The basic unit of the evolutionary tree for all organisms is the species, which can be defined as a genetically distinct group of individuals that reproduce and share genes and that are reproductively isolated from all other such groups (species). A species can be viewed as a group of populations of individuals within which exchange of genetic material occurs. Closely related species that share common characteristics are grouped together into the next higher category - the genus. All species within a single genus are more closely related to one another than to species in other genera (the plural of genus). The classification system works on the same principle for higher taxa. Related genera make up a family, closely related families are grouped into orders, and so on, to phyla and kingdoms.

Remember, though that the classification system is a human invention used to describe what we observe, and is therefore not set in stone. There is lively debate about where many organisms should be placed within this structure.