When first used in medicine in the 1940s, penicillin was uniformly effective in killing the bacterium Staphyloccocus aureus. Today, ____________________ has led to an increase in antibiotic-resistant alleles, and humans are increasingly at risk from untreatable Staphyloccocus aureus infections.
a) uncleanliness in hospitals
b) natural selection in Staphyloccocus aureus populations
c) natural selection in human populations
d) the substitution of methicillin for penicillin
e) molecular genetics
Ans: b) natural selection in Staphyloccocus aureus populations
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