Evolution in Action p. 58
This example illustrates evolution in action in modern times by demonstrating that the
Staphylococcus bacteria population as a whole has become more resistant to penicillin over the last 50 years due to the process of evolution (a result of which is the adaptation of the organism).
Since the first large-scale production and widespread use of penicillin in the 1940s (Scenario 2), it has become a major aspect of the Staphylococcus bacteria’s habitat and thus a source of
“pressure” for natural selection . Because of this selective pressure, a penicillin-resistance trait that was most likely first expressed as a random individual mutation in a single bacteria became expressed in larger and larger portions of the entire bacteria population with each successive generation, as bacteria which expressed that trait were more fit to survive and thus had more reproductive success. This process continued between Scenario 2 and Scenario 3, until the entire population of Staphylococcus bacteria was eventually penicillin-resistant. This is explains why
“the girl” dies in Scenario 3: the evolved bacteria now survive after exposure to penicillin.
Variation in the individual characteristics of the bacteria plays a critical role in the evolution of the penicillin-resistant bacteria population. The variation is the pool from which the process of natural selection selects, allowing the Staphylococcus population to remain sizable and competitive.
The difference in generation time for humans and bacteria makes a huge difference in their relative rates of evolutionary change. For every one generation of humans that lives (one per 20 years), bacteria live around 525600 generations (one per 20 minutes). Since evolution is the change in the inherited characteristics of populations over successive generations , bacteria will evolve on a very fast timescale. This means that a person will be able to observe significant results of evolutionary change in bacteria within a single lifetime.