Question

Appearance of antibiotic-resistant bacteria is an example of
A. Adaptive radiation
B. Transduction
C. Pre-existing variation in population
Divergent evolution

Answer 1

Antibiotic resistance is the mechanism of loss of susceptibility of bacteria to the killing or growth-inhibiting (bacteriostatic) properties of an antibiotic agent.

Complete answer:
Antibiotic resistance is the ability of a microorganism to stop the effects of an antibiotic. Genetic mechanisms develop resistance to antibiotics since they result in biochemical modifications that alter certain bacterial cell properties that normally render the cell sensitive to an antibiotic. Like inactivates the drugs, alteration of protein, enzyme etc.
There are two important types of genetic mechanisms that lead to antibiotic resistance: mutation and acquisition of new genetic material. Bacteria use mutation in their cell body because Bacteria reproduce rapidly, sometimes in as little as 20 minutes.
When a bacterial population encounters a particular antibiotic, the particular bacteria are sensitive to it and die. But some bacteria get mutated and develop resistance against antibiotics. Soon, the resistance providing genes become activated and the entire population becomes resistant. It is not due to adaptive radiation because adaptive radiation is development of different functional structures from a common ancestral form i.e. divergent evolution.
Antibiotics were the wonder discoveries of the 20th century. The first antibiotic was discovered in 1928, by Alexander Fleming who noticed that a mold inhibited the growth of staphylococcal bacteria and named the substance it produced "penicillin". The antibiotic resistance was recognized in the early 1940s. Overuse and repeated exposure to antibiotic agents favored the selection and replication of numerous strains of antibiotic-resistant bacteria

Therefore, Appearance of antibodies resistant bacteria is an example of pre-existing variation.

Note: Antibiotics are used to treat or prevent different types of bacterial infection. It kills bacteria or prevents them from reproducing and spreading. Antibiotics are not effective against viral infections, like the common cold, flu, most coughs and sore throats. Immune system also helps in killing very mild bacterial infections without using antibiotics. It's important that antibiotics should be prescribed and taken correctly to help prevent the progression of antibiotic resistance. Otherwise the strain of bacteria will no longer respond to treatment with one or more types of antibiotics.