User:Mutendere R

Introduction During the 19th century, a man known as Charles Robert Darwin, a graduate at the prestigious Cambridge University came up with a theory that every living thing on earth evolves independently from the guiding hand of the creator. In order for these theories to be proved he needed to do research. Most people believe that evolution challenges traditions and religions. This made Mr. Darwin to keep his research a secret for years until he had facts to prove that his research is base on true ideas but the most contributing factor that the research was kept a secret was that he did not wanted to lose respect from the Cambridge darns that taught him to be a scientists. Charles Darwin knew that he was dealing with was intellectual dynamite, what he was doing, where men came from etc. and kept it a secret. He stayed with this secret and did research alone for more than 20 years. The only support he had was from his wife Emma who was afraid that her husband was destined for eternal termination for challenging traditional beliefs. Together went through illnesses’, self doubt and family tragedy in all this years. Evolution can be defined as change over time of a specific organism in one or more inherited traits in a population. Inherited traits are distinguished characteristic for example; Anatomical, biochemical or behavioural that are passed on from generation to the next occurs when there is a variation of inherited traits within a population over time. The major sources of such inherited variants are mutation, genetic recombination and gene flow (Adds. J, et.al 2001). Evolution has led to the diversification of all living organisms from a common ancestor, (Tree of life) which is described by Charles Darwin as "endless forms most beautiful and most wonderful". His researched showed that evolution has four common mechanisms which will be explained later in this report. Evolution may in the long term lead to species splitting in different components, whereby a single ancestral species splits into two or more different species. This splitting is visible in anatomical, genetic and other similarities between groups of organism’s geographical distribution of related species, the fossil record and the recorded genetic changes in living organisms over many generations. Common decent stretches back over 3.5 billion years during which life has existed on earth. Both evolution within populations and speciation between them are thought to occur in multiple ways such as slowly, steadily and gradually over time or rapidly from one long static state to another. (Dyson. G.B, 1997)

Discussion The scientific study of evolution began in the mid-nineteenth century, when research into the fossil record and the diversity of living organisms convinced most scientists that species evolve. The mechanisms driving these changes remained unclear until the theory of natural selection was independently proposed by Charles Darwin in 1858. In the early 20th century, Darwinian theories of evolution were combined with genetics and systemetics.

In 1842 this sketch was penned by Charles Darwin of what became the origin of species that is shown in the video.

The critical break of biology from the concept of fixed species is generally seen as the theory of evolution of Charles Darwin. , Darwin noted that population growth would lead to a struggle for existence where favourable conditions could prevail as other conditions are damaged. Each generation has many offspring’s that fail to survive to an age of reproduction because of limited resources. This explains the diversity of animals and plants from a common ancestry through the working of natural laws working the same for all types of thing. Darwin developed his theory of Natural Selection from 1838 onwards until Alfred Russel Wallace sent him a similar theory in 1858. Both men presented their separate papers to the Linnean Society of London. At the end of 1859, Darwin's publication of On the Origin of Species explained natural selection in detail and in a way that lead to an increasingly wide acceptance of Darwinian evolution. Thomas Henry Huxley applied Darwin's ideas to humans, using a research known as palaeontology and comparative anatomy to provide strong evidence that humans and apes shared a common ancestry. Some people were disturbed by this since it implied that humans did not have a special place in the universe. (Wicander. R. and Monroe. J.S. 2004).

The four common mechanisms of evolution mentioned earlier in the report are: The first mechanism is natural selection, a process in which there is differential survival and reproduction of organisms that differ in one or more inherited traits. Selection can act at multiple levels of organization, for example differential survival and/or reproduction of organisms, populations, or gene differences. A second mechanism is genetic drift, a process in which there are random changes to the proportions of two or more inherited traits within a population. A third mechanism is biased mutation, which can affect phenotypes expressed across multiple levels of organisation. Finally, the fourth mechanism is gene flow, which is the incorporation of genes from one population into another (Dyson. G.B, 1997).

Heredity Precise mechanisms of reproductive heritability and the beginning of new traits remained a mystery. Darwin developed his provisional theory of pangenesis. In 1865 Gregory Mendel wrote that traits were inherited in a predictable manner through the independent assortment and segregation of elements (later known as genes). Mendel's laws of inheritance eventually supplanted most of Darwin's pangenesis theory. August Weismann made the important distinction between germ cells (sperm and eggs) and somatic cells of the body, demonstrating that heredity passes through the germ line only. Hugo de Vries connected Darwin's pangenesis theory to Wiesman's germ/soma cell distinction and proposed that Darwin's pangenesis were concentrated in the cell nucleus and when expressed they could move into the cytoplasm to change the cells structure. De Vries was also one of the researchers who made Mendel's work well-known, believing that Mendelian traits corresponded to the transfer of heritable variations along the geneline. To explain how new genes originate; De Vries developed a mutation theory that led to a temporary rift between those who accepted Darwinian evolution and biometricians who allied with de Vries. At the turn of the 20th century, pioneers in the field of population genetics, such as J.B.S. Haldane, Sewall Wright, and Ronald Fisher, set the foundations of evolution onto a robust statistical philosophy. The false contradiction between Darwin's theory, genetic mutations, and Mendelian inheritance was thus reconciled (Dyson. G.B, 1997).

DNA structure. Bases are in the centre, surrounded by phosphate–sugar chains in a double helix (Introduction to genetics). Evolution in organisms occurs through changes in heritable traits – particular characteristics of an organism. In humans, for example, eye colour is an inherited characteristic and an individual might inherit the "brown-eye trait" from one of their parents. Inherited traits are controlled by genes and the complete set of genes within an organism's genome is called its genotype (Adds. J. 2001)

Mechanisms of evolution Natural selection will only cause evolution if there is enough genetic variation in a population. Before the discovery of Mendelian genetics, one common problem was blending inheritance. But with blending inheritance, genetic differences would be rapidly lost, making evolution by natural selection impossible. The Hardy-Weinberg principle provides the solution to how variation is maintained in a population with Mendelian inheritance. According to this principle, the frequencies of alleles (variations in a gene) in a sufficiently large population will remain constant if the only forces acting on that population are the random reshuffling of alleles during the formation of the sperm or egg and the random combination of the alleles in these sex cells during fertilisation (Dyson. B.G, 1997). Variation comes from mutations in genetic material, reshuffling of genes through sexual reproduction and migration between populations (gene flow). Variation also comes from exchanges of genes between different species; for example, through horizontal gene transfer in bacteria and hybridisation in plants. Despite the constant introduction of variation through these processes, most of the genome of a species is identical in all individuals of that species. However, even relatively small differences in genotype can lead to dramatic differences in phenotype: for example, chimpanzees and humans differ in only about 5% of their genomes stated Dyson. B.G (1997). Gene flow is the exchange of genes between populations and between species. It can therefore be a source of variation that is new to a population or to a species as explained before in the report. Gene flow can be caused by the movement of individuals between separate populations of organisms, as might be caused by the movement of mice between inland and coastal populations.

Natural Selection Evolution by means of natural selection is the process by which genetic mutations that enhance reproduction become and remain, more common in successive generations of a population. • Heritable variation exists within populations of organisms. • Organisms produce more offspring than can survive. • These offspring differ in their ability to survive and reproduce. These conditions produce competition between organisms for survival and reproduction. Organisms with traits that give them an advantage over their competitors pass these advantageous traits on, while traits that do not have an advantage are not passed on to the next generation. The concept of natural selection, weather the organism has evolutionary fitness. Fitness is measured by an organism's ability to survive and reproduce, which determines the size of its genetic contribution to the next generation. Fitness is not the same as the total number of offspring: instead fitness is indicated by the proportion of subsequent generations that carry an organism's genes (Dyson. G.B, 1997). For example, if an organism could survive well and reproduce rapidly, but its offspring were all too small and weak to survive, this organism would make little genetic contribution to future generations and would have low fitness.

Natural selection within a population for a trait that can vary across a range of values, such as height, can be categorised into three different types. The first is directional selection, which is a shift in the average value of a trait over time — for example, organisms slowly getting taller. Secondly, disruptive selection is selection for extreme trait values and often results in two different values becoming most common, with selection against the average value. This would be when either short or tall organisms had an advantage, but not those of medium height. Finally, in stabilizing selection there is selection against extreme trait values on both ends, which causes a decrease in variance around the average value and less diversity. This would, for example, cause organisms to slowly become all the same height (Dyson. G.B, 1997). Natural selection most generally makes nature the measure against which individuals and individual traits are more or less likely to survive. "Nature" in this sense refers to an ecosystem, that is, a system in which organisms interact with every other element, physical as well as biological, in their local environment. Some define an ecosystem as: "Any unit that includes all of the organisms...in a given area interacting with the physical environment so that a flow of energy leads to clearly defined trophic structure, biotic diversity and material cycles (i.e.: exchange of materials between living and nonliving parts) within the system." Each population within an ecosystem occupies a distinct niche, or position, with distinct relationships to other parts of the system. These relationships involve the life history of the organism, its position in the food chain and its geographic range. This broad understanding of nature enables scientists to delineate specific forces which, together, comprise natural selection (Chapman and Rheiss, 1992). Conclusion Evolution influences every aspect of the form and behaviour of organisms. Most prominent are the specific behavioural and physical adaptations that are the outcome of natural selection. These adaptations increase fitness by aiding activities such as finding food, avoiding predators or attracting mates. Organisms can also respond to selection by co-operating with each other, usually by aiding their relatives or engaging in mutually beneficial symbiosis. In the longer term, evolution produces new species through splitting ancestral populations of organisms into new groups that cannot or will not interbreed. These outcomes of evolution are sometimes divided into macroevolution, which is evolution that occurs at or above the level of species, such as extinction and speciation and microevolution, which is smaller evolutionary changes, such as adaptations, within a species or population. In general, macroevolution is regarded as the outcome of long periods of microevolution. Thus, the distinction between micro- and macroevolution is not a clear one – the difference is simply the time involved. However, in macroevolution, the traits of the entire species may be important. For instance, a large amount of variation among individuals allows a species to rapidly adapt to new habitats, lessening the chance of it going extinct, while a wide geographic range increases the chance of splitting up, by making it more likely that part of the population will become isolated. In this sense, microevolution and macroevolution might involve selection at different levels – with microevolution acting on genes and organisms, versus macro evolutionary processes such as species selection acting on entire species and affecting their rates of speciation and extinction (Wicander. R, et.al. 2007).

A common misconception that people have is whether evolution has goals or long-term plans; realistically however, evolution has no long-term goal and does not necessarily produce greater complexity. Although complex species have evolved, they occur as a side effect of the overall number of organisms increasing and simple forms of life still remain more common in the biosphere. For example, the overwhelming majority of species are microscopic prokaryotes, which form about half the world's biomass despite their small size, and constitute the vast majority of Earth's biodiversity. Simple organisms have therefore been the dominant form of life on Earth throughout its history and continue to be the main form of life up to the present day, with complex life only appearing more diverse because it is more noticeable. Indeed, the evolution of microorganisms is particularly important to modern evolutionary research, since their rapid reproduction allows the study of experimental evolution and the observation of evolution and adaptation in real time. As this study of evolution still continues, the debate continues whether evolution is true or whether it is challenging beliefs and religion of those that don’t believe in it.

References

Adds. J, Larkcom. E and Miller. R (2001). Genetics, Evolution and Biodiversity. Neelson Thornes Ltd. United Kingdom Chapman. J.L and Rheiss. M.J. (1992).Ecology Principles and Applications.Cambridge University Press. New York. Dyson. G.B.(1997). Darwin among the Machines. Library of Congress Cataloging-in-Publication Data.United States of America. Wicander. R and Monroe J.S (2007). Historical Geology. Thomson Higher Education. Belmont.