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by the animal behaviorist temple grandin roosters bred for fast growth or heavy muscles did not know how to perform typical rooster courtship dances which alienated the roosters from hens and led the roosters to kill the hens after mating with them 13 a soviet attempt to breed lab rats with higher intelligence led to cases of neurosis severe enough to make the animals incapable of any problem solving unless drugs like phenazepam were used 14 the observable phenomenon of hybrid vigor stands in contrast to the notion of breed purity however on the other hand indiscriminate breeding of crossbred or hybrid animals may also result in degradation of quality studies in evolutionary physiology behavioral genetics and other areas of organismal biology have also made use of deliberate selective breeding though longer generation times and greater difficulty in breeding can make these projects challenging in such vertebrates as house mice 15 16 17 plant breeding edit main article plant breeding researchers at the usda have selectively bred carrots with a variety of colors the process of plant breeding has been used for thousands of years and began with the domestication of wild plants into uniform and predictable agricultural cultigens these high yielding varieties have been particularly important in agriculture as crops improved humans were able to move from hunter gatherer style living to a mix of hunter gatherer and agriculture practices 18 although these higher yielding plants were derived from an extremely primitive version of plant breeding this form of agriculture was an investment that the people who grew them were planting then could have a more varied diet this meant that they did not completely stop their hunting and gathering immediately but instead over time transitioned and ultimately favored agriculture 19 originally this was due to humans not wanting to risk using all their time and resources for their crops just to fail which was promptly called play farming due to the idea of farmers experimenting with agriculture 19 in addition the ability for humans to stay within one place for food and create permanent settlements made the process move along faster 20 during this transitional period crops began to acclimate and evolve with humans encouraging humans to invest further into crops over time this reliance on plant breeding has created problems as highlighted by the book botany of desire where michael pollan shows the connection between basic human desires through four different plants apples for sweetness tulips for beauty cannabis for intoxication and potatoes for control in a form of coevolution humans have influenced these plants as much as the plants have influenced the people that consume them 21 selective plant breeding is also used in research to produce transgenic animals that breed true i e are homozygous for artificially inserted or deleted genes 22 selective breeding in aquaculture edit selective breeding in aquaculture holds high potential for the genetic improvement of fish and shellfish for the process of production unlike terrestrial livestock the potential benefits of selective breeding in aquaculture were not realized until recently this is because high mortality led to the selection of only a few broodstock causing inbreeding depression which then forced the use of wild broodstock this was evident in selective breeding programs for growth rate which resulted in slow growth and high mortality 23 control of the reproduction cycle was one of the main reasons as it is a requisite for selective breeding programs artificial reproduction was not achieved because of the difficulties in hatching or feeding some farmed species such as eel and yellowtail farming 24 a suspected reason associated with the late realization of success in selective breeding programs in aquaculture was the education of the concerned people researchers advisory personnel and fish farmers the education of fish biologists paid less attention to quantitative genetics and breeding plans 25 another was the failure of documentation of the genetic gains in successive generations this in turn led to failure in quantifying economic benefits that successful selective breeding programs produce documentation of the genetic changes was considered important as they help in fine tuning further selection schemes 23 quality traits in aquaculture edit aquaculture species are reared for particular traits such as growth rate survival rate meat quality resistance to diseases age at sexual maturation fecundity shell traits like shell size shell color etc growth rate growth rate is normally measured as either body weight or body length this trait is of great economic importance for all aquaculture species as faster growth rate speeds up the turnover of production 25 improved growth rates show that farmed animals utilize their feed more efficiently through a positive correlated response 24 survival rate survival rate may take into account the degrees of resistance to diseases 24 this may also see the stress response as fish under stress are highly vulnerable to diseases 25 the stress fish experience could be of biological chemical or environmental influence meat quality the quality of fish is of great economic importance in the market fish quality usually takes into account size meatiness and percentage of fat color of flesh taste shape of the body ideal oil and omega 3 content 24 26 age at sexual maturation the age of maturity in aquaculture species is another very important attribute for farmers as during early maturation the species divert all their energy to gonad production affecting growth and meat production and are more susceptible to health problems gjerde 1986 fecundity as the fecundity in fish and shellfish is usually high it is not considered as a major trait for improvement however selective breeding practices may consider the size of the egg and correlate it with survival and early growth rate 24 finfish response to selection edit salmonids edit gjedrem 1979 showed that selection of atlantic salmon salmo salar led to an increase in body weight by 30 per generation a comparative study on the performance of select atlantic salmon with wild fish was conducted by akvaforsk genetics centre in norway the traits for which the selection was done included growth rate feed consumption protein retention energy retention and feed conversion efficiency selected fish had a twice better growth rate a 40 higher feed intake and an increased protein and energy retention this led to an overall 20 better fed conversion efficiency as compared to the wild stock 27 atlantic salmon have also been selected for resistance to bacterial and viral diseases selection was done to check resistance to infectious pancreatic necrosis virus ipnv the results showed 66 6 mortality for low resistant species whereas the high resistant species showed 29 3 mortality compared to wild species 28 rainbow trout s gairdneri was reported to show large improvements in growth rate after 7 10 generations of selection 29 kincaid et al 1977 showed that growth gains by 30 could be achieved by selectively breeding rainbow trout for three generations 30 a 7 increase in growth was recorded per generation for rainbow trout by kause et al 2005 31 in japan high resistance to ipnv in rainbow trout has been achieved by selectively breeding the stock resistant strains were found to have an average mortality of 4 3 whereas 96 1 mortality was observed in a highly sensitive strain 32 coho salmon oncorhynchus kisutch increase in weight was found to be more than 60 after four generations of selective breeding 33 in chile neira et al 2006 conducted experiments on early spawning dates in coho salmon after selectively breeding the fish for four generations spawning dates were 13 15 days earlier 34 cyprinids selective breeding programs for the common carp cyprinus carpio include improvement in growth shape and resistance to disease experiments carried out in the ussr used crossings of broodstocks to increase genetic diversity and then selected the species for traits like growth rate exterior traits and viability and or adaptation to environmental conditions like variations in temperature kirpichnikov et al 1974 35 and babouchkine 1987 36 selected carp for fast growth and tolerance to cold the ropsha carp the results showed a 30 40 to 77 4 improvement of cold tolerance but did not provide any data for growth rate an increase in growth rate was observed in the second generation in vietnam 37 moav and wohlfarth 1976 showed positive results when selecting for slower growth for three generations compared to selecting for faster growth schaperclaus 1962 showed resistance to the dropsy disease wherein selected lines suffered low mortality 11 5 compared to unselected 57 38 channel catfish edit growth was seen to increase by 12 20 in selectively bred iictalurus punctatus 39 more recently the response of the channel catfish to selection for improved growth rate was found to be approximately 80 that is an average of 13 per generation shellfish response to selection edit oysters edit selection for live weight of pacific oysters showed improvements ranging from 0 4 to 25 6 compared to the wild stock 40 sydney rock oysters saccostrea commercialis showed a 4 increase after one generation and a 15 increase after two generations 41 42 chilean oysters ostrea chilensis selected for improvement in live weight and shell length showed a 10 13 gain in one generation bonamia ostrea is a protistan parasite that causes catastrophic losses nearly 98 in european flat oyster ostrea edulis l this protistan parasite is endemic to three oyster regions in europe selective breeding programs show that o edulis susceptibility to the infection differs across oyster strains in europe a study carried out by culloty et al showed that rossmore oysters in cork harbour ireland had better resistance compared to other irish strains a selective breeding program at cork harbour uses broodstock from 3 to 4 year old survivors and is further controlled until a viable percentage reaches market size 43 44 over the years rossmore oysters have shown to develop lower prevalence of b ostreae infection and percentage mortality ragone calvo et al 2003 selectively bred the eastern oyster crassostrea virginica for resistance against co occurring parasites haplosporidium nelson msx and perkinsus marinus dermo they achieved dual resistance to the disease in four generations of selective breeding the oysters showed higher growth and survival rates and low susceptibility to the infections at the end of the experiment artificially selected c virginica showed a 34 48 higher survival rate 45 penaeid shrimps edit selection for growth in penaeid shrimps yielded successful results a selective breeding program for litopenaeus stylirostris saw an 18 increase in growth after the fourth generation and 21 growth after the fifth generation 46 marsupenaeus japonicas showed a 10 7 increase in growth after the first generation 47 argue et al 2002 conducted a selective breeding program on the pacific white shrimp litopenaeus vannamei at the oceanic institute waimanalo usa from 1995 to 1998 they reported significant responses to selection compared to the unselected control shrimps after one generation a 21 increase was observed in growth and 18 4 increase in survival to tsv 48 the taura syndrome virus tsv causes mortalities of 70 or more in shrimps c i oceanos s a in colombia selected the survivors of the disease from infected ponds and used them as parents for the next generation they achieved satisfying results in two or three generations wherein survival rates approached levels before the outbreak of the disease 49 the resulting heavy losses up to 90 caused by infectious hypodermal and haematopoietic necrosis virus ihhnv caused a number of shrimp farming industries started to selectively breed shrimps resistant to this disease successful outcomes led to development of super shrimp a selected line of l stylirostris that is resistant to ihhnv infection tang et al 2000 confirmed this by showing no mortalities in ihhnv challenged super shrimp post larvae and juveniles 50 aquatic species versus terrestrial livestock edit selective breeding programs for aquatic species provide better outcomes compared to terrestrial livestock this higher response to selection of aquatic farmed species can be attributed to the following high fecundity in both sexes fish and shellfish enabling higher selection intensity large phenotypic and genetic variation in the selected traits selective breeding in aquaculture provide remarkable economic benefits to the industry the primary one being that it reduces production costs due to faster turnover rates when selective breeding is carried out some characteristics are lost for others that may suit a specific environment or situation 51 this is because of faster growth rates decreased maintenance rates increased energy and protein retention and better feed efficiency 23 applying genetic improvement programs to aquaculture species will increase their productivity thus allowing them to meet the increasing demands of growing populations conversely selective breeding within aquaculture can create problems within the biodiversity of both stock and wild fish which can hurt the industry down the road although there is great potential to improve aquaculture due to the current lack of domestication it is essential that the genetic diversity of the fish are preserved through proper genetic management as we domesticate these species 52 it is not uncommon for fish to escape the nets or pens that they are kept in especially en masse if these fish are farmed in areas they are not native to they may be able to establish themselves and outcompete native populations of fish and cause ecological harm as an invasive species 53 furthermore if they are in areas where the fish being farmed are also native their genetics are selectively bred rather than being wild these farmed fish could breed with the natives which could be problematic in the sense that they would have been bred for consumption rather than by chance this results in an overall decrease in genetic diversity and rendering local fish populations less fit for survival 53 if proper management is not being undertaken then the economic benefits and the diversity of the fish species will falter 52 advantages and disadvantages edit selective breeding is a direct way to determine if a specific trait can evolve in response to selection a single generation method of breeding is not as accurate or direct the process is also more practical and easier to understand than sibling analysis selective breeding is better for traits such as physiology and behavior that are hard to measure because it requires fewer individuals to test than single generation testing however there are disadvantages to this process 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