Diversity of Life in Darwins Evolutionary Theory - Essay Example

21862 Academia Research 4 May 2009 Origin and Diversity of life based on Darwin’s evolutionary theory. The origin of species has always been referred to as the mystery of mysteries by many famous researchers throughout the history. After many years of work and gathering of facts that could possibly have some relation to this mystery, Charles Darwin drew conclusions in the form of short notes, and this has continued ever since. This was a landmark work in evolutionary biology. Darwin reflected on the mutual resemblances of organic beings based on their embryological relations, their geographical distribution, geological succession, and other facts that lead to a conclusion that each species had not been independently created, but had descended, like varieties from other species. However, this concept required proof of how the innumerable species inhabiting this world have been modified to acquire their particular shape and adaptation to natural habitat. Charles Darwin, in his Origin of Species (1859) wrote ‘probably all of the organic beings which have ever lived on this Earth have descended from some primordial form1’ Much focus was given to the fact that large amount of hereditary modification existed, and only those modifications or features selected by living beings thrived. Darwin’s study focused heavily on the variations that existed among various living organisms and the phenomenon of natural selection responsible for the extinction and elimination of less improved forms of life leading to divergence of character. He also strongly backed the fact that species are not immutable; and that, the way organisms belonging to one species are the descendants of that species, similarly those belonging to the same genera are lineal descendants of some other and generally extinct species (Darwin, C & Marcus, H; 30). Darwin drew many of his observations from the work of Linnaeus, Cuvier, Hutton, Lyell, Malthus and Lamarck; however, his own observations had formed the basis for his postulations. The basis for Darwin’s philosophy of evolution can be traced back to his voyages of the HMS Beagle in the 1830s during which he observed that the living organisms had many similarities with that of few fossils. Another observation was the diversity of life on the Galapagos Islands, such as a myriad of birds and insects. This was the basis for him to believe that the Earth was continually changing; same happened with living organisms, and also that they were a product of the earlier life-forms. Through his observations, Darwin proposed the two mechanisms associated with evolution, natural selection and variation that led to modifications in the living organisms. Variations Darwin’s observation of domestic plants and animals had been the precursor of his landmark theory. Based on his observations, he postulated that the domestic plants and animals of one variety differ hugely from other varieties of plants and animals respectively. These differences of plants and animals which have been cultivated are mainly due to the environmental conditions of life which were different in their parental stage. However, these differences or variations can appear only after exposure of these species to a particular environment for several generations. Once these variations appear, they continue for many generations, and no specific time can be allotted for these variations to change. However, further changes are also inevitable. Darwin attributed these causes of variations to the male and female reproductive elements being affected prior to act of conception. To this he further adds that many animals and plants tend to adapt to different conditions of environment and breed in confinement very well. On the other hand, many exotic plants and strong animals taken from natural conditions fail in adapting to new conditions and to produce offspring of respective nature. Also, many times seedlings of the same fruit, and the young of the same litter differ considerably from each other though both are exposed to same conditions of life. There are many similarities and differences attributed to change in conditions of life; however, the extent has not been defined. Variations have been evidenced due to habits, conditions of life, character of domestic varieties, reproducibility, and many other innumerable factors. The variations of heritable nature are not important; however, those with inheritable nature in terms of numbers and structures are extremely important. And, these cause evolution. The variations are seen to appear in the offspring during the same age as that of corresponding age of appearance in the parent species. For example, peculiarities in the silk worm appear at the corresponding caterpillar or cocoon stage2. The differences between domestic species and their races usually are seen as monstrous features of the domestic races. Yet, many similarities bring out the possible relation between the two. This is quite evident of evolution of species from earlier life forms and existence of variations that have brought certain changes according to the environment and life conditions. The amount of structural difference between the domestic races of the same species largely depends upon, both, due to domestication and based upon their descent from different species. Darwin quotes the examples of greyhound, bloodhound, terrier, spaniel, and bulldog, all belonging to the same race; however, may also be a possibility of descent from distinct species or wild stock. Darwin’s extensive study of many breeds of pigeons also depicted variations and modification from descent in many aspects. He later arrived at the conclusion that all these breeds had descended from the rock-pigeon (Darwin, C; 24). His experimental research with all kinds of pigeons led to many results which compelled him to stress on the fact that variations of important and useful characters are retained, and that of harmful or the useless ones are eliminated. These variations are true in case of all structural changes such as feathers, skull shape, beak shape, tail etc; in case of colour such as colour of feathers, body, beak etc; functional changes such as food habits, resting nature etc. Ultimately, he concludes that all these breeds have descended from many native species. Struggle for existence and Theory of natural selection Darwin’s proposal of evolution of all living organisms from common ancestors with modifications formed the bases for his theory of natural selection. His research included animal husbandry and extensive experiments with plants, finding evidence that species were not fixed and investigating many detailed ideas to refine and substantiate his theory. For more than a decade this work was in the background to his main occupation. Darwin’s observation of the universal nature of individual variability between any two forms living organisms had provided little evidence of origin of species in nature. This further gives rise to questions on adaptability of different or same organisms to different environmental conditions and to different organic beings to different parts of the world. These adaptations were most commonly observed in woodpeckers and misseltoes, in the parasites that exist on the feathers of birds, in the structure of a beetle that dives into water etc. These adaptations are impossible to name and number. Another strange and interesting fact is the constant change in these forms and adaptations. Darwin also observes that the incipient species further get converted to more stable and distinct species with many more differences than with those within their own species. All organisms continually struggle for life and owing to this struggle, any variation no matter how insignificant or how different, and if it helps the living being will continue to exist. These differences of adaptations or variations with respect to other complex beings or external environment will have a tendency to preserve that living being and will eventually be inherited by its offspring. The offspring will further pass it on to the next generations and these species will have a better chance to survive. This principle by which with each slight variation, if useful, is preserved, has been referred to as Natural Selection by Darwin (Darwin, C & Burrow, J W; p 115). The variation that is not useful will also be destroyed. Through this theory, Darwin inferred that selection is a power incessantly ready for action, and is immeasurably superior to any other forms of efforts by man, which nothing on this earth can equate. The fact that every living being is continually struggling for existence has been elaborately illustrated by Darwin through this theory of struggle for existence. Animals struggle for food continually. Trees too struggle for existence. Darwin gives the example of a tree on a desert which has to overcome the drought to survive. A plant may produce thousands of seeds annually of which only one seed matures after its struggle to compete for existence with other seeds. He further proposes that the struggle for existence is directly proportional to the rate at which organic beings tend to increase. As the production of individuals increase than that can possibly survive, struggle for existence intensifies in every case. During the struggle, only a few remain. In the absence of struggle and destruction, the whole earth would not be able to support all living beings. Darwin further explains the theory of natural selection in case of change in conditions of life which impact the reproductive system and cause or increase variability, giving preference to profitable variations lead to natural selection. There cannot be even one country where all individuals have equally adapted to the environment and in harmony with all other existing species; in such cases, none of them could improve due to absence of variability. Just like how man can produce excellent results through methodical and unconscious means of selection, so can nature. However, the difference between man and nature is that man can only act on the external and visible characters, whereas nature can act on every internal organ, on every shade of constitutional difference, on the whole machinery of life. Every selected character of nature is fully exercised to suit the living being. A few perfect examples of nature’s protection of living beings can be seen in the camouflage nature of few animals: green leaf-eating insects, grey bark-feeders, the alpine ptarmigan white in winter, heather colored red-grouse and peaty earth colored black-grouse etc. So, in nature, natural selection is the process that acts on and modifies organic beings at any age, by the accumulation of profitable variations at that age, and by their inheritance at a corresponding age. However, what natural selection cannot do is to modify the structure of one species, without giving it any advantage, for the good of another species. Darwin also included the theory of Sexual selection into the theory of natural selection wherein, not only struggle for existence, but also struggle between the mates for possession of females resulting in fewer or no offspring. This sexual selection, Darwin attributed to the female choice of the best male partners. Through sexual selection, some organisms with useful sexual variations have better chance of reproducing than organisms with sexually injurious variations (Ruse, M; p.20). Thus, through Darwin’s views on natural and sexual selection put together, a fairly integrated pattern of theory of evolution through selection arises. Adaptation Adaptation is the most important natural phenomenon for life to exist on the earth. Evolution is strongly connected to adaptation of different animals and plants to different environmental conditions and different ways of life in those conditions. Adaptive changes in structure in response to changed conditions are more common in plants than in animals. For example, plants are more flexible in response to light. Different plants have been grown in different and entirely opposite climates. Certain animals also have been transferred from one habitat to another, and have survived. The distinction between different categories of adaptation based on genetic, physiological, and developmental adaptations is vital for its survival in case of changed environment. The extent to which adaptation is possible by living organisms has been a difficult topic to explore. Darwin notes that, irrespective of the adaptation nature existing in living organisms, natural selection would inevitably preserve those living beings which were born with constitution best adapted to any country which they inhabited. To this, Darwin infers that habit, or use and disuse, have in some cases, played a considerable part in the modification of structure and function; however, the effects of these changes have been largely combined with natural selection (Darwin, C & Marcus, H; p 195). Use and Disuse Theory Adding more to the theory of natural selection, Darwin extends his explanation to structures of organic beings that are used and unused. According to this, if a structure or organ is used by a particular organic being if only once in its life time and if it is of high importance, this might be modified to any extent by natural selection. For example, the great jaws possessed by some insects and used exclusively for opening the cocoon, or the hard tip to the beak of nestling birds used for breaking the eggs3. The origin of species and early evolution of life. Evolution is the process of change through time. Evidence to prove that existing life forms have evolved from earlier life forms can be obtained by studying the various differences in structure, function, and behavior of various organisms. Discovery of various animals and their fossils made man believe that life existed for more than 6000 years on earth. Evidences showed that earth’s surface had been constantly shifting and changing, and the fossil rich rocks revealed the history of life on earth. Through natural selection, environment selected the traits useful for organic beings and discarded the harmful ones. In the 19th century studies, fossils showed that the kinds of living organisms on the earth have changed over millions of years. Even though Darwin knew nothing about inheritance, he showed that species have evolved from common ancestors and offered a means by which that could happen4. Another important factor for evolution, proposed by Darwin, was descent with modification, which stated that all existing living forms descended from earlier forms. Though neo-Darwinism advocates the concept of genetic heredity and mutation, Darwin’s theory also postulates the same outcomes of evolution, modification, and natural selection. For instance, a mutation i.e., a change in gene structure, whether deletion, addition, or modification, produces a different offspring. Through Darwin’s theory, this concept is explained on the basis of natural selection. The theory concludes that individuals or their characters best suited for the environment are more likely to survive and reproduce than those less well adapted. As a result, the proportion of individuals with favorable characters increases and populations gradually change in response to the environment. In modern terms, heritable genetic changes at the molecular level are responsible for modifications in the population; in other words, evolution occurs (Pevsner, J; p 216). Darwinian theories on origin of species help in discovering the great diversity of life forms and relation between various organic beings. To explain biological evolution, Darwin drafted comparative evolutionary distances between different species. These distances and relationships between different species of plants and animals supported Darwin to explain his theory of evolution. Darwin postulated that the great diversity of life can be understood to be the result of branching evolution or adaptive radiation, which means the diversification of a species into different lines as they adapt to new ecological conditions and ultimately evolve into a different species. Natural selection is the principal mechanism driving adaptive radiation5. In the hierarchical classificatory system of Linnaeus there is an implicit recognition of relatedness, for example, species belonging to one genus have more in common with each other than they do with species belonging to another genus, which again is very evident from Darwin’s explanations. The hierarchy of Linnaeus’s system, the species, the genera, the orders, and so on made sense to Darwin as evidence of evolution. To this, Darwin further added the principle of natural selection played by nature, further leading to variations (modifications) of all sorts. Owing to these factors, Darwin insisted on a geological classification that could explain evolution and the principle of inheritance6. Charles Darwins theory of evolution restructured comparative embryology and gave it a new focus. After reading Johannes Muller’s summary of von Baers laws in 1842, Darwin saw that embryonic resemblances would be a very strong argument in favor of the genetic connectedness of different animal groups. His statement, “community of embryonic structure reveals community of descent7.” He advocated that embryos of existing, for instance existing vertebrates, resemble more closely with the embryos of the parent stock of this great class, than with full grown existing vertebrates resemble their full-grown parent stock. He also covered that in the earliest and simplest condition of things, the parent and the embryo would always resemble each other, and that passage through embryonic forms is entirely due to (modifications) subsequent variations affecting only the more mature periods of life (Richards, R J; p124). Darwin also argued that adaptations that depart from the “type” and allow an organism to survive in its particular environment develop late in the embryo8 (Darwin, C; p 385). He noted that the differences between species within genera become greater as development persists, as predicted by von Baers laws. Thus, Darwin recognized two ways of looking at “descent with modification.” One could emphasize the common descent by pointing out embryonic similarities between two or more groups of animals, or one could emphasize the modifications by showing how development was altered to produce structures that enabled animals to adapt to particular conditions. Genomic Tree of Life Darwin’s vision of origin of life and species, and evolutionary theory clearly explained the evolutionary trees of animal and plant life. As stated by Darwin, “I believe that the arrangement of the groups within each class, in the subordination and relation to the other groups, must be strictly genealogical in order to be natural; but that the amount of difference in the several branches or groups, though allied in the same degree in blood to their common progenitor, may differ greatly, being due to the different degrees of modification they have undergone9’ clearly explains that he had envisioned understanding the evolutionary trees. The adaptive radiations explained by Darwin fit into this theory. In the modern classification of living organisms based on their structure and function, the broad categories formed are prokaryotes and eukaryotes. Prokaryotes have a simple internal architecture without a nucleus. Eukaryotes have a distinct internal structure with a nucleus containing the genetic material. It is assumed that prokaryotes predate eukaryotes, and that two preexisting prokaryotes contributed their genomes to the first eukaryotic genome. Eukaryotes consist of several kingdoms, including animals, fungi, plants, algae, protozoa, and others10. Linking this modern concept to Darwin’s theory may seem irrelevant. However, evidences are present that explained Darwin’s understanding, rather assumptions, of the basic building blocks of life, i.e., amino acids. In his letter to his friend, Joseph Hooker, he expressed his idea of what he thought how life would have started. He stated, ‘But if (and Oh! What a big if!) we could conceive in some warm little pond, with all sorts of ammonia and phosphoric salts, lights, heat, electricity etc., present that a protein compound was chemically formed ready to undergo still more complex changes, at the present day, such matter would be instantly absorbed, which would not have been the case before living creatures were found.11 This experiment was practically carried out by two scientists in 1953, which were able to synthesize ten different kinds of amino acids. These experiments though later proved that earth was seeded with organic compounds from space and that sparked the origin of life, were in a way, envisioned by Charles Darwin. Though not specified by Darwin, this essentially traces back to Darwin’s explanation of spark of life through chemical substances, thus explaining the origin of prokaryotes leading to the evolution of much complex eukaryotes (Prothero, .D R & Buell, C D; p 148) Conclusions Darwin’s significant scientific contribution presented an overwhelming mass of evidence demonstrating the occurrence of evolution through a logical and biologically proven mechanism that might account for evolutionary change through natural selection. Evolution being the unifying principle for biology provides an explanation for the difference in structure, function, and behavior among organisms. The three main dynamic aspects that Darwin addressed through his internal organization of organisms and their classificatory order are, the internal development of individual organisms, the distribution of species over space and in time, and the interaction of organisms within ecological systems (Darwin, C & Carroll, J; 14). His theory of natural selection was a landmark in the study of life sciences. The circumstances favorable for natural selection explain a large amount of inheritable and diversified variability. Darwin’s theory of evolution, referred to as Darwinism, can be listed with * main conclusions, that living things change, or evolve over generations, that living beings descended from other living beings with modifications, there is a natural selection operating upon all living beings which is responsible for the changes and adaptations in the living beings; organisms vary in many characteristics, and that these variations, if useful, will be inherited, and if not useful will be eliminated in due course of time; that natural selection is the mechanism of evolution. On the basis of natural selection, all those organic beings with favorable characteristics survive and others perish. Another phenomenon which operates in nature is the sexual selection which is extremely important for increase or decrease in particular species. Darwin’s explanation on adaptations, whether structural, functional or physiological adaptations provide clear proof of survival of the fittest, meaning those which are able to adapt to the life conditions can survive and perpetuate further life. The concept of Evolution is clearly explained by Darwin from study of fossils, anatomy, embryology, and biochemistry. Natural selection focused as the mechanism of evolution, acts mainly on variations produced in the organic beings. Darwin also explained evolution on the basis of genealogy and emphasized on the fact that significant changes in the genetic structure could lead to evolution of new species over time. Put together, the theories of evolution, modification, natural selection, variation, adaptation and the concepts of structural, physiological, functional, embryological changes and resemblances prove that all living beings evolved from their related earlier forms of living beings, ruling out the fact of chance in evolution. Thus, evolution has been proved to be a gradual process, starting from the smallest to the largest of animals and plants. Works cited Darwin, Charles. On the origin of species by means of natural selection or the preservation of favored races in the struggle for life. Edition 6. Published by Murray, 1864. Original from the Bavarian State Library. Digitized 12 Dec 2008 http://books.google.co.in/books?id=HIo5AAAAcAAJ&pg=PA420&dq=probably+all+of+the+organic+beings+which+have+ever+lived+on+this+Earth+have+descended+from+some+primordial+form Darwin, C. On the origin of species by means of natural selection, or, The preservation of favoured races in the struggle for life. Edition 4. Published by John Murray, 1866. Original from Oxford University, Digitized 11 Sep 2006. http://books.google.co.in/books?id=LUUDAAAAQAAJ&pg=PA99&dq=the+beak+of+nestling+birds+used+for+breaking+the+eggs+Charles+Darwin Darwin, C and Beer, G. The origin of species. Published by Oxford University Press, 1998. http://books.google.co.in/books?id=LDrPI52uFQsC&dq=origin+of+species+charles+darwin&printsec=frontcover&source=bn&hl=en&ei=APL_SfOGKYLY7AP3-ryZBg&sa=X&oi=book_result&ct=result&resnum=6#PPA3,M1 Darwin, Charles and Carroll, Joseph. On the origin of species by means of natural selection. Published by Broadview Press, 2003 http://books.google.co.in/books?hl=en&lr=&id=eTfRotZTXI0C&oi=fnd&pg=PA7&dq=origin+and+early+evolution+of+life+Charles+darwin&ots=rX21PTX1Z6&sig=qTLjqj2CMNs6kINQKdZ8fVORupU#PPA14,M1 Darwin, Charles and Marcus, Hans. Published by Plain Label Books, 1926. http://books.google.co.in/books?id=t0jb8-O6efoC&printsec=frontcover&dq=Origin+of+species+by+means+of+natural+selection&lr=#PPA31,M1 Darwin, Charles and Burrow, John W. The origin of species by means of natural selection, or, The preservation of favored races in the struggle for life. Edition 2. Published by Penguin Classics, 1985. http://books.google.co.in/books?id=Ue67xxvHqOEC&printsec=frontcover&dq=theory+of+natural+selection+proposed+by+darwin#PPA115,M1 Pevsner, Jonathan. Bioinformatics and Functional Genomics Edition 2. Published by John Wiley and Sons, 2009. http://books.google.co.in/books?id=Emr0ZABQUAIC&pg=PA215&dq=molecular+evolution+Charles+Darwin#PPA216,M1 .Prothero, Donald R. and Buell, Carl Dennis. Evolution: what the fossils say and why it matters. Published by Columbia University Press, 2007 http://books.google.co.in/books?id=QeKWpRX77JgC&pg=PA147&dq=Joseph+Hooker+letter+Charles+Darwin+Small+Little+pond&lr= Richards, Robert J. The meaning of evolution: the morphological construction and ideological reconstruction of Darwins theory. Published by University of Chicago Press, 1993. http://books.google.co.in/books?id=-tKjALjpYqMC&pg=PA140&lpg=PA140&dq=Charles+Darwins+classification&source=bl&ots=nsWku8Ow-f&sig=0T4HsHX40STkXy0BJrDLKoI5h7A&hl=en&ei=2DwFSorQE47A6QPplqnWCA&sa=X&oi=book_result&ct=result&resnum=6#PPA124,M1 Journals Ruse, Michael. Charles Darwin’s Theory of Evolution: an analysis. Department of Philosophy, University of Guelph, Guelph, Ontario. Published by Springer, Netherlands. Vol 8, Number 2/ September 1975. pp 219-241. Springer Linked on 6 November 2004. http://www.springerlink.com/content/p182j0w502606r8j/fulltext.pdf Website O’Neil, D. 17 February 2009. Classification of living things. Copyright © 1998-2009. http://anthro.palomar.edu/animal/animal_1.htm Read More