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NCERT Class 12 Biology Textbook Chapter 7 With Answer PDF Free Download
Chapter 7: Evolution
Evolutionary Biology is the study of the history of life forms on earth. What exactly is evolution?
To understand the changes in flora and fauna that have occurred over millions of years on earth, we must have an understanding of the context of the origin of life, i.e., the evolution of the earth, stars, and indeed of the universe itself.
What follows is the longest of all the construed and conjectured stories.
This is the story of the origin of life and the evolution of life forms or biodiversity on planet earth in the context of the evolution of earth and against the background of the evolution of the universe itself.
7.1 Origin of Life
When we look at stars in a clear night sky we are, in a way, looking back in time. Stellar distances are measured in light-years. What we see today is an object whose emitted light started its journey millions of years back and from trillions of kilometers away and reaching our eyes now.
However, when we see objects in our immediate surroundings we see them instantly and hence in the present time.
Therefore, when we see stars we apparently are peeping into the past. The origin of life is considered a unique event in the history of the universe. The universe is vast. Relatively speaking the earth itself is almost only a speck.
The universe is very old – almost 20 billion years old. Huge clusters of galaxies comprise the universe. Galaxies contain stars and clouds of gas and dust. Considering the size of the universe, the earth is indeed a speck.
The Big Bang theory attempts to explain to us the origin of the 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, the earth was supposed to have been formed about 4.5 billion years back.
There was no atmosphere on early earth. Water vapour, methane, carbon dioxide, and ammonia released from molten mass covered the surface The UV rays from the sun broke up water into Hydrogen and Oxygen and the lighter H2 escaped. Oxygen is combined with ammonia and methane to form water, CO2, and others.
The ozone layer was formed. As it cooled, the water vapor fell as rain, to fill all the depressions and form oceans. Life appeared 500 million years after the formation of the earth, i.e, almost four billion years back
Did life come from outer space? Some scientists believe that it came from outside. Early Greek thinkers thought units of life called spores were transferred to different planets including earth. ‘Panspermia’ is still a favorite 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-sterilized flasks, life did not come from killed yeast while in another flask open to the 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 the earth.
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NCERT Solutions Class 12 Biology Chapter 7 Evolution
1. Explain antibiotic resistance observed in bacteria in light of Darwinian selection theory.
As per Darwin’s observation, the environment selects entities with favorable variations, these entities are capable of surviving.
When a population of bacteria is attacked by a specific antibiotic, sensitive bacteria tend to perish, while other bacteria possessing favorable mutations turn resistant even in the presence of antibiotics and these survive, thriving, and rapidly multiplying as the other competing bacteria have died out.
Thus, the number of bacteria is on the rise. In addition to this, the mass transfer of these genes, resistant to bacteria, to other bacteria. Consequently, bacteria resistant to antibiotics widely spread across making the entire population become antibiotic-resistant.
2. Find out from newspapers and popular science articles any new fossil discoveries or controversies about evolution.
Fossil discoveries of dinosaurs had some interesting revelations. It conveys the evolution of reptiles in the Jurrasic era. This revelation gave rise to the discovery of the evolution of other animals such as mammals and birds.
Two unfamiliar fossils recently unearthed in China exploded a controversy over the evolution of birds. One such genus of primitive birds was Confuciusornis. These were crow-sized and thrived during the Cretaceous era in China.
3. Attempt to give a clear definition of the term species.
A species is a group or population of individuals having the potential to interbreed and produce sustainable and fertile offspring.
5. Find out through the internet and popular science articles whether animals other than man have self-consciousness.
Apart from humans, there are many other animals that possess self-consciousness. One such example is the Dolphin.
They are believed to have a high level of intelligence. Also, they have a sense of self and can identify themselves amongst others.
They whistle, tail-slap, and exhibit body movements to communicate with each other. Some other animals that exhibit self-consciousness are parrots, crows, gorillas, orangutans, chimpanzees, etc.
Evolution NCERT Textboo With Solutions PDF Free Download