Apply natural-selection rules to resistance and recognise conditions for speciation.
In a bacterial population, resistance may already exist through genetic variation before an antibiotic is applied. Treatment can kill susceptible bacteria while resistant ones survive and reproduce. The population’s resistant proportion can rise. The antibiotic did not teach bacteria to resist it. Resistance can also spread through gene transfer between bacteria. Similar selection logic applies to pest populations exposed to pesticides.
Speciation involves populations diverging until reproductive isolation develops. A geographical barrier can reduce gene flow, allowing different variants to accumulate under mutation, selection and chance. Separation alone does not instantly create a new species, and not every isolated population eventually becomes one. In the simplified biological-species model, inability to interbreed successfully to produce fertile offspring is a key criterion. Different organism groups may require other species concepts.
After antibiotic exposure, the resistant fraction rises from 2% to 80%. Which explanation fits selection?
Resistant variants survived and contributed more descendants while many susceptible bacteria died. Exposure selected variants; it did not cause purposeful learning.
A river divides one population. Are the two sides instantly different species?
No. The barrier may reduce gene flow, but reproductive isolation and divergence must develop; geography alone is insufficient.
Watch out: Antibiotics impose a selection pressure. Resistant inherited variants can leave more descendants.