Class Xii Biology Unit

CLASS XII BIOLOGY UNIT – VII: GENETICS AND EVOLUTION CHAPTER 7 Evolution : Theories and Evidences Syllabus Origin of Life; Evolution of Life Forms-A Theory; What are the Evidences for Evolution ? What is Adaptive Radiation ? Biological Evolution, Mechanism of Evolution; Hardy-Weinberg Principle; A brief account of Evolution; Origin and Evolution of Man Introduction A number of changes in flora and fauna have occurred over millions of years on earth. The study of the history of life form on earth is Evolutionary Biology. This chapter deals with origin of life and evolution of life forms of biodiversity on the planet earth, evidences of evolution, mechanism of evolution, Hardy Weinberg principle and human evolution. The theory of evolution maintains that the different kinds of organisms that we see today have evolved from common ancestors over millions of years. This theory is one of the most important concepts in biology. The distinguished scientist Theodosius Dobzhansky has said : “Nothing in biology makes sense except in the light of evolution”. For more than a century, the theory of evolution has exerted a very strong influence on our thinking about biology, also on developments in other disciplines such as sociology, politics, economics and religion. Life originated on the earth between 3000 and 4000 millon years ago, in the form the unicellular organisms. How did these simple cells lead to (or evolve into) organisms as large as a whale or a Sequoia (redwood) tree, and structures as complex and delicate as the eye and the brain? The “Theory of Evolution by Natural Selection”, was put forward by Charles Darwin and Alfred Russel Wallace towards the middle of the nineteenth century. It has provided us with a scientific framework for understanding the evolutionary changes that have occurred, and continue to take place in the biological world. 1. Basic unit of evolution is population. 2. Evolution is a slow, continuous and irreversible process of changes. 3. Earth is about 4.6 billion years old, and the oldest rocks that have persisted in recognizable form are about 3.8 billon years old. For many years, scientists believed that such ancient rocks did not contain any fossils, but they knew that fossils were simply too small to be seen clearly without an electron microscope. 4. The oldest microfossils discovered so far are that appeared 3.3 to 3.5 billion years ago. 5. Massive limestone deposits called Stromatolites became frequent in the fossil record about 2.8 billion years ago. Produced by cyan bacteria, stromatolites were abundant in virtually all freshwater and marine communities until about 1.6 billion years ago. 6. The fossil records indicates that unicellular protists – the first eukaryotes - appeared about 1.5 billion years ago. 7. Origin of Earth : The Big Bang Theory proposes that the universe had an explosive beginning. The universe originated about 20 billion years ago by a big bang (thermonuclear explosion) of a dense entity. About 4.5 billion years ago, our solar system was probably created when the gaseous cloud called Solar Nebula started to collapse under 1 the force of its own gravity, until it became a flattened spinning disc of atoms and particles. Its central region heated up and became a star. According to recent literature the first non-cellular forms of life could have originated3 billion years back. They would have been giant molecules (RNA, protein, polysaccharides etc.) These capsules reproduced their molecules perhaps. The first cellular form of life did not possibly originate till about 2000 million years ago. THERIES ON THE ORIGIN OF LIFE 1. Theory of special creation states that life was created by supernatural power in the form which has not undergone any change. It was given by Father Saurez. God has created life in six days from meteria prima and man was created by him on sixth day. According to this theory earth is about 4000 years old. 2. Theory of catastrophism was given by Cuvier, according to which after a gap of certain period (called age), the world undergoes a catastrophe (sudden calamity) to kill almost all the living organisms and then God created new generation or new life from inorganic matter. 3. Theory of biogenesis (i.e. life from life, Omnis vivum ex. vivo) was proved by Redi, Spallanzani and Pasteur independently. They disproved (refuted) theory of spontaneous generation (abiogenesis). Francesco Redi (1668) proved that files could not arisa from putrefying meat without their eggs. Spallanzani (1767) demonstrated that putrefaction of meat is due to microbes in the air it can be prevented by boiling and sealing the meat in air tight containers. Pasteur gave a definite proof of life arising from pre-existing life using microbes and sterilization methods. 4. Cosmozoic theory (Theory of panspermia) given by Richter (1865) , Helmholtz (1884), Arrhenius (1908) suggested that life reached the earth from some heavenly body through meteorites. Panspermia (primitive form of life, as suggested by Arrhenius, 1908) consisted of spores or seeds (sperms) and microbes that existed throughout universe and produced different forms of life on this earth. Abiogenic or Chemical Origin of Life Majority of the scientists are of the opinion that life originated from inanimate matter. Since the theory of abiogenic origin or chemical evolution of life is the only one that provides an explanation, which can be tested, most scientists have tentatively accepted it. Oparin-Haldane Hypothesis 1. Alexander 1. Oparin (1894-1980), a Russion biochemist , and J.B.S.H aldane (1892-1964), a British scientist put forward the concept that first organism evolved from non-living material. They also suggested that the sequence of events that might have occurred. In 1923, Oparin postulated that life originated on Earth at some point of time in the remote past, and under the conditions no longer observed. In his book, The Origin of Life (1938), Oparin submitted abiogenesis first, but biogenesis ever since. Oparin’s theory is know as primary abiogenesis. 2. According Oparin and Haldane (1929), spontaneous generation of early molecules might have taken place if the earth once had more reducing atmosphere compared to the present oxidising atmosphere . Oparin and Haldane agreed that the primeval Earth contained little, if at all, oxygen. Perhaps, in the primitive atmosphere oxygen in the free gaseous states was virtually absent. Therefore, no degradation of any organic compound arising in the primeval Earth could have taken place. 2 3. As there was no azone layer in the atmosphere any absorption of UV radiation, that is lethal to our present lives was possible in the primordial Earth. 4. The early gas cloud was rich in hydrogen ,being present in the form of methane (CH4), ammonia (NH3) and water (H2O). 5. Moreover, that atmospheric water vapour condensed into drops of water as rain that rolled down the rock surfaces and accumulated to form liquid pools and oceans. In the process, erosion of rocks and washing of mineral (e.g., chlorides and phosphates) into the oceans were inevitable. Thus, Halane’s hot dilute soup was produced and the stage was set for combination of various chemical elements. 6. Atmospheric chemicals and those in water produced small precursor molecules,like amino acids, sugars, nitrogenous bases etc. These precursor molecules then combined resulting in the appearance of proteins. 7. The energy sources for such reactions of organic synthesis were the UV radiation (solar radiation), cosmic rays, electrical discharges (lighting), intense dry heat (volcanic eruption) and radioactive decay of various elements on the Earth’s surface. Once formed, the organic molecules accumulated in water because their degradation was extremely slow in the absence of any life or enzyme catalysts. Such transformation is not possible in the present oxidizing atmosphere because oxygen of micro organisms will decompose or destroy the living particle that may arise by mere chance. Experimental Evidence for Abiogenic Molecular Evolution of Life Haroid C.Urey (1893-1981), an astronomer, accorded the first adequate recognition of Oparin-Haldane’s vview on the oringin of life in 1952,. Urey asked his student Stanely L. Miller, a biochemist, to replicate the primordial atmosphere as propounded by Oparin and Haldane. Miller (1953) made the first successful simulation experiment to assess the validity of the claim for origin of organic molecules in the primeval Earth condition. 1. Miller sealed in a spark chamber a mixture of water (H2O), methane (CH4), ammonia (NH3) and hydrogen gas (H2). He made arrangement for insertion of two electrodes to provide electrical energy (simulation of lightning) to the spark chamber. CH4, NH3, H2 were in 2: 1: 2 ratio and water vapour at 800ͦ C. Electric spark of 75,000 volt was provided to the mixture. 2. The spark chamber was connected to another flask with arrangement for boiling water (provision for evaporation). The other end of the spark chamber was connected to a trap by a tube that passed through a condenser (an arrangement for condensation and collection of aqueous solution, equivalent to rain and Haldane’s soup). The trap, in turn, was connected with the flask for boiling water (arrangement for circulation). 3. The control apparatus contained every arrangement except that it was devoid of energy source. 4. After eighteen days, significant amount of the simple organic compounds (monomers), such as amino acids and peptide chains, began to appear in the aqueous sample of the experimental set. Amino acids found were alanine, glycine and aspartic acid. Therefore, the obvious inference was that biotic synthesis of organic monomers occurred in the simulated experimental condition. By analogy, such synthesis could have occurred in the primitive atmospheric condition. Later on many scientists repeated Miller’s experiment using slightly different starting materials and UV radiation of other energy sources.

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