According to Darwin’s theory of evolution:

1. There is natural variation within any population, and some individuals have more favourable variations than others.

2. The population remains fairly constant even though all species produce a large number of off springs.

3. This is due to ‘competition’ or struggles for existence between same and different species.

Organisms produce more offspring than the available food and space, so there is a struggle for existence this ensures that the best among them survive.

Natural selection states that there is ‘struggle for existence’ within a population and variation exists in a population. The continuous competition between individuals for environmental resources creates ‘struggle for existence’, and this struggle makes sure that certain organisms would survive or reproduce leaving back the others that are not a better fit in the environmental conditions. Hence the less suited organisms are eliminated, and better-adapted organisms survive and pass their traits to the next generation leading to variation.

Struggle for existence preserves advantageous traits and weed out disadvantageous traits.