Why don't the spontaneous deleterious mutations that occur in an individual e. coli cause downward drops in fitness in this population? see section 23.6 ( page 475) ?

Please see below  

Explanation:

The reason behind this is the phenomenon of natural selection. The deleterious mutations do not provide a survival advantage so they are selected against, which results in these type of mutations to not be passed on - rather, they eventually get eliminated from the population.