How has biological evolution taken place?

(N/A) Evolution by natural selection in a true sense started when cellular forms of life with differences in metabolic capabilities originated on Earth.
The essence of the Darwinian theory of evolution is natural selection.
The rate of appearance of new forms is linked to the life cycle or life span of the organism.
Microbes that divide rapidly have the ability to multiply and become millions of individuals within hours.
$A$ colony of bacteria (say $A$) growing on a given medium has built-in variation in terms of its ability to utilize a feed component.
$A$ change in the medium composition would favor only that part of the population (say $B$) that can survive under the new conditions.
In due course of time,this variant population outgrows the others and appears as a new species.
This process can happen within days for microbes.
For the same process to occur in fish or fowl,it would take millions of years because the life spans of these animals are measured in years.
Here,we say that the fitness of $B$ is better than that of $A$ under the new conditions.
Nature selects for fitness based on characteristics that are inherited.
Hence,there must be a genetic basis for being selected and for evolution to occur.
Another way of expressing this is that some organisms are better adapted to survive in an otherwise hostile environment.
Adaptive ability is inherited and has a genetic basis.
Fitness is the end result of the ability to adapt and be selected by nature.
Branching descent and natural selection are the two key concepts of the Darwinian Theory of Evolution.