The essence of Darwinian theory about evolution is Natural selection.

$(i)$ Rapid multiplication : Microbes divide very fast. They multiply and become millions of individuals within hours.

$(ii)$ Limited food and space : $A$ colony of bacteria (say $A$) growing on a given medium has built in variations in terms of ability to utilize a feed component.

$(iii)$ Struggle for existence : $A$ change in the medium composition would bring out only thay part of the population (say $B$) which can survive under the new conditions.

$(iv)$ Variations: In due course of time this variant population outgrows the others and appears as new species.

$(v)$ Survival of the fittest : Here we can say fitness of $B$ is better than $A$ under new conditions.

$(vi)$ Natural selection : Nature selects for fitness. Fitness is based on characteristics which are inherited.

(vii) Speciation : Adaptive ability is inherited. It has genetic basis. Species get selected and evolved. Branching descent and natural selection are the two key concepts of Darwinian Theory of Evolution.