Why were some bacteria resistant to the antibiotic, based on the theory of evolution?

The correct answer highlights that some bacteria have developed resistance to antibiotics as a result of genetic mutations that have been inherited through generations. In evolutionary terms, mutations are random changes in the genetic material of an organism, which can sometimes confer advantages in specific environments, such as the presence of antibiotics. When an antibiotic is introduced, it creates a selective pressure on the bacterial population. Bacteria that possess mutations allowing them to survive in the presence of that antibiotic are more likely to survive and reproduce. Over time, their offspring inherit these beneficial genetic traits, leading to a population of bacteria that is resistant to the antibiotic.

This process is a clear demonstration of natural selection, where the bacteria that carry advantageous mutations become more prevalent in the population. It showcases the dynamic nature of evolution, where genetic variations can lead to notable changes in the fitness of organisms within their environments over generations. Other options do not accurately capture this mechanism; for example, rapid replication rates may lead to faster population growth but do not inherently cause antibiotic resistance. Environmental changes can influence the prevalence of resistance, yet they do not explain the origin of that resistance. Selective breeding typically applies to domesticated organisms under human influence, which is not relevant to the evolution of bacteria in response to antibiotics.