What is the main difference between the process of artificial selection and natural selection

Introduction

Artificial selection is the process by which humans choose individual organisms with certain phenotypic trait values for breeding. If there is additive genetic variance for the selected trait, it will respond to the selection, that is, the trait will evolve. All of our domesticated species, including crop plants, livestock, and pets, are the products of artificial selection for desirable traits, such as seeds and fruits that do not disperse readily, increased meat and milk production, and docile behavior. The earliest artificial selection may have been unconscious, but it developed into a sophisticated science of plant and animal breeding; indeed, much of the field of quantitative genetics was developed to improve breeding programs.

The importance of artificial selection to the field of evolutionary biology dates back to Darwin, who was likely the first to use the term artificial selection in the ‘Origin of Species’ (Darwin, 1859). Darwin used the obvious evolutionary results of domesticated species to show the power of selective breeding as an analogy to natural selection. One of the earliest uses of experimental artificial selection to address evolutionary questions was by Holtorp (1944). He selectively bred Brassica plants that produced an extra cotyledon and reported an increase in frequency of plants with three and even four cotyledons in subsequent generations. Similarly, Huether (1968) was able to increase and decrease the number of corolla lobes in Linanthus through five generations of artificial selection. These early studies established that even traits that are conserved at higher taxonomic levels could evolve.

Artificial selection differs from what has been called laboratory natural selection (Rose et al., 1990) or controlled natural selection (Conner, 2003). In artificial selection the experimenter chooses specific phenotypic traits to select upon, while in controlled natural selection an environmental factor is manipulated and evolution of the populations in response to this selective agent is monitored. While artificial selection is certainly a form of experimental evolution, often the meaning of the term ‘experimental evolution’ is confined to controlled natural selection, excluding artificial selection (e.g., Kawecki et al., 2012). Because artificial selection applies a known strength and direction of selection to specific phenotypic traits, it is one of the most powerful methods available for understanding the underlying genetic variation and thus evolvability of those traits; in controlled natural selection the strength and direction of selection cannot be determined by the investigator.

Introduction

Artificial selection is distinct from natural selection in that it describes selection applied by humans in order to produce genetic change. When artificial selection is imposed, the trait or traits being selected are known, whereas with natural selection they have to be inferred. In most circumstances and unless otherwise qualified, directional selection is applied, i.e., only high-scoring individuals are favored for a quantitative trait. Artificial selection is the basic method of genetic improvement programs for crop plants or livestock (see Selective Breeding). It is also used as a tool in the laboratory to investigate the genetic properties of a trait in a species or population, for example, the magnitude of genetic variance or heritability, the possible duration of and limits to selection, and the correlations among traits, including with fitness.

Abstract

Artificial selection has been practiced for thousands of years by humans to make improvements in plant species. Mass selection is one of the earliest methods of artificial selection that enabled domestication of crop plants. Another important type of selection was performed in landraces that are heterogeneous consisting of multiple genotypes and selection of individual plant progenies gives rise to pure lines. The pure line and mass selection theory, procedures, genetic basis, their merits, and limitations are described. The information on the application of mass selection and pure line selection for variety purity and breeder seed production are included.

3.3 Domestication

Artificial selection and inbred lines allow exploration of behavioral genetics by testing the responses of behavior to selection or to reduction of genetic variation.

Artificial selection, in scientific laboratories and in animal husbandry, has dramatic effects on behavior. Perhaps the broadest range of artificially selected behavior is seen in domestic dogs, which display a wide variety of behavioral attributes. These behavioral patterns are the result of selection for dogs that assist humans in work (e.g., retrievers, shepherds) or as companion animals. Most domestic livestock (such as chickens, horses, cattle, sheep, goats, and swine) reflect the results of artificial selection for manageability in confinement, ease of training, and docility (Figure 3.3). Strong artificial selection, such as that applied by animal breeders to domestic species (e.g., rabbits, chickens, dogs,12,13 cats, and cattle), can have substantial effects over three to five generations. This suggests that populations of species in new environments (such as invasive species) or species that are experiencing rapidly changing environmental conditions could have the flexibility to exhibit rapid evolutionary responses if sufficient genetic variation is present.

Artificial Selection

Artificial selection and inbred lines allow exploration of behavioral genetics by testing the responses of behavior to selection or to reduction of genetic variation. Recall from Chapter 1 that genetic variation is necessary for either natural or artificial selection to produce shifts in gene frequencies, and the only traits that can be selected are those found within the range of variation genetic variation present in the population. The potential for selection to modify a trait is assessed by measuring its heritability (see below), one estimate of genetic variation.

Key Term

An inbred line is a population in which closely related animals, such as siblings or parents and offspring, have been repeatedly mated so that nearly all genetic variation is lost. This is similar in effect to cloning.

Artificial selection, in scientific laboratories and in animal husbandry, has dramatic effects on behavior. Perhaps the broadest range of artificially selected behavior is seen in domestic dogs, which display a wide variety of behavioral attributes. These behavioral patterns are the result of selection for dogs that assist humans in work (e.g., retrievers, shepherds) or as companion animals. Most domestic livestock (such as chickens, horses, cattle, sheep, goats, and swine) reflect the results of artificial selection for manageability in confinement, ease of training, and docility (see Figure 3.9).

3.3 Domestication

Artificial selection, in scientific laboratories and in animal husbandry, has dramatic effects on behavior. Domestication of animal species has played a huge role in human cultural and economic history, and through practical experience, humans mastered many of the basic principles of genetics early in our pre-history. Perhaps the broadest range of artificially selected behavior is seen in domestic dogs, which display a wide variety of behavioral attributes. These behavioral patterns are the result of selection for dogs that assist humans in work (e.g., retrievers, shepherds) or as companion animals. Most domestic livestock (such as chickens, horses, cattle, sheep, goats, and swine) reflect the results of artificial selection for manageability in confinement, ease of training, and docility (Figure 3.3). Strong artificial selection, such as that applied by animal breeders to domestic species (e.g., rabbits, chickens, dogs,12,13 cats, and cattle), can have substantial effects over three to five generations. This suggests that populations of species in new environments (such as invasive species) or species that are experiencing rapidly changing environmental conditions could have the flexibility to exhibit rapid evolutionary responses if sufficient genetic variation is present. Artificial selection can allow exploration of behavioral genetics by testing the responses of behavior to selection or to reduction of genetic variation.

Another goal of artificial selection can be the production of inbred lines. This technique involves mating closely related animals (parents with offspring, brothers with sisters, and so on) so that genetic variation is reduced in each of the following generations. The outcome can be genetically identical, or nearly so, animals. These inbred lines allow detailed separation of genetic and environmental components of behavior. They can also be used in genetic engineering experiments in which small differences are introduced into otherwise identical genomes. Cloning has the same outcome of producing genetically identical organisms. Often the lack of genetic variation caused by inbreeding creates physiological and behavioral challenges for the animals in an inbred line, making such lines difficult to maintain.

Abstract

Artificial selection is a breeding process in which a population of organisms is screened for some quantitative trait or traits and those individuals rated highest are used as parents for the next generation. With selection on phenotype on a single trait, the response in the population is proportional to heritability and selection differential, and can be predicted from these population parameters. Predictions can be extended to include situations in which selection is practiced on an index incorporating data on relatives and/or on multiple traits, and on genomic information on the candidates and the population. Response over the longer term depends on unknown parameters such as the distribution of the frequencies and the effects on the trait of individual genes, on effective population size, and mutation. Artificial selection experiments have, for example, been widely used to test genetic assumptions and develop extreme lines.

Introduction

Artificial selection in livestock is primarily driven by human intervention in mate selection. Animal breeding essentially began with the first livestock domestication events approximately 10,000 years before the present (MacHugh et al., 2017). The first traits under artificial selection were related to tameness and docility and subsequently expanded to include those associated with food (meat, milk, eggs) and commodity production (wool), transportation and draught power (Fig. 1). Domestication enabled the growth of human populations and expansions into new environments (Toro and Mäki-Tanila, 2007). Emphasis on accurate record keeping of performance traits for individual animals began in the 1700s and quickly evolved into the establishment of herdbooks and pedigrees, progeny testing, and the development of numerous breeds (Oldenbroek and van der Waaij, 2015).

Breeders aim to improve animals by selecting for heritable traits of importance to the overall breeding objective (BO), or goal. They do this by choosing parents that are above average for the trait(s) under selection, resulting in better genetics in the next generation (Fig. 2). The rate of genetic gain (ΔG) depends on the four components of the breeders' equation and is proportional to 1) the intensity of selection, 2) the accuracy of selection, 3) the genetic variance in the population, and is inversely proportional to 4) the generation interval (i.e. the average age of the parents when their offspring are born).

ΔG = Intensity of selection × Accuracy of selection × Genetic variation in the population ÷ Generation interval

10.6 Artificial selection

Artificial selection is that which is practiced by man. Thereby, man determines, to a great extent, the animals that will be used to produce the next generation of offsprings. Some researchers have divided selection in farm animals into two kinds, one known as automatic and the other as a deliberate selection. Litter size in swine may be used as an illustration of the meaning of these two terms. Here automatic selection would result from differences in litter size even if parents were chosen entirely at random from all individuals available at sexual maturity. Under these conditions, there would be twice as much chance of saving offspring for breeding purposes from a litter of eight than from a litter of four. The automatic selection here differs from natural selection only to the extent that the size of the litter in which an individual is reared influences the natural selective advantage of the individual for other traits. Deliberate selection, in this example, is the term applied to selection in swine for litter size above that which was automatic. In one study by Dickerson and coworkers, involving selection in swine, most of the selection for litter size at birth was automatic and very little deliberate; however, the opportunity for deliberate selection among pigs was utilized more fully for growth rate.

Artificial Selection is a form of selection in which we actively choose the desirable traits that are passed on to the offsprings. Humans have used selective breeding long before Darwin's Postulates and the discovery of genetics. Farmers chose cattle with beneficial traits such as larger size or producing more milk, and made them breed; and although they may have known nothing about genes, they knew that the beneficial traits could be hereditary.

Farmers can breed animals in order to improve productivity, and thus, profits. For example, dairy farmers will look for the cows that can produce the most milk and only breed those cows. These cows then pass their genes that contribute to higher milk production onto their offspring, increasing productivity in each generation for the farmers. Selection based on many traits or multitraits selection in terms of progeny testing for male selection and selection indices for female selection becomes effective. A definite difference between breeds and types of farm animals within a species is proof that artificial selection has been effective in many instances. This is true, not only from the standpoint of color patterns that exist in the various breeds but also from the standpoint of differences in performances that involve certain quantitative traits. For instance, in dairy cattle, there are definite breed differences in the amount of milk produced and in butterfat percentages of the milk.

Selective Breeding

Artificial selection or selective breeding is the one of the oldest and most powerful methods in behavioral genetics. In the late 1940s, high and low preferring lines of rats were bred to drink alcohol solutions in preference to water at the University of Chile. Now there are a variety of rat and mouse lines selectively bred to differ in various responses to ethanol, including drinking preference, tolerance and withdrawal severity.42 One of the features of selective breeding is that selecting for one trait leads to the development of correlated pleiotropic differences in many other traits. For example, lines bred for differences in ethanol preference also differ in tolerance.40 Recently, some attempts have also been made to selectively breed lines differing in traits that are comorbid with addiction disorders. Anxiety, impulsivity, antisocial behavior, and depression can be modeled in rat and mouse behavioral assays, although some of these behavioral assays have a bit more than the usual level of difficulty in convincing nonbelievers they possess face validity. Given the intrinsic power of this method to assess genetic pleiotropy, it might be worthwhile developing additional lines of mice or rats that differ in some of the other traits correlated with drug abuse susceptibility in humans.