Which taxonomic level for a given genus includes the greatest number of species?

The answer is Phylum

A hierarchical system is used for classifying organisms to the species level. This system is called taxonomic classification. The broadest classifications are by domain and kingdom; the most specific classification is by genus and species. The hierarchical groupings in between include phylum, class, family, and order.

Species are the basic unit of classification. While there are different views on what defines a species, in sexually reproducing organisms, a species has traditionally been defined by the ability of its members to reproduce together to form fertile offspring. This definition is trickier in asexually reproducing organisms like bacteria, archaea, and protists, where scientists look at the similarity in DNA among individuals to tell whether they are in the same "species."

When identifying an organism, familiar names like human, fruit fly, or maple tree are most likely the names you use. However, each type of organism has a scientific name humans are called Homo sapiens, for example. Scientific names are derived from the genus and species names in a system known as binomial nomenclature "bi" = two; "nom" = name.

This system allows for a common language with which biologists can both classify and compare organisms. It also provides a basis for biologists to communicate their findings with other scientists. A key aspect of taxonomy is the ability to characterize each level of description with unifying features, thus relaying information about the organism.