Branching descent and Natural selection are the two key concepts in the evolutionary theory of Darwin.
Organisms have very high or biotic or reproductive potential . It produces a very large number of progeny.
Ecological niches , food and space are limited.
Due to large in number of individuals and limitations of food and space, a struggle for existence ensures.
Due to reproductive shuffling, a large number of random variations occur in each individual. Some variations have selective advantages and in particular environments, they survive and are able to reproduce.
Since there are several types of ecological niches, a number of variants would survive with time; variations providing adaptability to particular environments will accumulate and give rise to different species- branching descent.