile cross between individuals of different species. Examples of hybridism are mules, a cross between a horse and an ass, ligers and tigons, which are both crosses between lions and tigers, and the fictional jackalope, a humorous cross between a rabbit and a deer.

Hybrid Zone Case Study

Mussel populations show extensive hybridisation worldwide and are a well studied example of a marine hybrid zone. There are multiple sites of hybridisation between the species Mytilus edulis, M. trossulus and M. galloprovincialis across the Atlantic, Scandinavian and the Mediterranean Seas. These hybrid zones vary considerably. Some hybrid zones, such as the one in Newfoundland in Canada show remarkably few hybrids, while in Scandinavia most individuals are hybrids to some degree. Morphological and genetic differences are clear between these populations and it is believed that they are close to full speciation. They are probably maintained through a combination of hybrid unfitness, positive assortative mating and habitat segregation. In this summary focus will be on the Canadian hybrid zone in the North Atlantic, particularly that near Newfoundland which has been studied by researchers at Newfoundland Memorial University.

Based on the fossil record and genetic marker studies the following chronology is used to explain the Mussel hybrid zone:

The genus Mytilus is at one point restricted to the North Pacific but spreads to the Atlantic through the Bering Straight around 3.5 million years ago (Vermeij 1991).
M. trossulus evolves in the North Pacific and M. edulis in the Atlantic in near allopatry as migration across the Bering Straight is very low.
Subsequently M. galloprovincialis undergoes cladogenesis from M. edulis in the Mediterranean Sea after it is temporarily isolated from the Atlantic.
Recently M. trossulus from the Pacific enters the Atlantic and colonises shores on both sides. It spreads and forms secondary contact hybrid zones with M. edulis populations on coasts across Scandinavia and the eastern Atlantic.
Riginos and Cunningham (2005) includes a suggested pattern of migration of M. trossulus across the ocean based on a review of genetic marker studies.
The Canadian hybrid zone is unusual because both species are found along the entire shore (a mosaic pattern) instead of the typical cline found in most hybrid zones (Bates and Innes, 1995). Studies of mtDNA and allozymes in adult populations show that the distribution of genotypes between the two species is bimodal; pure parental types are most common (representing above 75% of individuals) while backcrosses close to parental forms are the next most prevalent. F1 hybrid crosses represent less than 2.5% of individuals (Saavedra et al., 1996).

The low frequency of F1 hybrids coupled with some introgression allows us to infer that although fertile hybrids can be produced, significant reproductive barriers exist and the two species are sufficiently deviated that they are now able to avoid recombinational collapse despite habitat sharing. Jiggins and Mallet (2000) have found that hybrid zones with low levels of F1 hybrids (bimodal distribution of hybrids) are highly stable and usually the result of assortative mating or fertilization, and not related to gross levels of genetic divergence or intrinsic genomic incompatibility. They hypothesized that such zones occurred between two populations that were close to full speciation and with some reinforcement. Toro et al. (2002) investigated whether different reproductive patterns and behaviour were the cause of this prezygotic isolation and discovered that M. edulis spawned over a 2-3 week period in July, while M. trossulus spawned over a more extensive period between late spring to early autumn. It was also found that hybrids were not infertile and exhibited normal reproductive development, allowing them to introgress with pure species. It was concluded that “differences in reproductive traits may partially explain the maintenance of the mussel hybrid zone in Newfoundland.”

The other likely candidate for hybrid zones stability is species segregation by habitat which has been investigated but not conclusively. Several studies have suggested that M. edulis are found in areas of lower salinity and less wave exposure at the heads of bays more than M. trosullus. M.trosullus appears to be favoured in habitats with higher wave exposure (Bates and Innes, 1995). The one subtidal (low wave action) site sampled by Bates and Innes had just 8% M. trossulus individuals. A similar segregation has been found in the Mediterranean hybrid zone with M. edulis also favouring more sheltered habitats compared to M. galloprovincialis (Bierne et al. 2003). If this is the case, this would provide partial habitat separation and reduce the probability of gametes of two species encountering one another and cross-fertilising. This would increase genetic distinctiveness despite the populations living in sympatry. However, conflicting results have been identified to this trend of habitat segregation and so these results are not conclusive (Riginos and Cunningham, 2005). It is suggested that differences in habitat are what has led to the very different type of hybrid zones in Scandinavia and Canada. Hybrid mussel fitness has not been properly investigated, so it is not possible to judge its effects on postzygotic isolation and whether it could cause reinforcement

Hybrid zone

A hybrid zone exists where the ranges of two interbreeding species meet. For a hybrid zone to be stable, the offspring produced by the cross (the hybrids) have to be less fit than members of the parent species, although this condition does not need to be met in the very first hybrid generation (F1 hybrid which can exhibit hybrid vigour). Some hybrid zones move, typically at a rate of 0.1-10 metres per year.

Hybrid zones are relatively rare, although a surprising number are now known to science. They present a problem to the taxonomy of the species involved, and the definition of species more generally. They are also important study systems for understanding how new species form (Hybrid speciation), as they are believed to be in transition to reproductive isolation.

These zones are often mapped including the current range of both species, with the overlap ranges highlighted.

Hybrid zones are locations where the hybrid offspring of two divergent populations (sometimes defined as subspecies or races) are prevalent and form a cline (Barton & Hewitt, 1985). Precise definitions of hybrid zones vary, some insist on increased variability of fitness within the zone, others that hybrids be identifiably different from parental forms and others that they represent secondary contact alone (Murray, 1985). They occur at the area of contact between two closely related but genetically different populations, each regarded as parental forms. Reviews of hybrid zones show varying widths between hundreds of metres and hundreds of kilometres and the presence of both gradual clines and stepped clines (Barton & Hewitt, 1985). They present a paradox for the biological definition of a species, usually “a population of actually or potentially interbreeding individuals that produce fertile offspring” (Mayr, 1942). Both parental forms by this definition are one species as they can produce fertile offspring at least some of the time. Despite this, the two populations remain identifiably different, conforming to an alternative definition of species as “taxa that retain their identity despite gene flow” (Barton and Hewitt, 1989). They are useful in the study of the processes of speciation as they provide natural examples of gene flow between populations that are at some point between representing a single species and representing multiple species in reproductive isolation.

The clines of hybrid zones can be observed by recording the frequency of certain diagnostic alleles or phenotypic characteristics for either population along a transect between the two populations. They almost always take the form of a sigmoid curve. They can be wide (gradual) or narrow (steep) depending on the ratio of hybrid survival to recombination of genes (Barton, 1983).

One form of hybrid zone results where one species has undergone allopatric speciation and the two new populations regain contact after a period of geographic isolation. The two populations then mate within an area of contact, producing 'hybrids' which contain a mixture of the alleles distinctive for each population. Thus novel genes flow from either side into the hybrid zone. Genes can also flow back into the distinct populations through interbreeding between hybrids and parental (non-hybrid) individuals (introgression) (Ridley 2003). These processes lead to the formation of a cline between the two pure forms within the hybrid zone.

Hybrid zones and gene flow do not inevitably lead to the recombination of the two populations involved, but can instead continue for thousands of years (White, et al., 1967.). The predominant explanation for this is that the hybrid zone represents a 'tension zone' between the conflicting effects of dispersal of parental forms and selection against hybrids (Bazykin, 1969.). Dispersal of individual parents leads to the creation of more hybrids within the hybrid zone. This results in gene flow between the two populations because of introgression. However, in many cases hybrids are less fit than parental forms because they lack the complete gene complexes of the parentals that make them well adapted to the environments either side of the hybrid zone. The more frequent death and sterility of hybrids forms a barrier to gene flow by making a 'hybrid sink' into which genes from parentals flow but rarely continue into the other population. Statistical models suggest that neutral alleles flow across this barrier very slowly while positively selected alleles will move across quite rapidly (Barton & Hewitt, 1985 p.135). An interesting outcome of this model is that hybrid zones are almost environment independent and can therefore move (Barton, 1979). Hybrids may not always be unfit in the very first generation, which can show hybrid vigour.

Several other models exist to explain hybrid zone stability, although the tension zone model is used in most cases. The dispersal-independent cline model does not consider dispersal at all, with the frequency of alleles finding different equilibria depending on the precise environmental conditions in a particular area. In each location, selection maintains a stable equilibria for each allele, resulting in a smooth cline. (Moore, 1977) The hybrids must therefore be fitter at some point along the cline. The wave of advance model sees multiple clines for individual alleles forming due to the progression of advantageous alleles from one population the other (Pialek and Barton, 1997).

Certain factors contribute to stability and steepness of hybrid zones within these models by reducing the frequency of inter-population mating and introgression. These include positive assortative mating within populations, habitat selection of different populations (examples of both these found in question 1 part B and question 2) and hybrid unfitness. Additionally, it is suggested that individuals in a populations near a tension zone (in which hybrids are less fit), will evolve methods of only mating with their own population to reduce the prevalence of unfit hybrids. This is dubbed reinforcement, and controversy remains as to its importance (Howard, 1993).

Hybrid zones can also occur across regions of primary contact in which parapatric speciation is taking place. As a population spreads across a contiguous area it may spread into an abruptly different environment. The populations will deviate and begin parapatric speciation – those in the new environment adapting to the different conditions. The point of contact between the older population and the newer population will be a stepped cline and a hybridisation zone can form. Despite this, the two populations will never have been fully isolated from one another, unlike in cases of secondary contact. This distinction may not be a very useful one as in practice it can be quite difficult

Hybrid vector

Hybrid vectors are vector viruses that are genetically engineered to have qualities of more than one vector.

How long should individual learning be run?
Individual learning intensity, til, is the amount of computational budget allocated to an iteration of individual learning, i.e., the maximum computational budget allowable for individual learning to expend on improving a single solution.

What individual learning method or meme should be used for a particular problem or individual?
In the context of continuous optimization, individual learning/individual learning exists in the form of local heuristics or conventional exact enumerative methods . Examples of individual learning strategies include the hill climbing, Simplex method, Newton/Quasi-Newton method, interior point methods, conjugate gradient method, line search and other local heuristics. Note that most of common individual learninger are deterministic.

In combinatorial optimization, on the other hand, individual learning methods commonly exists in the form of heuristics (which can be deterministic or stochastic), that are tailored to serve a problem of interest well. Typical heuristic procedures and schemes include the k-gene exchange, edge exchange, first-improvement, and many others.

Human fascination with hybridism

Many people become very interested in hybridism simply because of the fact that two species are blending together to create an entirely new species. This often ends up with interesting results, that simply fascinate people. Many cryptozoologists believe that there are thousands of undiscovered hybrids roaming the earth. Many cryptozoological monsters are believed to be hybrids. For example one theory about the origin of Bigfoot, or Sasquatch, is that if they do exist, they could be hybrids of men and apes.

Effect on biodiversity and food security

In agriculture and animal husbandry, the green revolution's use of conventional hybridization increased yields by breeding "high-yielding varieties". The replacement of locally indigenous breeds, compounded with unintentional cross-pollination and crossbreeding (genetic mixing), has reduced the gene pools of various wild and indigenous breeds resulting in the loss of genetic diversity. Since the indigenous breeds are often well-adapted to local extremes in climate and have immunity to local pathogens this can be a significant genetic erosion of the gene pool for future breeding. Therefore, commercial plant geneticists strive to breed "widely adapted" cultivars to counteract this tendency.

Limiting factors
A number of conditions exist that limit the success of hybridization, the most obvious is great genetic diversity between most species. But in animals and plants that are more closely related hybridization barriers can include morphological differences, differing times of fertility, mating behaviors and cues, physiological rejection of sperm cells or the developing embryo.[citation needed]

In plants, barriers to hybridization include blooming period differences, different pollinator vectors, inhibition of pollen tube growth, somatoplastic sterility, cytoplasmic-genic male sterility and structural differences of the chromosomes.

Mythical and legendary hybrids
Ancient folktales often contain mythological creatures, sometimes these are described as hybrids (e.g., Hippogriff as the offspring of a griffin and a horse, and the Minotaur which is the offspring of Pasiphaë and a white bull. More often they are kind of chimera, i.e., a composite of the physical attributes of two or more kinds of animals, mythical beasts, and often humans, with no suggestion that they are the result of interbreeding, e.g., Harpies, mermaids, and centaurs.

Genetic mixing and extinction

Regionally developed ecotypes can be threatened with extinction when new alleles or genes are introduced that alter that ecotype. This is sometimes called genetic mixing. Hybridization and introgression of new genetic material can lead to the replacement of local genotypes if the hybrids are more fit and have breeding advantages over the indigenous ecotype or species. These hybridization events can result from the introduction of non native genotypes by humans or through habitat modification, bringing previously isolated species into contact. Genetic mixing can be especially detrimental for rare species in isolated habitats, ultimately effecting the population to such a degree that none of the originally genetically distinct population remains.

Expression of parental traits in hybrids

When two distinct types of organisms breed with each other, the resulting hybrids typically have intermediate traits (e.g., one parent has red flowers, the other has white, and the hybrid, pink flowers). Commonly, hybrids also combine traits seen only separately in one parent or the other (e.g., a bird hybrid might combine the yellow head of one parent with the orange belly of the other). Most characteristics of the typical hybrid are of one of these two types, and so, in a strict sense, are not really new. However, an intermediate trait does differ from those seen in the parents (e.g., the pink flowers of the intermediate hybrid just mentioned are not seen in either of its parents). Likewise, combined traits are new when viewed as a combination.

In a hybrid, any trait that falls outside the range of parental variation is termed heterotic. Heterotic hybrids do have new traits, that is, they are not intermediate. Positive heterosis produces more robust hybrids, they might be stronger or bigger; while the term negative heterosis refers to weaker or smaller hybrids. Heterosis is common in both animal and plant hybrids. For example, hybrids between a lion and a tigress ("ligers") are much larger than either of the two progenitors, while a tigon (lioness × tiger) is smaller. Also the hybrids between the Common Pheasant (Phasianus colchicus) and domestic fowl (Gallus gallus) are larger than either of their parents, as are those produced between the Common Pheasant and hen Golden Pheasant (Chrysolophus pictus). Spurs are absent in hybrids of the former type, although present in both parents.

When populations hybridize, often the first generation (F1) hybrids are very uniform. Typically, however, the individual members of subsequent hybrid generations are quite variable. High levels of variability in a natural population, then, are indicative of hybridity. Researchers use this fact to ascertain whether a population is of hybrid origin. Since such variability generally occurs only in later hybrid generations, the existence of variable hybrids is also an indication that the hybrids in question are fertile.[citation needed]

Hybrids in nature

Hybridisation between two closely related species is actually a common occurrence in nature. Many hybrid zones are known where the ranges of two species meet, and hybrids are continually produced in great numbers. These hybrid zones are useful as biological model systems for studying the mechanisms of speciation (Hybrid speciation). Recently DNA analysis of a bear shot by a hunter in the North West Territories confirmed the existence of naturally-occurring and fertile grizzly–polar bear hybrids. There have been reports of similar supposed hybrids, but this is the first to be confirmed by DNA analysis. In 1943, Clara Helgason described a male bear shot by hunters during her childhood. It was large and off-white with hair all over its paws. The presence of hair on the bottom of the feet suggests it was a natural hybrid of Kodiak and Polar bear.

In some species, hybridisation plays an important role in evolutionary biology. While most hybrids are disadvantaged as a result of genetic incompatibility, the fittest survive, regardless of species boundaries. They may have a beneficial combination of traits allowing them to exploit new habitats or to succeed in a marginal habitat where the two parent species are disadvantaged. This has been seen in experiments on sunflower species. Unlike mutation, which affects only one gene, hybridisation creates multiple variations across genes or gene combinations simultaneously. Successful hybrids could evolve into new species within 50-60 generations. This leads some scientists to speculate that life is a genetic continuum rather than a series of self-contained species.

Where there are two closely related species living in the same area, less than 1 in 1000 individuals are likely to be hybrids because animals rarely choose a mate from a different species (otherwise species boundaries would completely break down). In some closely related species there are recognized "hybrid zones".

Some species of Heliconius butterflies exhibit dramatic geographical polymorphism of their wing patterns, which act as aposematic signals advertising their unpalatability to potential predators. Where different-looking geographical races abut, inter-racial hybrids are common, healthy and fertile. Heliconius hybrids can breed with other hybrid individuals and with individuals of either parental race. These hybrid backcrosses are disadvantaged by natural selection because they lack the parental form's warning coloration, and are therefore not avoided by predators.

A similar case in mammals is hybrid White-Tail/Mule Deer. The hybrids don't inherit either parent's escape strategy. White-tail Deer dash while Mule Deer bound. The hybrids are easier prey than the parent species.

In birds, healthy Galapagos Finch hybrids are relatively common, but their beaks are intermediate in shape and less efficient feeding tools than the specialised beaks of the parental species so they lose out in the competition for food. Following a major storm in 1983, the local habitat changed so that new types of plants began to flourish, and in this changed habitat, the hybrids had an advantage over the birds with specialised beaks - demonstrating the role of hybridization in exploiting new ecological niches. If the change in environmental conditions is permanent or is radical enough that the parental species cannot survive, the hybrids become the dominant form. Otherwise, the parental species will re-establish themselves when the environmental change is reversed, and hybrids will remain in the minority.

Natural hybrids may occur when a species is introduced into a new habitat. In Britain, there is hybridisation of native European Red Deer and introduced Chinese Sika Deer. Conservationists want to protect the Red Deer, but the environment favors the Sika Deer genes. There is a similar situation with White-headed Ducks and Ruddy Ducks.

Examples of species hybrids

Some plant hybrids include:

Leyland cypress, [x Cupressocyparis leylandii] hybrid between Monterey Cypress and Nootka Cypress.
Limequat, lime and kumquat hybrid.
Loganberry, a hybrid between raspberry and blackberry.
London plane, a hybrid between Plantanus orientalis Oriental plane and Plantanus occidentalis American plane (American sycamore). Thus forming [Plantanus x acerfolia]
Peppermint, a hybrid between spearmint and water mint.
Tangelo, a hybrid of a Mandarin orange and a pomelo which may have been developed in Asia about 3,500 years ago.
Triticale, a wheat–rye hybrid.
Wheat; most modern and ancient wheat breeds are themselves hybrids. Bread wheat is a hexaploid hybrid of three wild grasses; durum (pasta) wheat is a tetraploid hybrid of two wild grasses.
Triangle of U: cabbage, mustard etc.
Grapefruit, hybrid between a pomelo and the Jamaican sweet orange.
Some natural hybrids are:

White flag Iris, a sterile hybrid which spreads by rhizome division
Evening primrose, a flower which was the subject of famous experiments by Hugo de Vries on polyploidy and diploidy.
Some horticultural hybrids:

Dianthus x allwoodii, is a hybrid between Dianthus caryophyllus × Dianthus plumarius. This is an "interspecific hybrid" or hybrid between two species in the same genus.
x Heucherella tiarelloides, or Heuchera sanguinea × Tiarella cordifolia is an "intergeneric hybrid" a hybrid between two different genera.
Quercus x warei [Quercus robur x Quercus bicolor] Kindred Spirit Hybrid

heterosis

Plant hybrids, especially, are often stronger than either parent variety, a phenomenon which when present is known as hybrid vigor (heterosis) or heterozygote advantage. Plant breeders make use of a number of techniques to produce hybrids, including line breeding and the formation of complex hybrids. An economically important example is hybrid maize (corn), which provides a considerable seed yield advantage over open pollinated varieties. Hybrid seed dominates the commercial maize seed market in the United States, Canada and many other major maize producing countries.

Hybrid plants

Plant species hybridize more readily than animal species, and the resulting hybrids are more often fertile hybrids and may reproduce, though there still exist sterile hybrids and selective hybrid elimination where the offspring are less able to survive and are thus eliminated before they can reproduce. A number of plant species are the result of hybridization and polyploidy with many plant species easily cross pollinating and producing viable seeds, the distinction between each species is often maintained by geographical isolation or differences in the flowering period. Since plants hybridize frequently without much work, they are often created by humans in order to produce improved plants. These improvements can include the production of more or improved; seeds, fruits or other plant parts for consumption, or to make a plant more winter or heat hardy or improve its growth and/or appearance for use in horticulture. Much work is now being done with hybrids to produce more disease resistant plants for both agricultural and horticultural crops. In many groups of plants hybridization has been used to produce larger and more showy flowers and new flower colors.


A sterile Trillium hybrid between Trillium cernuum and Trillium grandiflorumMany plant genera and species have their origins in polyploidy. Autopolyploidy resulting from the sudden multiplication in the number of chromosomes in typical normal populations caused by unsuccessful separation of the chromosomes during meiosis. Tetraploids or plants with four sets of chromosomes are common in a number of different groups of plants and over time these plants can differentiate into distinct species from the normal diploid line. In Oenothera lamarchiana the diploid species has 14 chromosomes, this species has spontaneously given rise to plants with 28 chromosomes that have been given the name Oenthera gigas. Tetraploids can develop into a breeding population within the diploid population and when hybrids are formed with the diploid population the resulting offspring tend to be sterile triploids, thus effectively stopping the intermixing of genes between the two groups of plants (unless the diploids, in rare cases, produce unreduced gametes).

Another form of polyploidy called allopolyploidy occurs when two different species mate and produce hybrids. Usually the typical chromosome number is doubled in successful allopolyploid species, with four sets of chromosomes the genotypes can sort out to form a complete diploid set from the parent species, thus they can produce fertile offspring that can mate and reproduce with each other but can not back-cross with the parent species. Allopolyploidy in plants often gives them a condition called hybrid vigour, which results in plants that are larger and stronger growing than either of the two parent species. Allopolyploids are often more aggressive growing and can be invaders of new habitats.

Sterility in a hybrid is often a result of chromosome number; if parents are of differing chromosome pair number, the offspring will have an odd number of chromosomes, leaving them unable to produce chromosomally balanced gametes. While this is a negative in a crop such as wheat, when growing a crop which produces no seeds would be pointless, it is an attractive attribute in some fruits. Bananas and seedless watermelon, for instance, are intentionally bred to be triploid, so that they will produce no seeds. Many hybrids are created by humans, but natural hybrids occur as well.

Examples of hybrid animals

A "Zeedonk", a zebra/donkey hybrid
A "Liger", a Lion/Tiger hybrid
A "Jaglion", a Jaguar/Lion hybridHybrid Iguana is single cross hybrid, result of natural inbreeding from male marine iguana and female land Iguana since late 2000s.
Equid hybrids
Mule, a cross of female horse and a male donkey.
Hinny, a cross between a female donkey and a male horse. Mule and hinny are examples of reciprocal hybrids.
Zebroids
Zeedonk or Zonkey, a zebra/donkey cross.
Zorse, a zebra/horse cross
Zony or Zetland, a zebra/pony cross ("zony" is a generic term; "zetland" is specifically a hybrid of the Shetland pony breed with a zebra)
Bovid hybrids
Dzo, zo or yakow; a cross between a domestic cow/bull and a yak.
Beefalo, a cross of an American bison and a domestic cow. This is a fertile breed; this along with genetic evidence has caused them to be recently reclassified into the same genus, Bos.
Zubron, a hybrid between wisent (European bison) and domestic cow.
Sheep-goat hybrids, such as the Toast of Botswana.
Ursid hybrids, such as the grizzly-polar bear hybrid, occur between black bears, brown bears, and polar bears.
Felid hybrids
Savannah cats are the hybrid cross between an African serval cat and a domestic cat
A hybrid between a Bengal tiger and a Siberian tiger is an example of an intra-specific hybrid.
Ligers and tigons (crosses between a lion and a tiger) and other Panthera hybrids such as the lijagulep. Various other wild cat crosses are known involving the lynx, bobcat, leopard, serval, etc.
Bengal cat, a cross between the Asian leopard cat and the domestic cat, one of many hybrids between the domestic cat and wild cat species. The domestic cat, African wild cat and European wildcat may be considered variant populations of the same species (Felis silvestris), making such crosses non-hybrids.
Fertile canid hybrids occur between coyotes, wolves, dingoes, jackals and domestic dogs.
Hybrids between blacks and white rhinoceroses have been recognized.
Hybrids between spotted owls and barred owls
Cama, a cross between a camel and a llama, also an intergeneric hybrid.
Wholphin, a fertile but very rare cross between a false killer whale and a bottlenose dolphin.
A fertile cross between a king snake and an corn snake.
At Chester Zoo in the United Kingdom, a cross between an African elephant (male) and an Asian elephant (female). The male calf was named Motty. It died of intestinal infection after twelve days.
Cagebird breeders sometimes breed hybrids between species of finch, such as goldfinch x canary. These birds are known as mules.
Gamebird hybrids, hybrids between gamebirds and domestic fowl, including chickens, guineafowl and peafowl, interfamilial hybrids.
Numerous macaw hybrids are also known.
Red kite x black kite: five bred unintentionally at a falconry center in England. (It is reported[weasel words] that the black kite (the male) refused female black kites but mated with two female red kites.)
Hybridization between the endemic Cuban crocodile (Crocodilus rhombifer) and the widely distributed American crocodile (Crocodilus acutus) is causing conservation problems for the former species as a threat to its genetic integrity.[clarification needed]
Blood parrot cichlid, which is probably created by crossing a red head cihclid and a Midas cichlid or red devil cichlid
The mulard duck, hybrid of the domestic pekin duck and domesticated muscovy ducks.
Hybrids should not be confused with genetic chimeras such as that between sheep and goat known as the geep. Wider interspecific hybrids can be made via in vitro fertilization or somatic hybridization, however the resulting cells are not able to develop into a full organism. An example of interspecific hybrid cell lines is humster (hamster x human) cells.

Domestic and wild hybrids

Hybrids between domesticated and wild animals in particular may be problematic. Breeders of domesticated species discourage crossbreeding with wild species, unless a deliberate decision is made to incorporate a trait of a wild ancestor back into a given breed or strain. Wild populations of animals and plants have evolved naturally over millions of years through a process of natural selection in contrast to human controlled selective breeding or artificial selection for desirable traits from the human point of view. Normally, these two methods of reproduction operate independently of one another. However, an intermediate form of selective breeding, wherein animals or plants are bred by humans, but with an eye to adaptation to natural region-specific conditions and an acceptance of natural selection to weed out undesirable traits, created many ancient domesticated breeds or types now known as landraces.

Many times, domesticated species live in or near areas which also still hold naturally evolved, region-specific wild ancestor species and subspecies. In some cases, a domesticated species of plant or animal may become feral, living wild. Other times, a wild species will come into an area inhabited by a domesticated species. Some of these situations lead to the creation of hybridized plants or animals, a cross between the native species and a domesticated one. This type of crossbreeding, termed genetic pollution by those who are concerned about preserving the genetic base of the wild species, has become a major concern. Hybridization is also a concern to the breeders of purebred species as well, particularly if the gene pool is small and if such crossbreeding or hybridization threatens the genetic base of the domesticated purebred population.

The concern with genetic pollution of a wild population is that hybridized animals and plants may not be as genetically strong as naturally evolved region specific wild ancestors wildlife which can survive without human husbandry and have high immunity to natural diseases. The concern of purebred breeders with wildlife hybridizing a domesticated species is that it can coarsen or degrade the specific qualities of a breed developed for a specific purpose, sometimes over many generations. Thus, both purebred breeders and wildlife biologists share a common interest in preventing accidental hybridization.

Seed saving

In agriculture and gardening, seed saving is the practice of saving seeds or other reproductive material (e.g. tubers) from open-pollinated vegetables, grain, herbs, and flowers for use from year to year for annuals and nuts, tree fruits, and berries for perennials and trees. This is the traditional way farms and gardens were maintained. In recent decades, there has been a major shift to purchasing seed annually from commercial seed suppliers, and to hybridized or cloned plants that do not produce seed that remains "true to type"-retaining the parent's characteristics- from seed. Much of the grassroots seed-saving activity today is the work of home gardeners. However, it is gaining popularity among organic farmers, permaculturists and enthusiasts with cultural or environmentalist interests.

Open pollination is the key to seed saving. Plants that reproduce through natural means tend to adapt to local conditions over time, and evolve as reliable performers, particularly in their localities, known as landraces or "folk varieties". The modern trend to rely on hybridized and cloned plants negates these evolutionary processes. Hybrid plants are artificially cross-pollinated, and bred to favor desirable characteristics, like higher yield (in monocultures) and more uniform size to accommodate mechanized harvesting. However, the seed produced by the second generation (F2) of the hybrid does not reliably produce a true copy of that hybrid (it 'segregates') and often loses much of its yield potential. Likewise the cloned cultivars of many perennials such as seedless grapes have lost the ability to even reproduce by seeds. [citation needed]

While comprehensive figures are hard to come by, one popular view today holds that thousands of varieties of vegetables and flowers are being lost, due to reliance on commercial hybrid seed. Widespread use of a relatively few mass-marketed hybrid seed varieties, in both home gardening and commercial farming, is said to be eliminating many open-pollinated varieties, especially the local variations that were naturally developed, when local seed-saving was the common practice. The concern is that this erodes the gene pool, resulting at some point in less hardy, more vulnerable plants. Countering this trend (an environmental and sustainability issue), and an affinity for variety and tradition, are the principal motivations for many large seed-saving groups.

To be successful at seed saving, new skills need to be developed that enhance the capacity of growers to ensure that desired characteristics are retained in their landraces: learning the minimum number of plants to be grown which will preserve inherent genetic diversity, recognizing the preferred characteristics of the cultivar being grown so that plants that are not breeding true are selected against, understanding and promoting the breeding of improvements to the cultivar, using seed storage methods that maintain viability, learning the conditions that maximize germination, and detecting the presence of diseases that are seed-borne so that they can be eliminated.

Care must be taken, as training materials regarding seed production, cleaning, storage, and maintenance often focus on making landraces more uniform, distinct and stable (usually for commercial application) which can result in the loss of valuable adaptive traits unique to local varieties (Jarvis et al., 2000).

Additionally, there is a matter of localized nature to be considered. In the upper northern hemisphere, and lower southern, one sees a seasonal change in terms of a cooler winter. Many plants go-to-seed and then go dormant. These seeds must hibernate until their respective spring season.

However, in the tropics, such as Puerto Rico or nearer the equator, tropical fruits and plants reign. In this case, one eating a local fruit must exercise caution in tossing a seed out an open window after eating the flesh as it may sprout. These seeds having a great, if rather temporary, vitality, with no dormant period, and a viability of mere hours at best in most cases once the flesh of the fruit is removed.

To successfully save seed, understanding and mimicking nature of the plant seed in question, is the direction in which to go. Cold areas experience the dehumidifying aspects of winter, just as chapped hands and lips do, as well as cold. This is not seen in the tropics.

hybrid seed

In agriculture and gardening, hybrid seed is seed produced by artificially cross-pollinated plants. Hybrids are bred to improve the characteristics of the resulting plants, such as better yield, greater uniformity, improved color, disease resistance, and so forth. Today, hybrid seed is predominant in agriculture and home gardening, and is one of the main contributing factors to the dramatic rise in agricultural output during the last half of the 20th century. In the US, the commercial market was launched in the 1920s, with the first hybrid maize. Hybrid seed cannot be saved, as the seed from the first generation of hybrid plants does not reliably produce true copies, therefore, new seed must be purchased for each planting.

Interspecific hybrids

Interspecific hybrids are bred by mating two species, normally from within the same genus. The offspring display traits and characteristics of both parents. The offspring of an interspecific cross are very often sterile; thus, hybrid sterility prevents the movement of genes from one species to the other, keeping both species distinct. Sterility is often attributed to the different number of chromosomes the two species have, for example donkeys have 62 chromosomes, while horses have 64 chromosomes, and mules and hinnies have 63 chromosomes. Mules, hinnies, and other normally sterile interspecific hybrids cannot produce viable gametes because the extra chromosome cannot make a homologous pair at meiosis, meiosis is disrupted, and viable sperm and eggs are not formed. However, fertility in female mules has been reported with a donkey as the father.

Most often other processes occurring in plants and animals keep gametic isolation and species distinction. Species often have different mating or courtship patterns or behaviors, the breeding seasons may be distinct and even if mating does occur antigenic reactions to the sperm of other species prevent fertilization or embryo development. The Lonicera fly is the first known animal species that resulted from natural hybridization. Until the discovery of the Lonicera fly, this process was known to occur in nature only among plants

While it is possible to predict the genetic composition of a backcross on average, it is not possible to accurately predict the composition of a particular backcrossed individual, due to random segregation of chromosomes. In a species with two pairs of chromosomes, a twice backcrossed individual would be predicted to contain 12.5% of one species' genome (say, species A). However, it may, in fact, still be a 50% hybrid if the chromosomes from species A were lucky in two successive segregations, and meiotic crossovers happened near the telomeres. The chance of this is fairly high: (where the "two times two" comes about from two rounds of meiosis with two chromosomes); however, this probability declines markedly with chromosome number and so the actual composition of a hybrid will be increasingly closer to the predicted composition.

Hybrids are often named by the portmanteau method, combining the names of the two parent species. For example, a zeedonk is a cross between a zebra and a donkey. Since the traits of hybrid offspring often vary depending on which species was mother and which was father, it is traditional to use the father's species as the first half of the portmanteau. For example, a liger is a cross between a male lion and a female tiger, while a tigon is a cross between a male tiger and a female lion.

Hybrid (biology)

In biology and specifically genetics, hybrid has several meanings, all referring to the offspring of sexual reproduction.

In general usage, hybrid is synonymous with heterozygous: any offspring resulting from the mating of two distinctly homozygous individuals
a genetic hybrid carries two different alleles of the same gene
a structural hybrid results from the fusion of gametes that have differing structure in at least one chromosome, as a result of structural abnormalities
a numerical hybrid results from the fusion of gametes having different haploid numbers of chromosomes
a permanent hybrid is a situation where only the heterozygous genotype occurs, because all homozygous combinations are lethal.
From a taxonomic perspective, hybrid refers to offspring resulting from the interbreeding between two animals or plants of different taxa.

Hybrids between different subspecies within a species (such as between the Bengal tiger and Siberian tiger) are known as intra-specific hybrids. Hybrids between different species within the same genus (such as between lions and tigers) are sometimes known as interspecific hybrids or crosses. Hybrids between different genera (such as between sheep and goats) are known as intergeneric hybrids. Extremely rare interfamilial hybrids have been known to occur (such as the guineafowl hybrids). No interordinal (between different orders) animal hybrids are known.
The second type of hybrid consists of crosses between populations, breeds or cultivars within a single species. This meaning is often used in plant and animal breeding, where hybrids are commonly produced and selected because they have desirable characteristics not found or inconsistently present in the parent individuals or populations. This flow of genetic material between populations or races is often called hybridization.