Natural selection is a driving force for evolution-How ?

Explanation

a) Anterior and Posterior ends of the body are provided with suckers by which the animal attaches itself to the body of the host.
(b) The three Jaws, inside the mouth, auses a painless Y-shaped wound in the skin of the host.
(c) The salivary glands produce Hirudin, which does not allow the blood to coagulate. So,the continuous supply of blood is maintained.
(ii) Darwin published his observations under the name "origin of species". It elaborates on the theory of natural selection for evolutionary transformation. The principles of Darwinism tells that natural selection is a driving force for evolution
1. Overproduction:
Living beings have the ability to reproduce and have the capacity to multiply in a geometrical manner.
2. Struggle for existence:
Due to overproduction a geometric ratio of increase in population occurs. The space to live and food available for the organisms remain the same. This creates a competition among the organisms, for food and space, leading to struggle.
(a) The competition may be among the individuals of the same species (Intraspecific struggle)
(b) Competition between the organisms of different species living together (Interspecific struggle).
(c) Natural conditions like extreme heat or cold drought and floods can affect the existence of organisms (Environmental struggle).
3. Variations:
Small variations are important for evolution. According to Darwin, favourable variations are useful to the organisms and unfavourable variations are harmful or useless to the organisms.
4. Survival of the fittest or Natural selection:
During the struggle for existence, the organisms which can overcome the challenging situation,survive and adapt to the surrounding environment. Organisms, which are unable to face the challenges, are unfit to survive and disappear. The process of selection of organisms with favourable variation is called as Natural selection.
5. Origin of species:
According to Darwin, new species originates by the gradual accumulation of favourable variations for a number of generations.