Consider a mixture of bacterial species from the same genus, such as Staphylococcus. Some members of this genus are harmful pathogens, while others are not. Would a selective medium alone be a good tool to separate the harmful species from the non-harmful species

No. There are different types of media that may result useful to separate this species. Moreover, differential media can also be used to differentiate closely related species (e.g., by the change of color in the medium)

Explanation:

A selective media is a type of media capable of selecting for the growth of specific microorganisms while inhibiting the growth of others. For example, if a bacterium is resistant to an antibiotic (e.g., ampicillin), then this compound can be added to the medium in order to select bacteria that are resistant and simultaneously inhibit the growth of susceptible bacteria. Moreover, a differential media is a media that contain chemical indicators that allow certain microorganisms to be visually identified by the appearance of the colony and/or its surrounding media. Differential media have into account certain biochemical characteristics of target microorganisms (e.g., the change of color in the medium which is associated with the ability of the microorganism to ferment sugar molecules).

the wires in the toaster turning red and the toast crisping and turning brown