How Viruses Evade Immune Detection Biology Essay

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Viruses are able to avoid detection by the immune system because; viruses need to occupy host cells in order to survive. Once they have successfully infiltrated the host cell, the white blood cell immune response will register the virus as the original host cell. This allows the successful spread of infection unaffected by immune response. An example of this is the king of virus is the T4 bacteriophage; this is practically a protein sack harbouring the viral gene. It is shaped similarly to a lollypop with four spines that latch onto the host cell, once clamped it injects it genetic code directly into the host cell, thus allowing the virus to undertake the genetic takeover of the bacterium. Virus' ability to adapt does not end there, say for example the immune function is successful and the white blood cells come to the rescue and destroy the virus. Bacteria (registered as beneficial/native to the body) can absorb remnant genes of the defeated virus. This could take on the damaging trait of the virus and start a new more advanced strain. This was first proven during Griffiths experiment (published 1920) with mice. This occurrence is called transformation, a well-documented characteristic of viruses (they continually spread to new hosts, taking on new traits). This is why we need to produce more and more drugs to counter this ability (e.g. antibiotics).

[Campbell, N. A. B. (2008). Biology. San Francisco ; London, Person/Benjamin Cummings, Page 381]

Environmental Mutations

The traits of an organism do not only determine its performance it determines how well both it and its descendants interact with their environment. If for example an organism developed a feature that allowed it to evade predators, obtain food that was previously impossible to reach/digest or an adaption to climate conditions {i.e. extreme cold, dry weather). When these traits are expressed, it increased the survival rate of the offspring and reproduce. The trait will then appear in greater frequency in the next generation. From this, one can determine over a time-period the environmental conditions (from weather conditions, to predators) act as a natural selection process that can change the course of favourable traits in a population.

[Campbell, N. A. B. (2008). Biology. San Francisco ; London, Person/Benjamin Cummings, Page 459].

Unnatural environmental interaction can affect mutations; for example, radioactive waste can cause unpredictable cellular mutations in animal, causing cancers and birth defects. In addition, changes in temperature can kill off a species and welcome in a new one (i.e. the ice age).

Mutation causing Phenotypic Change

Each species has a distinctive genotype, this is expressed in individuals phenotypic variations, for example, skin, hair colour or height. If one was to imagine all the traits of a species mixed together in a pool and reproduction was a random allele selection from this gene pool. Then we could assume that all mating was completely random and the mating habits of male and female are equal (this describes the Hardy-Weinberg equilibrium).

If a mutation occurred in males that made them appear, more appealing it could change the gene pool drastically. For example if the trait was an allele expressing red feathers on male birds and the females preferred red feathers, they would choose to mate with males with red feathers over the males without. Thus, with each generation the red feathers trait will be expressed ever increasingly by the male offspring. This will continue until practically all male birds have mainly red feathers leaving most of the traits contained within the male birds without red feathers lost. The birds of paradise are a prime example of this phenotypic change. The males entice a female the male birds have adapted to express colourful plumage, distinct mating calls, decorative nests and ritual dances. The females are usually smaller with very bland colouration in contrast to the males. This illustrates the lengths at which evolution is willing to go in order to continue a species survival. [Campbell, N. A. B. (2008). Biology. San Francisco ; London, Person/Benjamin Cummings, Page 469].

Essay Section

Genetic Drift

Genetic drift it the chance balancing out to become equal. Events that happen by chance cause allele frequencies to fluctuate with each generation (more aggressively in smaller populations). This process is genetic drift (See Figure 1)

Figure 1

[Campbell, N. A. B. (2008). Biology. San Francisco ; London, Person/Benjamin Cummings, Page 469].

Figure 1 illustrates how genetic drift can affect a population of flowers. By chance, the Cw allele is lost from the gene pool. These random occurrences can happen due to survival or reproduction problems, for example, maybe they were eaten by cattle or were trampled.

This affected the breeding habits of the second generation then expressing the CR allele as the dominant gene in the population as a whole.

Genetic drift can cause alleles that are harmful to become fixed; it does this mainly in very small populations, when this happens the survival of the population is threatened.

Natural Selection

One famous example of natural selection is the case of the peppered tree moth during the industrial revolution. Around this time, copious amounts of factories were being build and operating. These factories were burning coal; the particulates that entered the surrounding environment blanketed everything in a layer of soot. Prior to the industrial revolution, the peppered tree moth was evenly dark coloured and white. The soot acted as camouflage for the darker moths and the white moths became more visible in contrast to their surrounding making them visible targets to predators. This random occurrence favoured the dark moths over the white ones is a famous example of natural selection. [Class lecture]

Founder Effect

The founder effect is when a small group is separated from a larger population and adapts to establish a new population, a derivative of the original containing a slightly different gene pool.

This can happen due to earthquake, volcanic activity of even spores carried by air currents or seeds by birds.

The founder effects causes genetic drift where chance occurrences alter genetic frequencies, only rather than the fluctuation caused by female mating preferences it is caused indiscriminately by a freak occurrence.

Inherited disorders found in humans tend to be of a much higher contrast in small populations due to founder effect. For example in 1814, British colonists settled on Tristan da Cunha (an Island located in the Atlantic between South America and South Africa). One of the 15 colonists carried a recessive allele for pigmentosa, (causes blindness) which affects homozygous persons. During the 1960's four of the 240 defendants was recorded as being afflicted with pigmentosa. This figure is ten times higher than found in population where the settlers originated.

Bottleneck Effect

The bottlenose effect is caused by a serious change in environment that kills off all but a few remaining populous that is able to adapt to the new circumstances. For example, this happened to humans during the last ice age. Humans adapted to the icy conditions by blending in to the white surroundings aiding in camouflage for hunting, those expressing white skin alleles took priority over those with darker traits. This drastically altered our genetic diversity, favouring those mainly with white skin. The advantage to this change is that the species was able to continue to thrive in the new environment; the disadvantage to this is that with less genetic diversity the population will have the same vulnerabilities to disease and weakness. In addition, they will not be able to adapt as quickly as when there were more genetic variants.

After the freak occurrence (i.e. ice age) has passed, the population will be able to recover its numbers but it will take much longer for the diversity to repair.

[Campbell, N. A. B. (2008). Biology. San Francisco ; London, Person/Benjamin Cummings, Page 476-477].

The bottlenose is an important in understand evolution because it is an effect that has been intestinally caused by humans for hundreds of year. The selective breeding of cattle (cow, sheep, pigs, chicken) and even crops (fruit, vegetables [especially bananas]) has caused a bottlenose effect in. This is why cattle and crops are so vulnerable to disease.

Conclusion

From the discussion, one can see how natural selection is caused by natural changes in environment, food, predators that alters the evolutionary steps of the species, whereas genetic drift is mainly caused by freak occurrences like extreme weather, geological conditions or human interaction. This causes fluctuations in genetic diversity by changing the living conditions thus killing off individuals with the weaker alleles (in contrast to the change) alternatively isolation can yet again causes a reduction of genetic diversity and creates new venerability's as it hinders the populations ability to adapt with a smaller gene pool.

One can conclude that human selective breeding will cause more and more problems unless we stop now or create synthetic alternatives [Class lecture]

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