Answer: MICROEVOLUTION
Explanation: Microevolution simply means those changes in the alleles frequencies of an organism over time.
In other words,we will say that microevolution is the generation to generation change in a population's frequencies of alleles.
These changes can occur due to natural or artificial selection,
Gene flow, mutation and genetic drift.
From the question asked,when a population of bacteria sees an increase in the number of bacteria resistant to a certain antibiotic over a number of generations,it is simply known as microevolution.