Darwin
Name of Author
Institutional Affiliation
Darwin
Introduction
Artificial selection refers to the practice of selecting and domesticating both plants and animals in order to develop or preserve their desirable features. Humans usually select organisms with desirable traits. The process of domesticating animals began in early prehistory where species such as rice, wheat and dogs were selected and domesticated. Darwin coined the phrase “selective breeding” since he was using the process to illustrate his proposed broad process of “natural selection” (Gregory, 2009). He observed that many domesticated plants and animals had unique properties developed by intentional breeding from individuals with desirable characteristics.
In artificial selection, the breeder (whether of dogs, cattle, pigeons et cetera) selected parents considered desirable for each generation and destroys the undesirable types (Hall, 2011). Since the chosen parents may give birth to a variety of offspring, the breeder has the option of selecting the breeds in a particular direction until the desired traits are consistently present. A common example is how humans selected dogs thousands of years ago. The various dog variants originate from a single species Canis lupus. Dogs can now range in size from St. Bernard to Chihuahua.
Robert Bakewell was a renowned breeder in the 19th century. He used native stock of sheep to breed to select large sheep with fine bones and long-lustrous wool. Using the Lincoln Longwool, he was able to develop the New Leicester. The sheep was widely exported in Australia and North America during the 18th century. However, due to a change in market demand and preferences, the New Leicester was not appealing anymore. Only a few of its species are present today. An example is the English Leicester, which is kept to provide wool. The change in meat preferences and textiles selected against the New Leicester. Other species of sheep with better quality meat and wool were developed; hence, they were able to survive in the market.
When individuals look for animals to breed, they look for appealing features in those animals that will suit their intended purpose. Sometimes, they may need crossbreeds to produce a different kind of stock with superior abilities. While in the market, they search closely for this features and would not take chances in finding the one that is desirable. For example, if the individual wants to breed chicken as layers, he intends to receive a certain number of eggs from the chicken. The layer must have the ability to withstand given environmental conditions and should resistant to diseases. Chicken lacking the desired features are selected against (Grandin & Johnson, 2005).
In plant breeding, the farmer selects for those plants that are high yielding and resistant to the environmental conditions present where they are cultivated. Plants which are resistant to pests and disease and give low quality output are uprooted and selected against. However, for plants which are high yielding but susceptible to pests and diseases attack, the farmer can decide to modify the environmental conditions of growth. By modifying the above mentioned processes, the farmer prolongs the lifespan of his plants. The environment cannot select against their growth.
Selective breeding in aquaculture improves the genetic potential of fish and shellfish. Selection of Atlantic salmon, for example, led to the increase of its body weight. The traits for which selection was carried out included growth rate, protein and energy retention, as well as feed consumption. The selected fish doubled in their growth rate with an increased feed intake as well as protein and energy retention. As a result, the selected salmonids had 20% better feed conversion efficiency than the wild type. Furthermore, these salmonids have a higher resistance to both and viral toxins. Storset et al (2007) exhibited 29.3% death rate relative to the wild salmons.
Taming cyprinids resulted in improvement of growth, shape as well as resistance to diseases. Kirpichnikov et al (1974), states that experiments conducted in USSR utilised crossings of brood stocks to elevate genetic assortment and qualities such as growth rate, viability and external traits. The outcome indicated faster development and forbearance to cold. There was a 30-40% improvement in cold tolerance. The selected cyprinids showed a lower mortality of 11.5%, while the unselected one showed a 57%.
Selection for pacific Oysters showed tremendous improvements compared to the wild species. Sydney oysters revealed a 4% growth rate by the end of one generation which subsequently improved to fifteen percent after the next. “Rossmore” Oysters in a harbour exhibited better resistance to pathogenic microorganisms; this was as opposed to different Irish strains. Over the years, the Oysters have shown the prevalence to infection and percentage mortality. Selectively bred Eastern Oyster showed resistance against multiple strains of parasites. After making dual selections year after year in four generations, they achieved successful dual resistance. The oysters exhibited reduced susceptibility to infections.
There is no doubt that Darwin considered the same process by which plants and animals were domesticated to develop the principle of natural selection. This process of adaptive evolution is present in the wild. Darwin (1859, p.4) gives an overview of how domesticated animals and cultivated plants can offer the best approach into natural selection. In his early pamphlet, Darwin worked out his theory of natural selection with reference to the domestication.
Darwin’s analogy between domestication and domestication is useful in providing a model for adaptive evolution, despite the controversy in both science and history. He considered two types of artificial selection, in addition to natural selection. Methodical selection guides man to modify a breed according to a predetermined standard (Gregory, 2009; p.6). Unconscious selection follows the act of naturally preserving the most valuable organisms while destroying the least valuables, without alteration of the breed.
The three types of selection: natural, methodical and unconscious, are linked by a common similarity. This similarity involves a non-random difference in the reproductive success of individual species based on heritable traits. The difference explains why some species will reproduce while others become extinct. Emshwiller (2006), states that the form of artificial selection which Darwin coined methodical selection, has been called “deliberate” or “intentional” selection.
Methodical selection involves a process where humans actively choose individuals from an available sample in order to preserve, and thereafter enhance traits of interest. Unconscious selection is much more passive form of selection than methodical selection and involves no intent whatsoever. Humans have the capacity to determine the individuals that will contribute most to the next generation. Natural selection dwells at this far continuum, where an individual’s reproductive success is not determined by selective breeding or cultivation.
Humans are responsible for creating various conditions in which natural selection operates. For instance, man can treat a bacterial colony with an antibiotic. This instigates natural selection where mutant bacteria in which mutants resistant to the antibiotic survive, reproduce and pass mutated genes to their offspring, a phenomenon which non-resistant cannot do. Humans can also impose selection in the form of selective predation. They can do this by capturing large fish and releasing small ones, to force natural selection smaller size at maturity.
By looking at animal breeding and plant cultivation as a means of artificial selection, Darwin was able to establish the power of selection in changing living organisms (Gregory, 2009; p. 6). He wished to convey that his studies of artificial selection formed the basis of his discovery of natural selection. His recognition of artificial selection as a key process in taming wild animals and plants primed him to conceive the concept of natural selection.
Artificial selection acts by allowing differential reproductive success to organisms with different genetically determined traits so as to increase the desirable features in the population. However, unlike natural selection, artificial selection does not essentially convey greater fitness. Artificially selected traits are based only on the qualities the breeder wants form the cultivated plants or domesticated animals. The traits are selected for by allowing only the individual that possesses the trait to reproduce, while those that lack the trait are prevented.
Since artificial selection involves the process of choosing traits for a long time, this can lead to genetic changes. Genomics can be used to ascertain various genes across the entire genome that has been altered during the intense period of domestication. Understanding these genomics offers a view into the genetic effects of both artificial selection and natural selection, the basis of Darwin’s evolution theory.
During the domestication, individuals undergo intense selective pressures which modify their genomes. The process of domestication largely focuses on core traits which define domesticated species. In seed or grain crops, these traits include seed size, reduction in natural seed dispersal, the annual cycle, as well as reduced lateral branching. The process of domestication, by which a few species are cultivated and selected against, results in very strong selective pressures. The genomes of these individuals usually lack diversity, as seen in the selective sweep.
The process of selection requires transmissible variation which arises by chance. Selection can be lacking if individuals are the same or if the differences among individuals are not inherited. It is crystal clear that Darwin recognised this principle, even though he knew the mechanism of inheritance. Furthermore, selection does not cause variation; hence, no severity of strength of preference can make the beneficial features appear. Although a man lacks the power to cause or prevent the variability, he can select, conserve, and accumulate the various traits given to him by nature in any way he chooses. Therefore, the desirable trait to be selected exist in a minority of organisms, or are introduced from outside by cross-breeding.
Artificial selection, as demonstrated by Darwin, states that continued selection was powerful enough to cause observable features in any species. His claim that natural selection could cause even greater changes than domestication and lead to speciation may be reasonable. Darwin saw selection and survival instead of extinction as important. He collected and analysed a huge amount of evidence for artificial selection. From this data and further enquiries, he obtained clear evidence that the artificial selection could have been marked by hereditary changes. Ergo, Darwin revolutionised the study of nature by taking real variations among actual things as central to the reality.
The problem with Darwin’s theory is its inability to explain how selection distinguishes each small modification. With many traits like size, a small change may hardly be enough to confer a significant advantage to an organism, whereas a large change can easily be selected. Another aspect of the problem is how to determine the initial adaptive level for which a trait a trait or organ must reach to be selected. If a trait already existed before selection, may be some quantitative expression of the trait may suffice for further evolution. If the trait barely existed, how could selection act on it (Hall, 2011)?
Natural selection is a means by which various species can adapt to changing conditions over a longer period of time. In any generation, the number of species born exceeds those who reach maturity to pass on their traits to the generation of offspring. These species struggle for existence. If we take wheat, for example, some species may grow into taller plants while others may grow into shorter ones. Among these plants, individual wheat output is varied. If the wheat is grown in a windy zone, chances are that the taller plants are likely to be destroyed or bent, leaving the shorter one.
However, among the tall plants, there may be those capable of resisting the wind. These varieties of plants can be harvested and genetically modified through engineering controls to produce plants that are both high yielding and withstand the harsh windy conditions. Therefore, Darwin’s theory is exploited in Agricultural production to produce high yielding species that are resistant environmental disturbances, making it easier for nature to select for them.
References
Hall, K.B. (August 24, 2011). Evolution: Principles and Processes. Jones & Bartlett Publishers,
Dalhousie University.
Gregory, R.T. (January 14, 2009). “Artificial Selection and Domestication: Modern Lessons
from Darwin’s Enduring Analogy.” Evolution Education Outreach, Vol. 2; pp. 5-27.
Grandin, T., & Johnson, C. (2005). Animals in Translation. New York, NY: Scribner. pp. 69-71.
Garland, T. Jr. (2003). Chapter 3, Vertebrate Biomechanics and Evolution. Selection Experiments: An Underutilized Tool in Biomechanics and Organismal Biology. Ed. Bels, VL., Gasc JP., Casinos, A.
Storset A, Strand C, Wetten M, Kjøglum S & Ramstad A. ( 2007). “Response to selection for
resistance against infectious pancreatic necrosis in Atlantic salmon (Salmo salar L.).” Aquaculture 272: 62–68
Kirpichnikov, V.S., IIYAsov, J.I., Shart, L.A., et al. (1993). “Selection of Krasnodar Common
Carp (Cyprinus carpio L.) for Resistance to Dropsy: Principal Results and Prospects.” Aquaculture, 111:7-20.
Darwin, C. (1859). The Origin of Species by Means of Natural Selection, or the Preservation of
Favoured Races in the Struggle for Life. London: John Murray; 1859.
Emshwiller E. (2006). Genetic data and plant domestication. In: Zeder MA, Bradley DG,
Emshwiller E, Smith BD, editors. Documenting domestication. Berkeley: University of California Press; pp.99–122.
