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Evolution Through Natural Selection Essay Sample

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Evolution Through Natural Selection Essay Sample

            Evolution refers to the succession of changes which the different generations of organisms have been undergoing “over time” since the beginning of life on earth. Biologists believe that the primary reason for these changes is survival. This is because the environment or nature in general is full of all kinds of diseases which are capable of wiping out whole species of organisms and is inhabited by predatory animals which prey on smaller and weaker animals. Nature has also been so exacting that organisms are required to possess certain specific capabilities before they could even feed properly. Because of these requirements for survival, organisms needed to evolve and adapt themselves to their surroundings in order to survive the harsh demands of their environment. One of the processes of evolution which allows organisms to achieve the “fitness” required to survive the exacting demands of nature is called evolution through “natural selection” (Lyal, 2005).

Since survival requires “fitness,” the process of natural selection has also been referred to as the “survival of the fittest” or the “elimination of the unfit.” It is considered responsible not only for the existence of about two million species of organisms on earth today but also the extinction of more than 99 percent of all the species of organisms that inhabited the earth since the beginning of time (Diaz, 1997). All things considered, evolution through natural selection is therefore a process which has enabled organisms to meet the challenges of nature by going through a series of changes over time. These changes have been necessary so that organisms could feed themselves according to the conditions of their environments, evade predators found in places where they lived, and live long enough in order to reproduce and perpetuate their species. However, three conditions should first exist before natural selection could occur. First, there should be a struggle for existence among organisms belonging to the same species. Second, there should be variation even among members of the same species. Third, the traits required in order to make organisms fit for survival should be heritable (The Open University, n.d.).

Darwin’s Galápagos experience

            The necessity of evolution to enable organisms to feed themselves in order to survive was established by Darwin based on his experiences in the Galápagos Islands located in the Eastern Pacific Ocean. His accounts of his observations brought the concept of evolution through natural selection to the attention of the world for the first time when these were published through his book entitled On the Origin of Species on November 24, 1859. Although the first edition of this book was published only in 1859 (with five subsequent editions until 1872), Darwin’s Galápagos experience actually occurred about 28 years earlier when he travelled with a mapping expedition which was led by British navy captain Robert Fitzroy. They were on board the H.M.S. Beagle which left port in December 1831 and travelled around the world for five years (O’Neil, 2008).

Upon reaching the Galápagos Islands, Darwin, in his capacity as the expedition’s “unpaid naturalist,” spent his five-week stay to study the birds which he found there. He observed that birds found in one island looked different from the birds found in other islands. One particular species which interested him was the finch. He already found one species of finch before they left the mainland South America. However, while in the islands, he was surprised to find 13 species whose beaks were different in sizes and shapes from one another as well as from the species which he found in the mainland.

He presumed that the finches came from the mainland but when they arrived in the islands, there were forced to evolve in order to adapt to the conditions they found there because he observed that the conditions in the islands were not exactly identical. In other words, because the environments differed from island to island, the finches needed to evolve differently in order to survive. For instance, he observed that the finch he found living in an island which was dry or arid had beaks which allowed them to feed on cactus because it was their only source of food. On the other hand, in islands were flowers grew in abundance, the beaks of the finches there were ideal either for gathering nectar from the flowers or for eating their seeds with hard shells. He concluded that when the finches were able to evolve according to the demands of the environment, they were able to survive to become productive adults, thus, were able to perpetuate their species. The different beaks of finches he found in the Galápagos Islands are shown below (O’Neil, 2008):                              

The finches found by Darwin in the Galápagos Islands (O’Neil, 2008)

            Based on his observations of the finches in the Galápagos Islands, Darwin theorized that “nature selected the best adapted varieties to survive and to reproduce” because he was not convinced that the changes which occurred in the beaks of the finches were actually caused by their environment. Put another way, Darwin’s theory was that the finches evolved in such a way as to have the kind of beak which would help them in feeding out of the available food sources found in their particular island. In other words, had they not evolved accordingly, the finches would not have lived long enough to reproduce and multiply. The process laid out by Darwin came to be known much later as the “natural selection.” He later concluded that this process, which he also called the “survival of the fittest,” was what was responsible for the variations which could be observed in any population, variations which actually allowed them to have the advantage as far as survival was concerned. These variations, according to Darwin, are being passed to the next generation in a process which he called “descent with modification” – meaning that the evolved generations are coming up with modified characteristics which are necessary for their survival (O’Neil, 2008).

The case of the peppered moth

            The case of the peppered moth in the British Isles was a good example of an organism which was able to evade its predators by evolving and changing its color. This moth is generally light in color with dark spots scattered all over. Around 1849, at the height of the industrial revolution in Europe, a mutant of the moth which had a darker coloration started appearing in polluted forests in Manchester, England. It was observed that in less than a century, this dark-colored variety already constituted around 90 percent of the moth population in the area. The moth, being a nocturnal organism, fed at night and rested during the day. In that specific region, their resting places were the trunks of trees in the forest.

Before being polluted, the trees in that forest were usually covered by lichens which were as light as the color of the moth. In other words, when the moth rested on the lichen-covered trunks, they were almost invisible to the birds which were preying on them. Unfortunately, with industrial revolution and its consequent pollution (soot and toxic gases from the industrial plants), the trees lost the lichens and then their barks darkened. The result was that when the moth rested on the trunks, their color stood out, making it easy for their predators to spot them. It eventually decimated the population of the peppered moth. After the darker-colored variety surfaced, it was able to survive longer and reproduce because predators found it hard to spot them. H.B.D. Kettlewell, an English geneticist, conducted his own investigation. After releasing peppered and dark-colored moths in the forest, he found that more light-colored moths were victimized by birds than their darker counterpart.

            Shown below are pictures of the moth in both environments (lichen-filled trunks and the soot-blackened trunks) furnished by Kettlewell:

Note that the moth resting on the lichen-filled trunk in the picture on the left (encircled with red) is almost invisible while it is very recognizable in the soot-blackened trunk as shown in the picture on the right. Evidently it was much easier for birds to feed on them while they rested on the pollution-darkened trunks. The population of the lighter moths was able to recover only after the Second World War when environmental programs were already implemented to curb pollution in the Manchester area. In other words, the lichens again started appearing on the trunks of trees, thereby giving refuge to the lighter peppered moth (Evolution and Adaptation, 2007).

            The finches of the Galápagos Islands and the peppered moth of Manchester, England, showed the importance of natural selection in these organisms’ struggle for survival. However, natural selection could not exist by itself. It could not come about unless certain conditions are present. The first of these conditions is that organisms should find themselves in an atmosphere where they have to struggle to survive. Without this need, natural selection would not be serving any practical purpose. For instance, if the available food supply could only support two million organisms and the population is already four million, a struggle-to-survive situation occurs. In this case, natural selection would act to allow only the “fittest” organisms to survive. As shown by the finches in Galápagos which found themselves in an island where the available food supply was in the form of flowers, it was the finches with beaks which were suitable for collecting nectar who survived. On the other hand, in the island with an arid climate, the finches with beaks suited for eating cactus were the ones who survived and lived long enough to reproduce (The Open University, n.d.).

            Second, there should be variation even among the members of the same species. Some people believe that organisms could never be identical. In fact, even twins have differences, no matter how slight and indistinguishable. It is this variation which allows natural selection to occur. This is explained by the fact that when variations occur, some characteristics are more advantageous than others. In the case of the peppered moth of the British Isles, a dark-colored mutant surfaced. It was that mutant which eventually survived the predatory birds and lived to reproduce (The Open University, n.d.).

In this connection, mutants are produced by mutations which represent alterations in the genetic properties of organisms. Mutations take place under two circumstances. The first is when genes are not copied exactly before being transmitted during the process of reproduction. The second is when the genes are exposed to radiation or some other mutagens (chemicals or viruses which are capable of causing changes in the genes). If mutation happens to produce a more desirable organism or mutant, it lives long enough to reproduce (Lyal, 2005). This actually happened in the case of the dark-colored mutant of the Manchester moth. Its altered coloration enabled it to survive the preying birds so it was able to breed. After almost a hundred years, its number exceeded that of the lighter-colored moths which failed to reproduce fast enough because they were constantly eaten by predators.

The third condition for natural selection to take place is “inheritance.”  This means that natural selection could only be possible if the advantageous characteristic or the characteristic which caused the variation is heritable. In the case of the mutant moth, it was clear that the advantageous characteristic, which was its darker coloration, was heritable so it was able to breed and increase in number, enabling the process of evolution through natural selection to complete itself. There are characteristics that could cause variations in organisms but are not heritable. One specific example is the body size of toads which is determined by age and the availability of food. Since toads are known to be capable of growing as long as they live, they could be expected to add to their body size as long as there is food available. Now, since age and food availability are considered external factors, the body size of toads is therefore not an inherited characteristic. Therefore, toads could not increase their body size through natural selection (The Open University, n.d.).

The primacy of these conditions to natural selection could also be verified by looking at experiments performed on guppies in the 1970s. Guppies are small fishes which, in their natural habitat, are being eaten by larger, predatory fishes. While female guppies are plain-looking, the males are brightly colored. However, depending upon their origin, the colorful spots found in male guppies vary in their brightness, in their sizes, and in their number. This is clearly shown by the picture which has been reproduced from The Open University (n.d.) below:

The two plain guppies shown in the picture above are females while the colored ones are males. It is plain to see that the male guppies vary in their brightness as well as in the sizes of their colored spots. The biologists who have been working on guppies have already claimed that these variations are being determined by whether predators were present in the rivers and streams where they lived (The Open University, n.d.).

However, in order to verify this claim, John Endler, an American zoologist, performed several experiments. In his first experiment, he first acquired guppies from different habitats in Trinidad. Then he placed them in several ponds where no other fishes or predators were present. At the start of the experiment, the average number of spots in the male guppies was ten. After six months, when reproduction had already taken place several times, the average spots in the male guppies in all three ponds had increased to 11.8. This already indicated that the spots are, indeed, inherited. The results of the succeeding experiments are shown in the chart reprinted from The Open University (n.d.) below:

            After six months, Endler divided the ponds into three groups, namely: groups A, B, and C. Then in each of the ponds under group C, he added one Crenicichla alta, which is classified as “a voracious predator of guppies.” In the ponds under group B, six predator fishes (Rivulus hartil) were added. However, this kind of predator does not specifically prey on guppies. Then no other fishes were added to the ponds under group A. After another five months and several breeding periods, the colored spots on the male guppies were again counted. It was found that the average of the colored spots for guppies in the ponds under group A where no fishes were added, as well as in the male guppies of the group B ponds where the predator fish did not prey on the guppies, increased from 11.8 to between 12.5 and 13.0 spots.

The average colored spots in the male guppies in the ponds under group C where one voracious predator was added to each pond, decreased from 11.8 to 10.5 spots. In other words, a variance in the number of spots was found as result of the presence of predators. The ponds were left alone for another nine months after which the divergence in the number of spots further widened. While the guppies in groups A and B had an average of 13 spots, the male guppies in the ponds under group C had an average of only 9.5 spots for every fish. The experiment, therefore, validated the theory that the average number of colored spots in male guppies is reduced by the process of natural selection in reaction to the presence of a voracious predatory fish (The Open University, n.d.).

            The foregoing discussion validates Darwin’s theory of evolution through natural selection as a logical explanation for the perpetuation of organisms which possess the characteristics necessary for survival. These characteristics enable them to feed themselves from the food supply available in their habitat and help them evade predators long enough to breed and reproduce. Natural selection occurs through successive changes which generations of organisms undergo in their struggle for survival.


Diaz, E.D. (1997). Evolution. Retrieved December 8, 2008, from


Evolution and Adaptation. (2007). Retrieved December 8, 2008, from


Lyal, C. (2005). How does evolution work? Natural History Museum. Retrieved December 8,

2008, from http://www.nhm.ac.uk/nature-online/evolution/what-is-evolution/how-does-evol-work/

O’Neil, D. (2008). Darwin and Natural Selection. Retrieved December 8, 2008, from


The Open University. (n.d.). Evolution through natural selection. Retrieved December 8,

            2008, from http://openlearn.open.ac.uk/course/view.php?id=1646

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