How are our red blood cells similar to a bus or taxi?

Both erythrocytes and bus/taxi drivers' main job is to transport important components. Erythrocytes transport oxygen and carbon dioxide through the body. Likewise, taxi/bus drivers carry citizens through the city. Citizens are important to the city just as oxygen is important to the body. Taxi drivers move through, and reach all parts of the city; similar to how erythrocytes transport oxygen all throughout the body. In addition, there are great amounts of both Erythrocytes in the body and taxi/bus drivers in the city.

not necessarily. higher fitness requires the organism to be reproductive successful. for example, an organism at age 10 with 15 offspring is much more fit than an organism at age 15 with only 5 offspring.