Why do evolutionists think that resistant bacteria is proof of evolution?

Normally, antibiotic resistance does not confer any advantage to the organism; however, when exposed to antibiotics, to which some organisms are naturally resistant, the organisms gain an advantage that distinguishes them from the rest of the bacterial population. Antibiotic resistance may occur naturally as a unique variation in a small number of organisms within the bacterial population.

Differential reproduction allows resistant bacteria to multiply at a higher rate, and their progeny bacteria thrive in the absence of nonresistant bacteria, allowing resistant organisms to survive (survival of the fittest) longer than nonresistant ones.

The bacterial population eventually develops antibiotic resistance due to the rapid increase in the frequency of the genetic allele that confers resistance. A population is considered to be evolving whenever the frequency of an allele changes.

The fact that all adaptations begin as pre-adaptive characters (due to random mutation) and become selected in a suitable environment is another thing that these evolutionary processes help us understand.

Evolutionists consider resistant bacteria as evidence of evolution because it demonstrates the process of natural selection in action. When bacteria are exposed to antibiotics, those with genetic variations that confer resistance to the antibiotics are more likely to survive and reproduce. Over time, this leads to an increase in the proportion of resistant bacteria in the population, illustrating how organisms can adapt to their environment through genetic changes, which is a fundamental principle of evolutionary theory.