When first used in medicine in the 1940s, penicillin was uniformly effective in killing the bacterium Staphylococcus aureus. Today, has led to an increase in antibiotic-resistant alleles, and humans are increasingly at risk from untreatable Staphylococcus aureus infections.

When first used in medicine in the 1940s, penicillin was uniformly effective in killing the bacterium Staphylococcus aureus. Today, the Darwinian selection has led to an increase in antibiotic-resistant alleles, and humans are increasingly at risk from untreatable Staphylococcus aureus infections.

Explanation:

Those bacterias which were affected by the penicillin were filtered off from the environment since the offspring of those susceptible bacterias were decrease by time until total extinction.

But by the natural mutation of bacterias, some bacterias can originate a offspring of bacterias which were capable of resist the action of an antibiotic. Those genes could also be carried out by a plasmid conjugation creating new colonies which are resistant to the penicillin.