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Selective breeding
Selective breeding
Selective breeding
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Evolutionary biology
Darwin's finches by John Gould
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Mutation and selection
A Belgian Blue cow. The defect in the breed's myostatin gene is maintained through linebreeding and is responsible for its accelerated lean muscle growth.
This Chihuahua mix and Great Dane shows the wide range of dog breed sizes created using selective breeding.
Selective breeding transformed teosinte's few fruitcases (left) into modern maize's rows of exposed kernels (right).

Selective breeding (also called artificial selection) is the process by which humans use animal breeding and plant breeding to selectively develop particular phenotypic traits (characteristics) by choosing which typically animal or plant males and females will sexually reproduce and have offspring together. Domesticated animals are known as breeds, normally bred by a professional breeder, while domesticated plants are known as varieties, cultigens, cultivars, or breeds.[1] Two purebred animals of different breeds produce a crossbreed, and crossbred plants are called hybrids. Flowers, vegetables and fruit-trees may be bred by amateurs and commercial or non-commercial professionals: major crops are usually the provenance of the professionals.

In animal breeding artificial selection is often combined with techniques such as inbreeding, linebreeding, and outcrossing. In plant breeding, similar methods are used. Charles Darwin discussed how selective breeding had been successful in producing change over time in his 1859 book, On the Origin of Species. Its first chapter discusses selective breeding and domestication of such animals as pigeons, cats, cattle, and dogs. Darwin used artificial selection as an analogy to propose and explain the theory of natural selection but distinguished the latter from the former as a separate process that is non-directed.[2][3][4]

The deliberate exploitation of selective breeding to produce desired results has become very common in agriculture and experimental biology.

Selective breeding can be unintentional, for example, resulting from the process of human cultivation; and it may also produce unintended – desirable or undesirable – results. For example, in some grains, an increase in seed size may have resulted from certain ploughing practices rather than from the intentional selection of larger seeds. Most likely, there has been an interdependence between natural and artificial factors that have resulted in plant domestication.[5]

History

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Selective breeding of both plants and animals has been practiced since prehistory; key species such as wheat, rice, and dogs have been significantly different from their wild ancestors for millennia, and maize, which required especially large changes from teosinte, its wild form, was selectively bred in Mesoamerica. Selective breeding was practiced by the Romans.[6] Treatises as much as 2,000 years old give advice on selecting animals for different purposes, and these ancient works cite still older authorities, such as Mago the Carthaginian.[7] The notion of selective breeding was later expressed by the polymath Abu Rayhan Biruni in the 11th century. He noted the idea in his book titled India, which included various examples.[8]

The agriculturist selects his corn, letting grow as much as he requires, and tearing out the remainder. The forester leaves those branches which he perceives to be excellent, whilst he cuts away all others. The bees kill those of their kind who only eat, but do not work in their beehive.

— Abu Rayhan Biruni, India

Selective breeding was established as a scientific practice by Robert Bakewell during the British Agricultural Revolution in the 18th century. Arguably, his most important breeding program was with sheep. Using native stock, he was able to quickly select for large, yet fine-boned sheep, with long, lustrous wool. The Lincoln Longwool was improved by Bakewell, and in turn the Lincoln was used to develop the subsequent breed, named the New (or Dishley) Leicester. It was hornless and had a square, meaty body with straight top lines.[9]

These sheep were exported widely, including to Australia and North America, and have contributed to numerous modern breeds, despite the fact that they fell quickly out of favor as market preferences in meat and textiles changed. Bloodlines of these original New Leicesters survive today as the English Leicester (or Leicester Longwool), which is primarily kept for wool production.

Bakewell was also the first to breed cattle to be used primarily for beef. Previously, cattle were first and foremost kept for pulling ploughs as oxen,[10] but he crossed long-horned heifers and a Westmoreland bull to eventually create the Dishley Longhorn. As more and more farmers followed his lead, farm animals increased dramatically in size and quality. In 1700, the average weight of a bull sold for slaughter was 370 pounds (168 kg). By 1786, that weight had more than doubled to 840 pounds (381 kg). However, after his death, the Dishley Longhorn was replaced with short-horn versions.

He also bred the Improved Black Cart horse, which later became the Shire horse.

Charles Darwin coined the term 'selective breeding'; he was interested in the process as an illustration of his proposed wider process of natural selection. Darwin noted that many domesticated animals and plants had special properties that were developed by intentional animal and plant breeding from individuals that showed desirable characteristics, and discouraging the breeding of individuals with less desirable characteristics.

Darwin used the term "artificial selection" twice in the 1859 first edition of his work On the Origin of Species, in Chapter IV: Natural Selection, and in Chapter VI: Difficulties on Theory:

Slow though the process of selection may be, if feeble man can do much by his powers of artificial selection, I can see no limit to the amount of change, to the beauty and infinite complexity of the co-adaptations between all organic beings, one with another and with their physical conditions of life, which may be effected in the long course of time by nature's power of selection.[11]

— Charles Darwin, On the Origin of Species

We are profoundly ignorant of the causes producing slight and unimportant variations; and we are immediately made conscious of this by reflecting on the differences in the breeds of our domesticated animals in different countries,—more especially in the less civilized countries where there has been but little artificial selection.[12]

— Charles Darwin, On the Origin of Species

Animal breeding

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Main article: Animal breeding

Animals with homogeneous appearance, behavior, and other characteristics are known as particular breeds or pure breeds, and they are bred through culling animals with particular traits and selecting for further breeding those with other traits. Purebred animals belong to a single, recognizable breed, and purebreds with recorded lineage are called pedigreed. Crossbreeds are a mix of two purebreds, whereas mixed breeds are a mix of several breeds, often unknown. Animal breeding begins with breeding stock, a group of animals used for the purpose of planned breeding. When individuals are looking to breed animals, they look for certain valuable traits in purebred stock for a certain purpose, or may intend to use some type of crossbreeding to produce a new type of stock with different and presumably superior abilities in a given area of endeavor. For example, to breed chickens, a breeder typically intends to receive eggs, meat, and new, young birds for further reproduction. Thus, the breeder has to study different breeds and types of chickens and analyze what can be expected from a certain set of characteristics before he or she starts breeding them. Therefore, when purchasing initial breeding stock, the breeder seeks a group of birds that will most closely fit the purpose intended.

Purebred breeding aims to establish and maintain stable traits, that animals will pass to the next generation. By "breeding the best to the best," employing a certain degree of inbreeding, considerable culling, and selection for "superior" qualities, one could develop a bloodline superior in certain respects to the original base stock. Such animals can be recorded with a breed registry, the organization that maintains pedigrees and/or stud books. However, single-trait breeding, breeding for only one trait over all others, can be problematic.[13] In one case mentioned 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

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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

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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

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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

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Salmonids

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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

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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

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Oysters

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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

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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

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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 in mass. 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 native too 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. Resulting in an overall decrease in genetic diversity and rendering local fish populations less fit for survival.[53] If proper management is not taking place then the economic benefits and the diversity of the fish species will falter.[52]

Advantages and disadvantages

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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. This is because a single experiment done in selective breeding cannot be used to assess an entire group of genetic variances, individual experiments must be done for every individual trait. Also, due to the necessity of selective breeding experiments to require maintaining the organisms tested in a lab or greenhouse, it is impractical to use this breeding method on many organisms. Controlled mating instances are difficult to carry out in this case and this is a necessary component of selective breeding.[54]

Additionally, selective breeding can lead to a variety of issues including reduction of genetic diversity or physical problems. The process of selective breeding can create physical issues for plants or animals such as dogs selectively bred for extremely small sizes dislocating their kneecaps at a much more frequent rate then other dogs.[55] An example in the plant world is the Lenape potatoes were selectively bred for their disease or pest resistance which was attributed to their high levels of toxic glycoalkaloid solanine which are usually present only in small amounts in potatoes fit for human consumption.[56] When genetic diversity is lost it can also allow for populations to lack genetic alternatives to adapt to events. This becomes an issue of biodiversity, because attributes are so wide-spread they can result in mass epidemics. As seen in the Southern Corn leaf-blight epidemic of 1970 that wiped out 15% of the United States corn crop due to the wide use of a type of Texan corn strain that was artificially selected due to having sterile pollen to make farming easier. At the same time it was more vulnerable to Southern Corn leaf-blight.[57][58]

See also

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References

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Bibliography

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Further reading

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Selective breeding

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Selective breeding, also termed artificial selection, is the process by which humans deliberately mate or animals exhibiting desired heritable traits to produce offspring that more consistently display and enhance those characteristics across generations, thereby accelerating genetic changes aligned with human objectives such as improved yield, size, or utility. This method contrasts with by substituting human choice for environmental pressures, enabling rapid adaptation of domesticated species to agricultural, industrial, or companion roles. Practiced since approximately 9000 BCE, selective breeding underpinned the of staple crops like and , as well as such as and dogs, transforming wild progenitors into highly productive forms that supported human and . In , it has yielded diverse varieties, from high-yield kernels vastly larger than teosinte ancestors to multicolored carrots bred for nutritional content and . For animals, it produced the morphological diversity among dog breeds, from diminutive Chihuahuas to massive Great Danes, alongside enhanced traits like milk production in or rapid growth in broiler chickens. These advancements have dramatically boosted global , with selective breeding contributing to exponential increases in crop and productivity over millennia. Despite its successes, selective breeding carries risks, including narrowed from intense focus on few traits, which can amplify deleterious recessive alleles and heighten vulnerability to diseases or environmental stresses, as observed in inbred lines prone to declines and skeletal disorders. In companion animals , prioritizing extreme conformations—such as flattened faces in brachycephalic breeds—has led to chronic health issues including respiratory distress, , and reduced lifespan, prompting debates on welfare standards in breeding practices. Modern genomic tools now complement traditional selection to mitigate these pitfalls, enabling precise trait enhancement while preserving broader genetic health.

Fundamentals

Definition and Core Principles


Selective breeding, also known as artificial selection, is the process by which humans intentionally select organisms possessing desirable traits for reproduction, thereby increasing the frequency and expression of those traits in subsequent generations. This human-directed method contrasts with , as it substitutes deliberate choice for environmental pressures in determining which individuals contribute genes to the next generation. The core principles of selective breeding rest on three foundational elements derived from : , , and differential . provides the raw material, arising from mutations, recombination, and , ensuring a range of phenotypic differences within a upon which selection can operate. quantifies the proportion of phenotypic variance attributable to genetic variance transmissible to offspring, typically estimated as h2=VGVPh^2 = \frac{V_G}{V_P}, where VGV_G is genetic variance and VPV_P is total phenotypic variance; traits with higher respond more predictably to selection. Differential , imposed by selection, favors individuals with superior trait values, generating a selection differential that translates into genetic gain over generations via the breeder's : R=h2SR = h^2 S, with RR as response to selection and SS as selection differential. These principles enable cumulative improvement but are constrained by genetic limits, such as or , where selection for one trait may inadvertently alter others. from long-term breeding programs, such as those in yielding annual genetic gains of 1-5% in traits like milk yield, validates the efficacy of these mechanisms when applied systematically.

Underlying Genetic Mechanisms

Selective breeding achieves genetic improvement by exploiting heritable variation in traits, where phenotypes emerge from interactions between genotypes and environments, with enabling predictable across generations. The foundational mechanism involves differential reproduction of individuals carrying that contribute favorably to selected traits, thereby shifting population frequencies toward those enhancing the target . This process parallels but is directed by human choice, amplifying additive effects at quantitative trait loci (QTL) or numerous small-effect polymorphisms underlying polygenic traits common in breeding programs. Narrow-sense heritability (h²), defined as the ratio of additive genetic variance (V_A) to total phenotypic variance (V_P), quantifies the proportion of trait variation transmissible via additive allelic effects and directly predicts the response to selection via the breeder's equation: R = h² S, where R represents the generational change in mean trait value and S is the selection differential (the difference between the population mean and the mean of selected parents). Estimates of h², derived from methods like parent-offspring regression or genomic relatedness matrices, typically range from 0.1 to 0.5 for agriculturally important traits such as growth rate or yield, indicating moderate predictability of breeding outcomes despite environmental influences and non-additive effects like dominance or that contribute to broader genetic variance but less reliably to long-term gain. Genetic variation sustaining selection arises primarily from standing allelic diversity maintained by segregation and recombination during , with rare de novo mutations providing novel inputs over longer timescales; however, intense in closed breeding populations can erode this variation through fixation of favorable alleles or increase , potentially leading to reduced and diminished response after initial gains. Quantitative genetic models account for these dynamics by partitioning variance components, enabling breeders to sustain progress through strategies like maintaining diverse or incorporating genomic selection to capture cryptic variation at causal loci. itself is context-dependent, varying with environmental uniformity and population structure, underscoring that empirical estimation in target conditions is essential for accurate forecasting of genetic gain.

Historical Development

Prehistoric and Ancient Practices

The earliest evidence of selective breeding appears in the period, approximately 12,000 years ago, coinciding with the of and animals in the and other regions as humans shifted toward sedentary . This process initially involved unconscious selection, where individuals with traits favoring human use—such as reduced in or docility in animals—were preferentially propagated, gradually altering genetic profiles over generations. Archaeological and genetic data indicate that these practices marked the onset of artificial selection, distinct from natural processes, as humans actively influenced reproduction to enhance utility for food, labor, and companionship. Among animals, dogs represent one of the earliest cases, domesticated from gray wolves (Canis lupus) between 17,000 and 13,000 years before present in , with selection favoring traits like reduced aggression and enhanced hunting or sentry capabilities. In the , sheep domestication began around 12,000 BP, goats around 11,000 BP in southeastern Anatolia and the , cattle between 11,000 and 10,500 BP from in similar locales, and pigs around 10,500 BP, all through selection for herding suitability, meat yield, and milk production, evidenced by morphological changes like smaller body sizes and curved horns in remains. Cattle domestication specifically originated in the mid-9th millennium BC in Southwest , involving a genetic bottleneck from an estimated 81 female founders, with subsequent selection reducing diversity as populations spread to by 6400 BCE. For plants, selective breeding is documented in the transformation of teosinte grass into around 10,000 years ago in , where early farmers chose variants with larger, non-shattering ears for easier harvesting and higher yields, as confirmed by archaeological cobs from sites like Guilá Naquitz Cave. In the Near East, wild and underwent rapid by 9500 BCE, with selection against seed shattering—favoring "sticky" mutants that retained grains—evident in archaeological assemblages showing increased non-dehiscent forms within centuries. These changes, driven by repeated of favored phenotypes, demonstrate early human intervention yielding domesticated species adapted to cultivation, distinct from wild progenitors. In ancient civilizations, selective breeding practices grew more systematic, incorporating deliberate hybridization and trait-specific mating. In around 2500 BCE, the kunga—elite hybrid equids used for warfare and transport—were produced by crossing female domestic donkeys with male hemiones (wild Asiatic asses), representing the earliest archaeogenetically confirmed human-engineered animal hybrids. Egyptian records from the New Kingdom (1550–1070 BCE) describe royal breeding programs for suited to warfare, selecting for speed and adaptation to the environment amid imports from the . Similarly, third-millennium BC Iberian sites yield DNA evidence of managed sheep flocks with genetic signatures of selection for and , indicating organized herding beyond mere capture. These examples reflect advancing causal understanding of , prioritizing empirical outcomes like productivity over wild variability.

Enlightenment to Industrial Era Advancements

During the 18th century, as part of the , selective breeding evolved into a more deliberate and systematic practice, driven by the need to boost productivity to support growing urban populations and industrial demands. English agriculturist Robert Bakewell (1725–1795) led this shift by applying controlled mating and to enhance specific traits in sheep, , and horses on his Dishley estate. For Leicester Longwool sheep, Bakewell selected rams and ewes for faster weight gain, finer bones, and superior carcass quality, resulting in animals that reached marketable weight in under a year—compared to longer times in unimproved breeds—and produced mutton with higher fat content prized by markets. His progeny testing, where potential sires were evaluated over multiple seasons before breeding, minimized variability and amplified , with records showing his improved sheep fetching premiums up to three times those of common stock by the 1770s. Bakewell's methods extended to Dishley Longhorn cattle, selected for lean meat yield and milking ability, and New Leicester horses for draft power, spreading via leased rams that disseminated elite genetics across and influencing . By the late 1700s, contemporaries like Thomas Coke of implemented similar progeny-based selection on Norfolk Horn sheep and Southdown breeds, achieving documented yield increases of 20-30% in wool and meat through targeted crossing and culling of inferiors. These practices correlated with broader agricultural output gains, as enclosure acts consolidated land for specialized farming, enabling data-driven herd management that foreshadowed quantitative genetics. In the , amid industrialization's demand for standardized breeds, pedigree herd books and societies formalized selective breeding, emphasizing purity and measurable traits. The Colling brothers in Durham developed from 1780 onward by crossing local stock with Bakewell-influenced sires, selecting for dual-purpose milk (up to 10 gallons daily in top cows) and beef conformation, with the first herd book published in 1822 tracking over 1,000 animals by descent. Sheep breeding advanced similarly, with imports from selectively crossed for fine suited to mills, as in Australian and programs yielding fleece weights doubling pre-1800 averages by mid-century. , though less individualized, saw empirical selection for yield in root crops like turnips and cereals under four-field rotations, with farmers propagating high-tillering varieties that supported 20-50% productivity rises in enclosed fields. These advancements rested on observational rather than genetic , yet yielded verifiable economic impacts: British output rose approximately 150% from 1700 to 1850, underpinning from 5.5 million to 21 million. Central European breeders, such as those in , refined Bakewell's techniques for wool sheep, prioritizing genealogical records to sustain improvements amid variable climates. Limitations persisted, including noted in some closed herds, prompting hybrid vigor experiments by the 1840s.

Modern Scientific and Genomic Era

The integration of Mendelian genetics into selective breeding practices accelerated following the independent rediscovery of Gregor Mendel's laws of in 1900 by , , and , which established the particulate nature of genetic transmission and refuted earlier blending models prevalent in breeding theory. This foundational shift enabled breeders to predict patterns for discrete traits, laying the groundwork for systematic programs in crops and . Concurrently, the emergence of in the early 20th century, pioneered by in his 1918 paper partitioning phenotypic variance into additive, dominance, and epistatic components, provided mathematical frameworks to model polygenic traits central to most breeding objectives. Sewall Wright's path analysis and J.B.S. Haldane's statistical contributions further refined estimates of and response to selection, formalized as R=h2SR = h^2 S (where RR is the response, h2h^2 is narrow-sense , and SS is the selection differential), allowing for optimized phenotypic selection in populations with continuous variation. Advances in from the mid-20th century onward transformed selective breeding by enabling indirect selection via genetic markers. The 1953 elucidation of DNA's double-helix structure by and facilitated the development of DNA-based tools, culminating in the first use of restriction fragment length polymorphisms (RFLPs) as markers in the . (MAS), which links molecular markers to quantitative trait loci (QTLs) for early or indirect trait selection, gained traction in the after initial applications with isozymes in the for introgressing traits like disease resistance in tomatoes. MAS improved selection efficiency for traits difficult or costly to measure phenotypically, such as yield components in or milk production in , though limited by marker density and decay. By the late , single nucleotide polymorphisms (SNPs) replaced earlier markers, enhancing resolution in QTL mapping and breeding programs across species. The genomic era, marked by dense marker arrays and computational models, represented a with the proposal of genomic selection (GS) by Theo Meuwissen, Ben Hayes, and Mike Goddard in their 2001 paper, which demonstrated that genome-wide SNP data could predict total genetic values with accuracies rivaling or exceeding pedigree-based methods. GS estimates breeding values using statistical models like genomic best linear unbiased (GBLUP), capturing both major and minor effect loci without prior QTL identification, thereby accelerating genetic gain by enabling selection on juveniles before phenotypic expression. In , the U.S. Department of Agriculture released the first official genomic evaluations in January 2009 for Holsteins and Jerseys, followed by other breeds, resulting in generation intervals shortening from 4.6 to 2.5 years and genetic gain rates increasing by up to 50% for traits like fat yield. adopted GS similarly, with applications in and reducing cycle times from 10-12 years to under 5 years in some programs, as denser via next-generation sequencing lowered costs from thousands to under $10 per sample by the . These tools have unified animal and methodologies, prioritizing empirical genomic data over sole reliance on phenotypes, though challenges persist in maintaining and model accuracy across environments.

Applications

In Animal Breeding

Selective breeding in entails the systematic selection of individuals exhibiting superior heritable traits for , resulting in populations with amplified characteristics such as enhanced growth rates, , and adaptability to human needs. This process, applied across terrestrial , companion species, and , has yielded substantial genetic gains, though it also introduces risks like reduced and associated health vulnerabilities. In , breeding programs have focused on economic traits, with documented annual genetic improvements in milk yield for exceeding 1% since the mid-20th century, accelerating further after bovine sequencing in the early enabled . Similar advances in have increased carcass weight and feed efficiency through targeted selection initiated in the 1700s by figures like Robert Bakewell, who emphasized and progeny testing for traits like meat quality. Pigs have seen a 50% rise in litter size and 37% increase in lean meat yield from the 1960s to 2005 via selective breeding, alongside improvements in sheep for wool production and lamb growth. However, intense selection for production often correlates with , including higher incidences of genetic defects and fertility declines in breeds like fast-growing broilers and dairy cows.

Terrestrial Livestock

Terrestrial breeding exemplifies selective breeding's role in agricultural intensification. In , programs have boosted annual milk production per cow from approximately 5,000 pounds in the to over 23,000 pounds by 2020 in the U.S., driven by selection indices incorporating traits like fat and protein content alongside health metrics. selection has enlarged mature body size and marbling, with breeds showing marked improvements in final product quality since the through within-breed and crossing. breeding has prioritized rapid growth and feed conversion, yielding annual genetic gains of 2-3% in average daily gain, though this has elevated risks of skeletal issues and reduced longevity. Sheep programs target dual-purpose traits, enhancing fleece weight by up to 5% per generation while mitigating through diverse sire lines.

Companion and Working Animals

Companion and working animals demonstrate selective breeding's customization for behavioral and morphological traits. Dogs, descended from wolves through initial around 15,000–40,000 years ago, underwent intensified artificial selection post-agriculture to produce over 400 breeds differentiated by function, such as herding in Border Collies or retrieval in Retrievers. This has fixed extreme phenotypes—like in Pugs—but at the cost of health compromises, including respiratory distress and prevalent in 20-50% of certain purebreds due to reduced . Working equines, including Thoroughbreds for , trace speed enhancements to 18th-century foundational sires, achieving average race times reduced by 10-15% over two centuries via pedigree-based selection. Cats exhibit less breed diversification, with selective efforts mainly amplifying coat patterns and temperament since the , yielding fewer genetic bottlenecks than in dogs.

Aquaculture Species

Aquaculture selective breeding, formalized in the 1970s, targets farmed fish and for traits like growth and resistance amid expanding global production. programs in have delivered 10-12% annual genetic gains in harvest weight since inception, with 12 generations yielding doubled growth rates and halved feed conversion ratios through family-based selection. Similar efforts in and emphasize pathogen resistance, reducing mortality from by up to 50% in selected lines via genomic tools. These advances project potential production doublings within a decade if scaled, though challenges persist in maintaining effective population sizes to avert , which can erode gains in later generations.

Terrestrial Livestock

Selective breeding in terrestrial livestock has focused on enhancing traits such as milk yield, meat production, growth rate, litter size, and resistance in species including , sheep, goats, and pigs. These efforts, rooted in artificial selection of desirable phenotypes across generations, have driven substantial genetic gains, often quantified through estimated breeding values and estimates for key production traits. For instance, in , selective breeding has prioritized high milk volume and fat content, leading to average annual genetic progress of approximately 100-150 kg in milk yield per since the mid-20th century. In breeds like Holsteins, which dominate global production, selective breeding since 1960 has nearly tripled milk output per cow, from around 5,000 kg to over 14,000 kg annually in high-yielding herds, attributed to genetic selection accounting for about 50% of total productivity gains when combined with improved and . breeding programs have similarly targeted carcass weight, feed efficiency, and marbling, with breeds such as Angus achieving genetic gains of 10-20 kg in yearling weight per decade through selection indices incorporating ultrasound-measured traits. Swine breeding has yielded marked improvements in reproductive and carcass traits; from the to 2005, selective breeding increased litter sizes by 50% (from roughly 8 to 12 piglets) and lean meat percentage by 37%, reducing backfat while enhancing growth rates to market weight in under 150 days. Sheep and goat breeding emphasizes dual-purpose outcomes, including wool yield, lamb growth, and kidding rates; in meat-focused sheep lines, genetic selection has boosted weaning weights by 1-2 kg per generation and carcass lean yield, though rapid growth selection has occasionally correlated with higher dystocia rates. Goats, often bred for milk or mohair, have seen heritability-driven gains in daily milk production of 0.2-0.3 kg via progeny testing, with crossbreeding strategies further amplifying hybrid vigor for meat traits. These advancements rely on quantitative genetic principles, where narrow-sense (typically 0.1-0.4 for production traits) guides selection intensity, enabling annual genetic progress of 1-3% in targeted metrics across populations. Modern integrations of genomic selection have accelerated gains by 20-50% over traditional methods, reducing intervals through early-accuracy predictions. However, intensive selection for yield has prompted balanced indices incorporating and to mitigate correlated declines, such as reduced reproductive efficiency in high-milk lines.

Companion and Working Animals

Selective breeding has transformed wolves into over 300 recognized dog breeds, with lineages diverging for companionship and work since domestication approximately 20,000 to 40,000 years ago. Early selection emphasized functional traits like herding instinct in breeds such as Border Collies, where genetic markers for high intelligence and eye-stalking behavior have been intensified through generations of targeted matings. Similarly, hunting breeds like Pointers were developed by 17th-century English breeders to exhibit strong pointing posture and scent-tracking ability, enhancing efficiency in retrieving game. For working roles beyond dogs, have undergone selective breeding since for traits like in Arabians, documented in breeding records from the 7th century onward, and draft power in Clydesdales, where muscle mass and calm temperament were prioritized in 18th-century . These efforts rely on of polygenic traits, such as stamina linked to variations, allowing populations to outperform wild ancestors in human-directed tasks. Companion animals, particularly dogs and cats, have been bred for aesthetic and docile traits, often converging on juvenile features like shortened muzzles in pugs and Persian cats, a pattern confirmed by genomic analysis showing parallel selection pressures since the . standards, established by organizations like the in 1884, guide breeders to emphasize conformation over utility, resulting in breeds like Chihuahuas selected for diminutive size under 6 pounds. However, closed pedigrees foster coefficients exceeding 25% in some lines, correlating with reduced —e.g., a 2021 study found Golden Retrievers with high runs of homozygosity exhibited 15-20% lower litter sizes. Intense selection in working breeds maintains utility but introduces vulnerabilities; for instance, dogs show elevated risks of from conformational extremes, with prevalence up to 20% in German Shepherds per orthopedic registries. Responsible practices, advocated since the by veterinary bodies, incorporate genomic testing to mitigate , preserving traits like guarding vigilance in breeds such as Rottweilers while reducing congenital defects. Overall, these programs demonstrate causal links between human-directed selection and amplified phenotypic extremes, balanced against empirical evidence of welfare trade-offs from diminished .

Aquaculture Species

Selective breeding in aquaculture targets commercially important species of fish, crustaceans, and mollusks to improve traits such as growth rate, survival, disease resistance, and feed conversion efficiency, enabling higher production yields and . Family-based selection programs, often spanning multiple generations, have demonstrated genetic gains of 10-15% per generation for growth in many , surpassing traditional farming practices reliant on wild stocks. These programs leverage to select superior breeding families, with heritability estimates for growth traits typically ranging from 0.2 to 0.4, allowing predictable progress despite challenges like mass spawning and high fecundity in aquatic organisms. Atlantic salmon (Salmo salar) exemplifies successful application, with Norwegian programs established in the 1970s yielding cumulative gains where modern strains exhibit 3-4 times faster growth than wild progenitors after 10-12 generations of selection. has improved by approximately 30% relative to base populations, reducing resource inputs for equivalent output and contributing to Norway's annual production exceeding 1.5 million tonnes by 2023. Disease resistance to pathogens like sea lice and infectious salmon anemia has also advanced through multi-trait selection, lowering mortality rates and use in net-pen farming. In ( niloticus), the Genetically Improved Farmed (GIFT) strain, developed via selective breeding since the , achieves 10-17% per-generation increases in body weight, enhancing yields in pond and cage systems across and . (Litopenaeus vannamei) breeding programs, such as those originating in in 1997 and expanded commercially, focus on resistance to virus and Taura syndrome, with growth gains of 10-20% per generation improving survival from 20-30% in unselected stocks to over 70% in advanced lines. For mollusks like Pacific oysters ( gigas), selection for faster shell growth and meat yield has boosted harvest indices by 15-20% per generation, supporting sustainable bivalve amid environmental pressures. Overall, these efforts underscore selective breeding's role in scaling global output, projected to double by 2050 with intensified genetic programs, though realization depends on integrating genomic tools for polygenic traits.

In Plant Breeding

Selective breeding in entails the deliberate of individuals exhibiting favorable traits, such as enhanced yield, uniformity, or resistance to environmental stresses, to propagate those characteristics in subsequent generations. This process, initiated during the approximately 10,000 years ago, fundamentally altered wild species into domesticated crops by favoring like non-shattering seed pods in cereals, which prevented natural dispersal and facilitated human harvesting. Evidence from archaeological and genetic studies confirms that such selections reduced in domesticated lineages while amplifying adaptive traits absent or rare in wild ancestors. A prominent example is the domestication of from teosinte, a wild Mexican grass, where over 9,000 years of selection increased kernel size and cob development, yielding plants with up to 1,000 kernels per ear compared to teosinte's few scattered grains. Similarly, wild cabbage underwent selective breeding to produce diverse vegetables including , , and , demonstrating how artificial selection can derive multiple morphologies from a single progenitor through targeted trait enhancement. In modern agriculture, selective breeding has driven substantial productivity gains, contributing roughly 50% of observed improvements in crop output through yield-enhancing varieties and better resource utilization. For , breeding efforts during the mid-20th century incorporated semi-dwarfing genes and disease resistance, elevating global yields from under 1 metric ton per hectare in the 1950s to averages exceeding 3 metric tons per hectare by the 2000s in major producing regions. Rice breeding programs have similarly realized genetic gains of 1-2% annually, underpinning for billions. These advancements underscore selective breeding's causal role in amplifying and allocation, though they often necessitate complementary agronomic practices for full realization. Applications extend to non-food plants, where selection has optimized fiber production in —doubling lint yields since the 1900s—and ornamental traits in flowers like roses, achieving novel colors and forms through hybridization. Empirical data from long-term trials indicate that while breeding narrows genetic bases in elite lines, strategic incorporation of wild mitigates risks of vulnerability to pests or climate shifts. Overall, selective breeding's efficacy in derives from their reproductive plasticity, including and , enabling rapid fixation of desired alleles across populations.

Crops for Food and Fiber

Selective breeding has profoundly shaped food and by enhancing traits such as yield, uniformity, and environmental adaptability through deliberate human selection over millennia. In food crops, ancient began with cereals like , transformed from the wild teosinte grass with small, hard s to modern varieties producing large, edible kernels, a process initiated around 9,000 years ago in . Similar selection in and prioritized non-shattering heads and larger grains, enabling reliable harvests and supporting early agricultural societies. The 20th-century accelerated these gains via targeted breeding for semi-dwarf varieties that resisted under high inputs, dramatically boosting . yields in developing countries rose 208%, 109%, and 157% from 1960 to 2000, averting widespread amid . In the U.S. , yields tripled since 1950 through breeding for hybrid vigor and disease resistance, maintaining water inputs while expanding output. These advancements relied on empirical selection of heritable traits, yielding staple crops that supply the majority of global caloric needs. For fiber crops, selective breeding focused on fiber length, strength, and yield. Cotton domestication in regions like the Indus Valley and , dating back over 5,000 years, involved selecting for longer, more abundant lint fibers from wild species, culminating in modern upland cotton varieties with fibers up to 1.5 inches long. , cultivated for linen since the period in the , underwent artificial selection for taller stems and finer fibers, as seen in Russian landraces bred for height exceeding 1 meter to maximize bast fiber extraction. These efforts improved processing efficiency and quality, underpinning industries from ancient to contemporary apparel.

Ornamental and Industrial Plants

Selective breeding of ornamental has primarily targeted aesthetic and functional traits, including flower color, shape, size, fragrance, architecture, and post-harvest longevity, to meet demands for , , and interior decoration. In , systematic collection, , and breeding of ornamentals intensified from the 17th to 18th centuries, building on earlier ancient practices and leading to thousands of cultivars across like (Rosa spp.), ( spp.), and orchids (Orchidaceae). For instance, sweet alyssum (), originally a wild Mediterranean , has been selectively bred to produce a of flower colors, including and variants, enhancing its appeal in . These efforts have resulted in over 30,000 registered cultivars by the early 21st century, achieved through cross-pollination and selection for traits like repeat blooming and disease resistance. In industrial plants, selective breeding emphasizes yield and quality of non-food products such as , resins, fibers, and bioactive compounds, often prioritizing economic viability over edibility. The para rubber tree (), native to and widely planted in since the late , exemplifies this: decades of from high-yielding seedlings increased annual productivity from approximately 650 kg per in unselected populations to over 2,000 kg per in modern elite clones by the mid-20th century. Similarly, guayule (), a drought-tolerant evaluated as a domestic U.S. rubber alternative, has undergone breeding programs since the 1940s to elevate rubber content from 3-7% in wild plants to higher levels in improved lines, supporting production for medical applications. Russian dandelion () has also been selectively bred for root yield, with extractable rubber quantities optimized through field trials to rival synthetic alternatives during periods of shortages. These advancements demonstrate how targeted selection enhances industrial output while adapting plants to specific agro-climatic conditions.

Techniques and Methodologies

Traditional Selection Approaches

![Corn selection process showing phenotypic variation in maize ears][float-right] Traditional selective breeding approaches, predating the genomic era, primarily relied on phenotypic observation and manual selection of individuals exhibiting desirable traits, without knowledge of underlying genetic mechanisms. These methods, originating from early practices around 8000 BCE, involved humans identifying and propagating variants that enhanced utility, such as increased yield or docility. By the , systematic techniques emerged, exemplified by Robert Bakewell's and progeny testing in sheep breeding around 1760, which accelerated trait fixation through controlled mating. Mass selection, one of the simplest and earliest methods, entails harvesting seeds or selecting breeders from a large based on visible phenotypic superiority, such as plant height or size, then bulking progeny for the next generation. This approach is effective for traits with high and minimal environmental influence, as seen in improving crops where superior are chosen en masse post-pollination. In , mass selection ranks individuals by own-performance metrics like milk yield in , assuming genetic correlation with , though success depends on estimates often ranging from 0.2 to 0.5 for quantitative traits. Pedigree selection builds on mass selection by maintaining detailed records of ancestry to select from family lines with proven superiority, reducing and enabling targeted improvement in self-pollinated crops or . Developed prominently in the early , this method tracks parent-offspring performance, as in breeding where lines are isolated and evaluated over generations for traits like yield stability. Pure-line selection, suited to homozygous self-pollinators, isolates and propagates individual plants to establish uniform varieties, a technique refined by in 1903 for eliminating segregation and stabilizing traits like seed size in peas. In practice, these approaches were applied across ; for instance, breeders selected for root color and shape over centuries, yielding diverse varieties from wild precursors by 16th-century . Limitations arose from polygenic trait complexity and environmental masking, often requiring multiple cycles—typically 5-10 generations—to achieve modest gains of 1-2% annually in yield. Despite inefficiencies compared to modern tools, traditional methods laid the foundation for , with empirical evidence from long-term experiments showing cumulative capture through repeated phenotypic .

Contemporary Genomic and Data-Driven Methods

Genomic selection (GS), a of contemporary methods, employs genome-wide molecular markers, typically single nucleotide polymorphisms (SNPs), to estimate breeding values through statistical models trained on reference populations with phenotypic and genotypic data. This approach, theoretically outlined by Meuwissen, Hayes, and Goddard in 2001, enables prediction of genetic merit in unphenotyped individuals, facilitating early selection and reducing generation intervals compared to phenotypic selection alone. Models such as genomic best linear unbiased prediction (GBLUP) or Bayesian methods capture polygenic effects, yielding prediction accuracies often ranging from 0.5 to 0.85 for . In contrast to (MAS), which relies on a set of markers linked to specific quantitative trait loci (QTL) for targeted traits, GS integrates markers across the entire to account for minor-effect loci, improving accuracy for low-heritability, polygenic phenotypes like yield or disease resistance. MAS remains useful for monogenic traits but underperforms for quantitative ones, whereas GS has demonstrated 2-4 times higher genetic gains in cycles shortened by juvenile selection. Implementation in accelerated post-2009, particularly in , where GS doubled annual genetic progress for production traits; for U.S. Holsteins, rates for milk yield and component traits increased by up to 192% since adoption, alongside fitness improvements like . In plants, GS optimizes doubled-haploid pipelines in , reducing cycle times, and enhances parental selection in , with historical data enabling accuracies up to 0.85 for grain yield. Data-driven enhancements leverage from high-throughput sequencing, , and environmental sensors, incorporating algorithms like for multi-trait genomic prediction and genotype-by-environment modeling. These methods boost accuracy by 3-5% through microbiome or multi-omics integration and support decentralized breeding via optimized training sets. Recent tools, including decision support platforms, further refine selections by simulating breeding scenarios, as seen in programs harnessing pangenomes for imputation accuracy.

Demonstrated Benefits

Productivity and Yield Enhancements

Selective breeding programs have demonstrably increased crop yields through targeted selection for higher grain output and improved harvest indices. In the United States , maize grain yields rose from 5.2 megagrams per hectare in 1970 to 11.1 megagrams per hectare in 2020, representing a 114% increase attributable in large part to genetic improvements via breeding. Breeding efforts have also driven over a threefold increase in average yields for since the 1930s, with modern varieties outperforming historical ones by substantial margins under comparable conditions. Similar gains occurred in staple crops like and , where adoption of selectively bred cultivars, combined with fertilization, yielded dramatic productivity surges in regions such as and starting in the mid-20th century. In livestock, selective breeding has elevated production metrics for meat, milk, and eggs. Genetic selection has produced estimated 20-30% gains in carcass weight for beef, sheep, and pigs, alongside comparable improvements in dairy cow milk yields and laying hen egg output. These advancements stem from multi-generational selection prioritizing traits like growth rate and output efficiency, with programs focusing on both production and resilience to sustain industrial-scale farming. For dairy cattle, evaluations of genetic trends from 1970 to 2020 highlight breeding's role in countering environmental factors to boost overall herd productivity. Aquaculture species have seen accelerated growth and yield enhancements through selective breeding, often achieving 10-20% improvements per generation in key traits. In , breeding programs have delivered 10-17% increases in body weight per generation, enhancing fillet yield and overall biomass production. and programs similarly report rapid gains in growth rates and feed efficiency, enabling higher stocking densities and harvest volumes without proportional input increases. These genetic gains, realized through family-based selection and performance testing, have expanded commercial viability for species like , where breeding accounts for substantial portions of annual production upticks.

Adaptability and Resilience Improvements

Selective breeding has enhanced the adaptability of domesticated species to environmental stressors, including , , , and flooding, by prioritizing traits that enable sustained performance under suboptimal conditions. This process targets heritable physiological mechanisms, such as improved water-use efficiency, osmotic adjustment, and deeper root systems in plants, or thermoregulatory adaptations in animals. from breeding programs demonstrates that such selections yield lineages with quantifiable reductions in stress-induced losses, often measured through field trials comparing selected versus unselected populations. In crop species, selective breeding for has produced varieties that maintain yields during prolonged dry spells. hybrids developed through recurrent selection since the 1930s exhibit concurrent gains in grain yield and drought resistance, with recent genomic-informed programs projecting 17.8% lower yield penalties under severe drought by 2100 relative to unimproved lines. breeding for resistance, initiated after the epidemics, has deployed cultivars like those carrying the , which reduced regional yield losses from 20-50% in susceptible varieties to near zero in resistant ones during outbreaks. varieties selected for submergence tolerance, such as FR13A derivatives, survive up to 14-17 days of flooding—far exceeding non-selected types—through traits like quiescence and rapid post-stress recovery, as validated in multi-location trials across . Livestock breeding programs have similarly bolstered resilience to diseases and climatic extremes. In sheep, European national schemes selecting against scrapie-susceptible PrP genotypes since the early 2000s have decreased prevalence by over 90% in targeted flocks, correlating with lower mortality and reduced needs. lines bred for resistance in subtropical regions show 20-50% fewer infestations and associated cases compared to susceptible breeds, achieved via phenotypic selection on infestation scores over multiple generations. breeding for resistance has incorporated genomic selection, yielding lines with 15-30% higher survival rates post-challenge in controlled studies, without compromising growth metrics. These outcomes underscore selective breeding's capacity to fortify populations against recurrent threats, though gains depend on genetic variance and ongoing .

Identified Risks and Criticisms

Biological and Genetic Drawbacks

Selective breeding often narrows by favoring specific traits, leading to where offspring from related parents exhibit reduced biological fitness, including lower , , and growth rates. A 2014 meta-analysis of quantified this effect, revealing an average trait decline of 0.137% per 1% increase in the inbreeding coefficient, with production-related traits suffering a steeper 0.351% reduction due to the expression of recessive deleterious alleles that would otherwise remain masked in outbred populations. This depression arises from decreased heterozygosity, which diminishes hybrid vigor and exposes homozygous harmful mutations accumulated over generations of artificial selection. In animals, these genetic constraints manifest as heightened prevalence of heritable disorders, particularly in closed breeding populations like dogs, where selection for aesthetic or functional extremes prioritizes morphology over physiological robustness. Genome sequencing of dog breeds has identified elevated loads of deleterious variants, including loss-of-function mutations, that correlate with breed-specific bottlenecks and contribute to conditions such as , , and portosystemic shunts, with many breeds showing risks exceeding those in outbred dogs by factors of 2 to 10 for orthopedic and cardiac issues. Similarly, in , intense selection for traits like milk yield in has inadvertently amplified linked genetic defects, resulting in drops of up to 20-30% in high-producing lines and increased lameness incidence from skeletal imbalances, as selection pressures disrupt homeostatic gene networks without counterbalancing for overall viability. For plants, selective breeding erodes allelic diversity by fixating elite varieties, rendering crops more susceptible to biotic and abiotic stresses through homogenized gene pools that lack the buffering polymorphisms found in wild progenitors. Empirical assessments of major staples like and demonstrate that domesticated lineages retain only 10-20% of ancestral , heightening vulnerability to pathogens, as evidenced by rapid disease sweeps in uniform fields where a single resistance locus failure can devastate yields, akin to the 1970 southern corn leaf blight that destroyed 15% of U.S. corn production due to cytoplasmic uniformity from breeding practices. This reduced evolvability constrains to novel environmental challenges, such as shifting climates or emerging pests, by limiting standing variation for natural or assisted selection responses. Beyond immediate fitness costs, selective breeding can entrench polygenic trade-offs, where gains in targeted traits impose correlated declines elsewhere via or , fostering long-term genomic instability. In and , for example, rapid selection for body mass has correlated with immune suppression and metabolic disorders, as quantitative genetic models show antagonistic effects where alleles boosting growth divert resources from reproductive or stress-response pathways, potentially accelerating or susceptibility in production environments. Such outcomes underscore the causal limits of unidirectional selection in finite genomes, where unchecked focus on few loci permits drift of unselected harmful variants, necessitating ongoing interventions like to mitigate accumulating .

Ethical and Societal Concerns

Selective breeding in livestock has been associated with unintended health consequences, such as increased incidence of lameness, mastitis, and reduced fertility in dairy cattle due to selection for high milk yield, leading to higher rates of culling and shortened lifespans. In companion animals, breeding for aesthetic traits like brachycephaly in dogs has resulted in chronic respiratory distress, higher rates of surgical interventions, and diminished quality of life, with studies documenting elevated morbidity from conditions such as brachycephalic obstructive airway syndrome. These outcomes stem from prioritizing production or appearance over physiological robustness, prompting calls from veterinary ethicists for breeding programs that incorporate welfare metrics, such as calving ease and longevity, to mitigate suffering. Intensive selection practices contribute to erosion of within breeds, heightening vulnerability to diseases and environmental stressors. For instance, pedigree analyses of breeds from 1980 to 2012 revealed an average loss of 70% of founder alleles, accelerating and fixation of deleterious mutations. In farm animals, focus on narrow traits like rapid growth in or high yield in has reduced intra-breed variation, exemplified by socioeconomic drivers diminishing traditional populations and fostering monocultures susceptible to pests, as seen in historical failures from uniform varieties. While some conservation efforts preserve diversity through breed banks, critics contend that market-driven breeding exacerbates these risks without adequate safeguards, potentially undermining long-term adaptability. Societal apprehensions extend to parallels with human applications, invoking the movement of the early , where selective breeding principles were coercively applied via forced sterilizations —affecting over 60,000 individuals deemed "unfit" under laws upheld by the in 1927—based on flawed assumptions of hereditary inferiority. This era's pseudoscientific racism, influencing policies in multiple countries until post-World War II repudiations, underscores risks of state or institutional overreach in directing reproduction, even as modern voluntary genetic selection via technologies like preimplantation diagnosis raises debates over equity and unintended . Proponents of ethical frameworks argue that while non-coercive selection could enhance , historical abuses necessitate stringent oversight to prevent or erosion of individual autonomy.

Comparative Analysis

Relative to Natural Selection

Selective breeding, also known as artificial selection, parallels in relying on heritable variation within populations and differential reproductive success to propagate advantageous traits over generations. drew this analogy explicitly in (1859), using examples of domesticated pigeons and dogs to illustrate how human-directed changes mirror undirected environmental pressures, demonstrating that substantial modifications could accumulate under either mechanism. Both processes require as raw material, sourced from and recombination, and favor traits enhancing propagation—survival and in , or to humans in breeding. However, selective breeding diverges fundamentally in agency, intensity, and objectives. Humans impose selection criteria based on desired phenotypes, such as milk yield in cattle or docility in pets, unconstrained by wild survival demands, whereas natural selection filters traits strictly through fitness in native environments. This human mediation accelerates trait fixation: breeding programs can achieve near-100% selection intensity for target traits per generation by restricting reproduction to elite individuals, yielding changes in years or decades, compared to natural selection's typical millennial timescales for comparable shifts, as evidenced by the rapid divergence of dog breeds from wolves since domestication around 15,000–40,000 years ago. In domesticated animals like dogs, this process amplifies genetic differentiation through artificial barriers imposed by controlled breeding, resulting in higher FST values (0.15–0.52 between breeds) than those in wild canid populations under natural selection, which rely on geographic isolation but permit ongoing gene flow. For instance, maize (Zea mays) evolved from teosinte via human selection over approximately 9,000 years, far outpacing natural adaptation rates in wild grasses. Selective breeding often entails greater risks to and long-term adaptability than . Intensive focus on narrow traits reduces allelic variation through bottlenecks and , increasing homozygosity and vulnerability to diseases or environmental shifts, as seen in pedigreed where effective population sizes drop below 100, amplifying recessive deleterious alleles. , by contrast, sustains polymorphism via balancing forces like or fluctuating environments, pruning only unfit variants while preserving adaptive potential; studies in wild populations, such as , show trait evolution without equivalent diversity erosion. Artificially selected lineages may exhibit reduced fitness outside human control—e.g., many chickens require assistance for due to exaggerated growth—highlighting a dependency absent in naturally evolved forms. Thus, while selective breeding amplifies short-term gains, it can compromise the robustness fosters through undirected, holistic pressures.

Relative to Genetic Engineering

Selective breeding, or artificial selection, differs fundamentally from in methodology, as it involves iteratively mating organisms with desirable traits to amplify existing genetic variations over multiple generations, whereas directly modifies DNA sequences using techniques such as or CRISPR-Cas9 to insert, delete, or edit specific genes, often achieving changes in a single generation. In terms of efficiency, enables faster trait development; for instance, the introduction of herbicide-tolerant traits in soybeans via Agrobacterium-mediated transformation occurred within years of research starting in the 1980s, contrasting with selective breeding's decades-long process to enhance similar resistances through cross-pollination and selection. Precision is another advantage of , targeting individual genes with known functions, as opposed to selective breeding's reliance on polygenic traits that may inadvertently propagate linked undesirable alleles, such as reduced resistance in highly inbred lines. However, selective breeding operates within the organism's natural , avoiding the introduction of transgenes from unrelated species, which permits—exemplified by incorporating bacterial Cry genes for insect resistance, a trait unattainable through breeding alone. This expanded scope in facilitates novel adaptations, like papaya ringspot virus resistance developed in the 1990s via coat protein gene insertion, rescuing Hawaii's papaya industry where breeding failed due to insufficient natural variation. Regarding risks, both methods can reduce , but raises concerns over off-target mutations and , though empirical data from over 25 years of commercial GM crops show no verified risks beyond those of conventional counterparts, per regulatory assessments. Selective breeding, by contrast, has a millennia-long track record with observable drawbacks like the Cornish hen's skeletal deformities from rapid growth selection, yet lacks the ethical debates surrounding edits in . favors selective breeding, requiring minimal infrastructure compared to genetic engineering's need for specialized labs and bioinformatics, rendering it more feasible for small-scale or resource-limited applications.

References

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