NCERT Solutions Class 12 Biology Chapter 7 PDF
Evolution is defined as the process by which the organisms changes over time as a result of changes in genetic, physical appearance and behavioral traits over several generations and it mainly depend on the process of natural selection mutation, and genetic drift. The study of evolution is used for understanding about the different living species, including plants, animals, reptiles and birds, how the species became extinct, about their natural habitats and how are new species produced.Charles Darwin, an English biologist, was the first person to propose the theory of evolution in the year 1859. According to this theory, there are more than 2 million existing species of organisms which have been identified, named and described. Apart from these, there are other 10 to 30 million species which are yet to be discovered.
The Big Bang theory attempts to explain to us the origin of universe. It talks of a singular huge explosion unimaginable in physical terms. The universe expanded and hence, the temperature came down. Hydrogen and Helium formed sometime later.The gases condensed under gravitation and formed the galaxies of the present day universe. In the solar system of the milky way galaxy, earth was supposed to have been formed about 4.5 billion years back. There was no atmosphere on early earth. Water vapour, methane, carbondioxide and ammonia released from molten mass covered the surface.The UV rays from the sun brokeup water into Hydrogen and Oxygen and the lighter escaped.Early Greek thinkers thought units of life called spores were transferred to different planets including earth.Panspermia is still a favourite idea for some astronomers. For a long time it was also believed that life came out of decaying and rotting matter like straw, mud, etc. This was the theory of spontaneous generation. Louis Pasteur by careful experimentation demonstrated that life comes only from pre-existing life.He showed that in pre-sterilised flasks, life did not come from killed yeast while in another flask open to air, new living organisms arose from killed yeast.Spontaneous generation theory was dismissed once and for all.However,this did not answer how the first life form came on earth.
|Section Number||Topics Covered Name|
|7||Origin of Life|
|7.1||Evolution of Life Forms – A Theory|
|7.2||What are the Evidences for Evolution?|
|7.3||What is Adaptive Radiation?|
|7.5||Mechanism of Evolution|
|7.6||Hardy – Weinberg Principle|
|7.7||A Brief Account of Evolution|
|7.8||Origin and Evolution of Man|
Conventional religious literature tells us about the theory of special creation. This theory has three connotations. One, that all living organisms (species or types) that we see today were created as such. Two, that the diversity was always the same since creation and will be the same in future also. Three, that earth is about 4000 years old. All these ideas were strongly challenged during the nineteenth century. Based on observations made during a sea voyage in a sail ship called H.M.S. Beagle round the world, Charles Darwin concluded that existing living forms share similarities to varying degrees not only among themselves but also with life forms that existed millions of years ago. Many such life forms do not exist any more.Let us also remember that Alfred Wallace, a naturalist who worked in Malay Archipelago had also come to similar conclusions around the same time. In due course of time, apparently new types of organisms are recognisable. All the existing life forms share similarities and share common ancestors. However, these ancestors were present at different periods in the history of earth (epochs, periods and eras).The geological history of earth closely correlates with the biological history of earth. For more information on NCERT Solutions for Class 12 Biology, Chapter 7 Evolution, check the solutions given below.
NCERT Solutions for Class 12 Biology Evolution
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