Phenomenon of Evolution - Research Paper Example

?Evolution Evolution can be defined as the modification of living organisms through time as they descend from their ancestors, generation after generation. The ancestors of various species look different in their structure, behavior and physiology back in time. Prior to Darwin, though some naturalists had speculated about modification of species, they failed to explain why and how species change. They also believed that evolution began with special creation of only a fixed number of species. Partially influenced by Thomas Malthus' essay on the Principle of Population and stimulated by a letter from Alfred Russell Wallace, Charles Darwin in 1859 discussed the evolution of species through natural selection in his famous work titled On the Origin of Species, which entirely revolutionized the earlier concepts of evolutionary biology. Natural selection refers to a process in which species compete and struggle for their survival according to the limited resources and conditions of their natural environment with different adaptive abilities. As individuals in a population are not the same due to difference in inherited characteristics, nature only selects those individuals that are best suited to the environmental conditions, and thus rest of the population dies over time (Starr et al., 2012, p. 263, 264, 265). As all the offspring in a population acquire characteristics from their ancestors, produced more than nature can support and have different reproductive characteristics, only those organisms will survive that are better adapted to the living conditions. This means that organisms with higher reproduction ability will survive due to higher probability of their descendants to survive, and others will eventually become extinct due to less survival rate of their offspring with the passage of time. Since environmental conditions are different from place to place, there will be variation in characteristics of species at different locations. Darwin concluded that populations extending over large areas or through migration might have been isolated resulting in variation of their characteristics according to varying environments. Over long periods of time, they may have diverged or evolved into different species. For instance, Darwin found that finches on the Galapagos Islands were more similar to each other than to finches of the mainland. He also noticed that some varieties were only existent on the archipelago islands. So, he proposed that all species might have descended from a common ancestor and increase in number of species occurred through evolutionary natural selection over time rather than special creation (Strickberger, 2005, p. 22, 23). Darwin and other naturalists believed that variations among individuals of a species were due to mixing of traits from both the male and female. He was not aware of the heredity mechanism and different traits were regarded to be the result of blending of characteristics through generations over time. However, the concept of blending inheritance failed to describe the survival of variety as they descended through generations with time. It also failed to describe the maintenance of specific characteristics in varieties and how new species would emerge through blending. It was 1866, when Gregor Mendel published his experimental findings on garden peas. To experiment with pure seeds, he selected a self pollinating plant. He experimented with garden peas that were different from each other in many characteristics such as their flowers were either red or white, had green or yellow seeds, and tall or dwarf. After cross-breeding generations having different characteristics, he observed that descendants from each cross possessed characteristics of only one of the parents and blending did not happen. Mendel concluded that instead of blending of certain fluids, heredity from parents was passed on to offspring through independent discrete units, particles or factoren, which were later termed as genes. The characteristic that appeared in a descendant after cross breeding was termed as the dominant trait, while the suppressed characteristic that receded in descendant was termed as the recessive trait. This described a vital evolutionary question that characteristics in a variety do not totally disappear but could reappear in next generations. The descendant generations inherited one factor from each parent for expressing a single characteristic (such as color), and the three possible outcomes (variants) for factors were dependent on the dominant and recessive traits contained by those factors (Starr et al., 2012, p. 170, 171). The discovery of DNA proved Mendel’s observation that inheritance is passed by discrete units or factoren (genes) and nullified the Darwinian concept of blending. The double helical structure of DNA showed the presence of heredity information in form of sequence of nucleotides on DNA strands and provided with further evidence that how inheritance is passed on to generations. The genetic changes can affect the growth potential of cells in accordance with exposure to particular environmental conditions, and these changes are selected for or against those conditions increasing variety among species (Starr et al., 2012, p. 178). A wide variety of fields including plate tectonics, biogeography, stratigraphy, fossils and molecular biology present with various evidences to explain the change in species through time or evolution. In the 1800s, during the age of canals, William Smith recorded the mineral composition and fossils present in various layers of the rock strata and later on his nephew Phillips regarded lower layers as the oldest and upper ones as youngest in a sequence. Later on, James Hutton concluded that only those varieties best adapted to the environment maintained their existence and continued to perfect themselves according to natural conditions with time. Plate tectonic evidence today suggests that all the continents once existed in united form and gradually moved apart by continental drift, and younger water bodies or oceans are shallower than older ones. This explains the distribution of the same species over different continents. The evidence for existence of a single continent has been also supported by biogeography. For instance, the fossils of extinct species such as Mesosaurus, Lystrosaurus and Cynognathus have been discovered in water separated lands of India, America, Africa and Australia. In comparative anatomy, it has been found that ancestors of the jawed fish had six gill slits and over six brachial arteries. However, evolution in jawed fish tends to be toward less but more efficient gills (Strickberger, 2005; Starr et al., 2012). Hardy-Weinberg population genetics also provide evidence in support of the evolutionary theory through natural selection. Hardy Weinberg explored that a population can maintain equilibrium only if the gene mutations do not occur, they do not migrate, are very large, have random and equal participation in mating, there is absence of natural selection and has same number of descendants. If even one or few of these principles are violated, populations will evolve i.e. gene frequencies are altered (Starr et al., 2012, p. 281). It is evident that the fore mentioned principles are mostly in a continuous violation supporting evolutionary concepts of Darwin and Mendel. Thus, a population of a single species may divide and become isolated on different islands. As all islands would have different environmental conditions and resources, the isolated population will acquire different characteristics to adapt to that particular environment. The gradual changes or evolution in species through generations over long periods of time may result in divergence of one species into two or more distinct species. This suggests that all organisms have evolved from a single ancestor through changes expanding over million of years. References Starr, C., Taggart, R., Evers, C. & Starr, L. (2012). Biology: The unity and diversity of life. 12th ed. Belmont, CA: Brooks/Cole. Strickberger, M. W. (2005). Evolution. 3rd ed. London: Jones and Bartlett. Read More