Antibiotic resistance is the loss of susceptibility in bacteria to drugs like penicillin and erythromycin. The reason this is interesting from an evolutionary perspective is that it demonstrates evolution occuring in real time—that is, within a period easily observable by humans. The variation in response to antibiotics within a population of bacteria is similar to that described by Darwin and Wallace. When some bacterial cells survive after an incomplete course of antibiotic treatment, they form the basis for a new drug-resistant strain. What is troubling, however, is that different types of bacteria can actually share the genes that make them drug resistant so that this ability is becoming more prevalent among different strains of disease-causing organisms.