Evolution

Evolution
1.	The diversity of living world is enormous and a large number of them have not been identified so far. Not only this, we also know about the fossils, which are the remains of dead organisms, which lived in remote past. How and from where did such a wide variety of organisms come to exist? How did human evolve? The branch of biology that deals with all these problems is called evolution. Evolution is a slow, continuous and irreversible process. Origin of life: J.B.S. Haldane, a British scientist (who became a citizen of India later), suggested that the first life developed from the simple inorganic molecules which existed on primordial earth. The primordial earth had a reducing atmosphere and was rich in CH4, NH3 and H2 and water. Harold C. Urey an astronomer asked his student Stanley L. Miller to perform an experiment to prove Oparin and Haldane's hypothesis. He constructed an apparatus of glass tubes and chambers. In the spark chamber he added CH4 : NH3 : H2 in the ratio of 2 : 1 : 2. After 18 days, significant amount of the simple organic compounds such as amino acids appeared. Amino acids formed were alanine, glycine and aspartic acid.
2.	Evidences of Evolution Homologous organs are those which have the same basic structure and developmental origin but perform different functions. The organisms which possess such organs are said to have originated from common ancestor. Example: The forelimb of a frog, a bird and a human being, show structural similarity; but the function of these fore limbs is entirely different from that of each other. The similarities indicates that all these vertebrates had common ancestor. The wing of insects and birds are structurally different but perform the same function i.e., flying. They are termed as analogous organs.
3.	If we study the embryos of different groups of vertebrates, they show striking similarities. All have gill slits, notochord and tail. This laid Ernst, Haeckel to propose a law, called as biogenetic law. The law, states that "Ontogeny recapitulates phylogeny. Which means that during the embryonic development of any organism, it repeats ancestral history".
4.	Vestigial organs are functionless remnants of the organs which were once functional in ancestors e.g., vermiform appendix, nictitating membrane of human eye. Appendix is remnant of large caecum, functional in reptiles.
5.	Fossils also provide evidence of evolution for example, the fossil Archaeopteryx looks like a bird, but it bears a number of other features, which are found in the reptiles. This indicates birds have evolved from reptiles.
Theories of Evolution
6.	Jean Baptiste Lamarck, gave the theory of inheritance of acquired characters. The theory was put forward in the book Philosophies zoologique. According to this theory use of an organ leads to strengthening and disuse leads to weakening. All these changes or variations are inherited by the offspring's. August Weismann, discarded the theory of inheritance of acquired characters.
7.	Later, Charles Robert Darwin, had put forward the theory of natural selection or Darwinism, in his famous book "The Origin of Species". "Darwin was most influenced by the essay on population" written by Malthus. He made the observations of nature during his voyage on the ship H.M.S Beagle. According to Darwin all the organisms have a high reproductive potential, but there is constancy in number. There is struggle for existence in nature and due to variations there is survival of fittest.
8.	Now the most accepted theory of evolution, is known as synthetic theory of evolution, in which origin of spices is based on the interaction of genetic variation i.e., population genetics and natural selection.
9.	Natural selection is not the only force responsible to bring about changes in gene frequencies. There is the role of chance or genetic drift also e.g., in the environment on bushes beetles are present. They show variations, most of the beetles are green in color, some red and some blue, Crows can not see green colored beetles on the green leaves of the bushes, and therefore can not eat them. This variations becomes common because it had survival advantage and thus was naturally selected. Natural selection is exerted by crows. Suppose an elephant comes and stamps on the bushes where beetles lives. This kills most of the beetles by chance, the few beetles that have survived are mostly blue. Now blue colored beetles become common, through there is no survival advantage. So genetic drift, provides diversity without any adaptations.
10.	Micro-evolution occurs at genetic level due to differences in the alleles of the same gene. They simply change the common characteristics of the species.
11.	Speciation may take place when variation is combined with geographical isolation. Example, if between the two sub-populations of beetles a large river comes into existence the two populations are further isolated. The levels of gene flow between them will decrease. Over generations, genetic drift will accumulate different changes in each sub-population. Together, the process of genetic drift and natural selection will result in these, two isolated sub-populations of beetles to become more and more different and slowly give rise to new spices.
12.	Artificial selection some genetic variability is always present in the population, Some alleles make organisms live and reproduce successfully. In artificial selection the role of nature is played by man, e.g., cabbage, cauliflower, kohlrabi are descendants of a common ancestor colewort, obtained through artificial selection.
13.	Human Evolution: Study of human evolution indicates that all of us belong to a single spices that evolved in Africa and spread across the world. The earliest members of the human species, Homo sapiens, have been traced in Africa. So, all human races have originated from a common Homo sapien ancestor in Africa.