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According to the Theory of Evolution, Natural Selection is the mechanism by which evolution occurs.
The process of Natural selection is based on the following four postulates (listed in your textbook on page 288): See attachment of page 288
Postulate 1 - Individual members of a population differ from one another in many respects.
Postulate 2 – At least some of the differences among members of a population are due to characteristics that may be passed from parent to offspring. That is, they are heritable or genetic.
Postulate 3 - In each generation, some individuals in a population survive and reproduce successfully but others do not.
Postulate 4 – The fate of individuals is not determined by luck. Instead, an individual’s likelihood of survival and reproduction depends on its characteristics. Individuals with advantageous traits survive longest and leave the most offspring, a process known as Natural Selection.
Choose an animal species. Your goal is to describe the way in which an evolutionary change might occur for a particular characteristic (trait) of that species as a result of natural selection. The characteristic could be something like coloration pattern, length of the limbs, or size of the teeth or beak, or any measurable trait that is inherited.
The organism you choose should be a real one but the evolutionary change you describe can be real or theoretical.
Remember: evolution occurs at the level of the population. Individuals do not evolve, populations evolve.
Explain how the change occurs in terms of the four postulates listed in the book.
You will describe what the population starts out like at time zero and then what it will look like at a later time, as a result of natural selection. Make sure to explain the process by which the changes occurred.
What is the selective pressure? Explain why the change might confer more fitness to a particular environment?
The length of neck of giraffes can be considered as a particular characteristic trait to explain the evolutionary change occurred through natural selection. Lets us consider a population of giraffe holding various lengths of necks. Due to the feeding habit and competition the giraffe with long-neck seemed to survive and pass on the characteristic trait to its offspring while the short-necked giraffes along with various sized necks did not survive. A population of long-necked eventually survived the competition.
According to the postulate 1 explained above here in this example the individual members not only differed in the length of the neck, it might also differ in color pattern, tail length, limb length etc., As per postulate 2 as the offspring’s at the end of the generation were found to be long-necked the characteristic trait have been passed from parents to offspring to survive and become the fittest members of a population.
As per postulate 3 the dominant heritable characteristic holding long-necked trait passed its characteristics to the offspring while the short-neck holding traits could not reproduce successfully and failed to survive. Due to the process of natural selection the advantageous trait with long-neck survived the population. Genetic variants that aid survival and reproduction are much more likely to become common than variants without this ability and so the long-necked survived.
Process and outcomes of Natural Selection:
Three general outcomes are possible depending on which phenotypes are
favored. They are
(i) Directional Selection
(ii) Diversifying selection
(iii) Stabilizing selection
(a) Graph showing stabilizing selection
(b) Graph showing directional selection
(c) Graph showing diversifying selection
It shifts the phenotypic characteristics curve of a population by selecting in favor of some extreme phenotype. Here in our example this selection has been taken over and the long-necked population survived due to its feeding habit, as it is the common selection pattern. At time zero the original population will have a bell shaped curve wherein the right ends will hold short-necked giraffe, centre of the bell holds the intermediate neck sized and the other end holds the long-necked giraffe.
It can lead to balance between two or more contrasting morphs (phenotypic forms) in a population. Both extremes are favored at the expense of intermediate varieties. This is uncommon, but of theoretical interest because it suggests a mechanism for species formation without geographic isolation. Diversifying selection, like directional selection, favors the extremes traits in a population. Disruptive selection differs in that sudden changes in the environment create sudden forces favoring that extreme.
It maintains variation for a particular trait within a narrow range. It culls extreme variants from the population.
The bell shaped curve helps to explain the population starts out at time zero and then the relative variation at a later time, as a result of natural selection. At time zero there will be more or less equal or unequal distribution of the species of giraffe holding three different neck sizes. As time goes by due to increase in the number of species that best suits the environment and gets adapted accordingly will try to increase in number.
Due to the survival of the fittest concept the population holding individuals with a tendency to pass its characteristics to its offspring reaches the maximum curve level and all other species with these ability will gets reduced in their size in the population. At a time where natural selection ahs occurred the giraffe with long- neck would have reached the maximum level in the graph whereas the small and intermediate sized would have steepen down in its curve level.
Selective pressure represent a profile of selection across species that is predictive of gene function: pairs of genes with correlated selective pressures are more likely to share the same cellular function, and genes in the same pathway can evolve in concert.
Natural selection promotes the survival of the fittest individuals within a species. Over many generations, this may result in the maintenance of ancestral traits (conservation through purifying selection), or the emergence of newly beneficial traits (adaptation through positive selection). At the genetic level, long-term purifying or positive selection can cause genes to evolve more slowly, or more rapidly, providing a way to identify these evolutionary forces.
While some genes are subject to consistent purifying or positive selection in most species, other genes show unexpected levels of selection in a particular species or group of species—a pattern we refer to as the “selective pressure” of the gene. The change might confer more fitness to a particular environment because those adaptations of particular phenotypic traits were developed depending on that particular environment
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