What are hierarchies Hierarchies are nested groupings. Fullerton Notice how each group is completely subordinate to any group on its left. Nature is hierarchical A comparison of an organism with its nearest to more distant relatives will reveal that attributes of organisms are nested in hierarchical patterns.
Organisms resemble their closest relatives more than distant relatives.
In other words, we hypothesize that this similarity is due to common ancestry. If this were not so then we would have to postulate that placental nourishment evolved independently in lineages leading to humans and mice. A clade is a grouping of organisms that includes an ancestor and all of its descendents. Birds would be birds even if we did not call them such. We recognize clades by their shared evolutionary novelties, such as the placenta in all eutherians, or feathers in all birds.