Define 'evolution'. Describe Darwin's theory of evolution.

Evolution is the process of gradual change in the heritable characters generation by generation. These variations are occurring due to change in the environmental conditions and also these variations help the organism to survive in this challenging environment.

Darwin's theory - This theory is also known as the theory of natural selection. According to this theory, the population has variations only those organism will able to survive in the environment, which will perfectly fit in the environmental situations. That's why this theory is also known as the survival of fittest.