Explain Darwin's views on natural selection regarding biological evolution.

(A) In the true sense,natural selection for evolution on Earth began when cellular forms of life with different metabolic capabilities originated.
The core essence of Darwinian evolution is natural selection. The rate of appearance of new forms is linked to the life cycle or life span. Microbes that divide rapidly have high multiplication ability and reach millions in numbers within hours.
$A$ colony of bacteria (say $A$) growing in a given medium has variations in terms of their ability to utilize feed components. If the composition of the medium is changed,only that part of the population (say $B$) will survive that can withstand the new changed conditions. Over a period of time,this variant population will outgrow the others and appear as a new species.
This happens within a few days. However,when the same is applied to fish or fowl,it takes millions of years because their life span is in years. Here,we can say that the fitness of $B$ is better than $A$ under the new conditions. Nature selects for fitness. It is important to remember that the so-called fitness is based on characteristics which are heritable.
Therefore,there must be a genetic basis for selection and evolution. In other words,some organisms are better adapted to survive in an otherwise changing environment. Adaptability is inherited. It has a genetic basis. Fitness is the end result of the ability to adapt and get selected by nature.
Branching descent and natural selection are the two key concepts of Darwinian theory of evolution.