A bit on the lowest taxonomical classification.

Species

• Any two individuals of appropriate mating types can produce a fertile offspring.
• Typically defined as groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups.

Genus

They are formed on the basis of three different rules:

• Monophyly: The group excludes non-descendants of the most recent common ancestor of the group.
• Compactness: No needless expansion of a genus.
• Distinctness: DNA sequences are a consequence rather than a condition of diverging evolutionary lineages except in cases where they directly inhibit gene-flow.