The Theory Of Evolution By Natural Selection - Origins And Evidence

This essay will be looking into the theory of evolution by natural selection, by first looking into the origin of the theory and the supporting information. Then progressing on to look at more recent evidence to reinforce it. Concluding with an assessment of the current validity of the theory.

The theory of evolution by natural selection was first expounded by Charles Darwin and was co-published by Alfred Russel Wallace. They suggested if an organism had a particular characteristic which would aid their survival and reproduction, it would mean that the organism would be better suited to its environment, opposed to that of a lesser adapted species. Thus meaning that it could have more offspring which would likely inherit the characteristic and therefore continuing it, until it would be present in all of the species occupying that niche.

Chapter 1 of Darwin`s book `Origin of species` was assigned to `Variation under Domestication`. The information given within this chapter supports his theory of evolution. He speaks about the vast diversity of plants and animals which have been cultivated under domestication. In particular he focuses on variations in pigeons such as; beak and feather size, bone structure and tail formation and how breeders can select these variations in order to create a particular breed (Darwin, C, 1859 p.1-37). Darwin used this information to try and suggest that if breeders can create these varieties, what would prevent natural selection from doing the same. However this only spoke about microevolution (changes within a species) whereas Darwin wanted to suggest that an accumulation of these changes over a long period of time could lead to a change from one species to another (macroevolution) which he explained as 'decent with modification'. Thus causing all the diversity present today. He addressed this by an illustrated diagram, the only diagram within the 'Origin of Species'. This diagram is known as the 'Tree of life'. The 'Tree of life' tries to explain how all species are related from the 'trunk' of the tree which is the LUCA (last universal common ancestor) progressing upwards to more sophisticated organisms, like plants and animals. What Darwin tried to do, is bridge a gap between each species to try and suggest how over a very long time period each characteristic change will morph a species so much so, that it will be largely different to the original, thus forming another species.

Since Darwin, the 'Theory of Evolution by Natural Selection' has progressed and been developed in a dramatic way, by not only biologists. Geneticists, paleoanthropologist, botanists and also geologists have all added to help support this claim. In particularly genetics, the father of the field is undoubtedly Gregor Mendel who was the first to identify the rules of heredity, and how traits are passed through generations of organisms in which he used pea plants as a model. He selectively crossed pea plants together in order to note the expression on the offspring. What he found is that particular traits/characteristics could skip a generation and 'lost' traits could reappear in another generation, and discovered that this was due to recessive and dominant traits. He knew this because the phenotypical expression on the offspring could be mathematically predicted. After the discovery of DNA it opened up a new world of genetics in which Thomas Morgan used to further Mendel's work. Thomas Morgan, followed Mendel's work and looked into the genetics on Drosophila melanogaster a species of fruit fly. He discovered that the random mutations of genes and also the varying structure and arrangement of chromosomes lead to these characteristic change which are passed on due to hereditary. These gene mutations and changes in chromosome arrangement would alter a certain characteristic and therefore act as the driving mechanism for evolution. This was particularly important as it filled a gap in Darwin's theory of how these characteristic changes appeared and what was making evolution progress.

Due to the processes discussed, it would mean that there should be evidence of species transition from one to the next. Fossil records have been able to bridge gaps in historical species in order to find the correct linage for today's species evolution. The phylogeny of the cetacean has been under a deep scrutiny and provided many examples of intermediate species. For example fossils show us that cetaceans have all evolved from one common ancestor (Pakicetus). Pakicetus was a terrestrial species thought to be the first cetacean, however a species known as Ambulocetus thought to be the next species towards whale evolution, displayed more aquatic features such as; shorter legs with enlarged feet and hands to act as paddles and a larger muscular tail. As this phylogeny progressed to become more of an aquatic species, more and more features became present to aid this way of life, the nostril positioning was pushed further back on to the top of the head (present in extant sp.) shown in (Fig.1)

(Fig.1)

Other anatomical features of the terrestrial animals such as the hind legs have been adapted to better suit the aquatic environment, they had been shortened over time, until no longer present externally. However these bones are still present in modern cetaceans but much smaller, but why if not for evolution would these bones still be present, as these bones serve no purpose for current cetaceans (Zimmer, C. 2010).

Biologists have theorised that evolutionary changes act in a clock like way, meaning that mutations will build up in a stretch of DNA in fairly reliable rate. Therefore if this rate is reliable then we can use the genes as a molecular clock, meaning that we can estimate the date of lineage splitting on a phylogenetic tree. This being the case we can use fossil records previously discussed and the molecular clock to try and date back from extant species to find historical species in the lineage. Therefore if we can estimate these changes fairly certainly this is again proof that supports Darwin's claim of evolution.

Evolution by natural selection revolutionised the understanding of the modern world and opened up many new windows of science. Allowing us to utilize the theory and apply it in areas of most importance for example; Pharmacology. The theory is now widely accepted in the scientific community as fact, and although it may only be a theory, but so is that of gravity. Due to scientific discoveries like those mentioned in this essay like fossil records, hereditary, and molecular clocks, all of which point to the theory of natural selection and therefore refuting any claims of species 'creation' and intern supporting Charles Darwin and his claims of the origin of species.

References

Darwin, C (1859). Origin of Species. London: John Murray. 1-37

Dobzhansky, T (1937) Genetics and the Origin of Species, Columbia University Press

Zimmer, C (2010). The Tangled Bank. Colorado: Roberts & Company, Greenwood Village. p8-15.

Images

(Fig.1) Thewissen, J.G.M. From Land To Water: The Origin Of Whales, Dolphins, And Porpoises.. 2009. Web. 8 Mar. 2015.