Assertion (A) : The rate of appearance of new forms is linked to the life cycle or the life span.
Reason (R) : In microbes variant population would appear as new species within days, where as fish or fowl would take million of years as life span of these animals are in years.

The rate of appearance of new forms is linked to the life cycle or the life span.
For example, the microbes have shorter life spans and divide faster and become millions of individuals within hours, therefore microbes have more chances to evolve faster.  where as fish or fowl would take million of years to evolve,  as life span of these animals are much longer than microbes.