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What are hierarchies

Hierarchies are nested groupings.

Examples of hierarchies

Levels of organization in our bodies:

Organism: Organ Systems: Organs: Tissues: Cells: Organelles


Biome: Community: Population: Organism

Political boundaries:

USA: California: Orange County: Fullerton

Notice how each group is completely subordinate to any group on its left.

Political boundaries happen to be arranged hierarchically, but can you imagine a situation in which they are not? For example, what if part of Orange County was outside of California?

Is this a hierarchy?

Red Cars: American Cars: Vehicles with 4 wheels

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.

Vertebrata; Tetrapoda; Amniota; Mammalia; Eutheria; Homo sapiens

[or more completely: Eukaryotae; Metazoa; Eumetazoa; Bilateria; Coelomata;
Deuterostomia; Chordata; Vertebrata; Gnathostomata; Osteichthyes;
Sarcopterygii; Choanata; Tetrapoda; Amniota; Mammalia; Theria;
Eutheria; Archonta; Primates; Catarrhini; Hominidae; Homo; sapiens]

We recognize that both humans and mice belong to Eutheria (placental mammals) because, like other members of Eutheria, they have placental nourishment of their developing embryos. In other words, we hypothesize that this similarity is due to common ancestry. Humans and mice share a common mammalian ancestor (the ancestor of Eutheria) and this ancestor had a placenta. 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. Eutheria is the clade that includes the eutherian ancestor and all its descendents (including humans and mice). Clades are real historical "individuals" in the sense that they have a birth, the change throughout life, and eventually they die. 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.


Eutheria has a "sibling" (or "sister") clade called Metatheria (marsupial mammals). Kangaroos and opossums belong to Metatheria. They nourish their newborns with a marsupium, not a placenta. Again we hypothesize that the common ancestor for Metatheria had a marsupium.


Eutheria also has "children" clades. Primates and rodents are two of the children of Eutheria. Likewise, Eutheria and Metatheria are children of Mammalia.

Lineages are hierarchical

Siblings Eutheria and Metatheria are each subgroups of more inclusive groupings such as Mammalia, the common ancester of which is believed to have had fur and mammory glands. Likewise, Mammalia

is a subgroup of Amniota, whose ancestor had an amnion,

is a subgroup of Tetrapoda, whose ancestor had four limbs,

is a subgroup of Vertebrata, whose ancestor had vertebrae,

and so on.

In the left column below is the entire detailed lineage for Eutheria and to the right are siblings and children of Eutheria.


Why does nature produce hierarchical groups?

Evolution generally results in lineage splitting. The human lineage is forever separated from the mouse lineage because each has evolved for many millions of years along separate paths. If separated lineages could commonly fuse back together again to form a single lineage, then we would not expect patterns of similarities that are hierarchical.

Languages are mostly hierarchical. French, Italian, and Spanish words can usually be traced back to a root word in a presumed common ancestral language. However, melding together of long separate languages can also occur. When the Romans invaded England, the necessity to communicate with each other had lasting impacts on each language. This phenomenon complicates the historical reconstruction of word origins. Judging from the neat hierarchical arrangement of similarities in organims, it is likely that such fusion is rare in lineages of organisms.

Consider an analogy to a maple tree. When two twigs first split on the tree, they may later go on to become "sibling" branches or trunks. Humans and mice are separated twigs on the same major branch (Eutheria). Kangaroos and opossums are twigs on a neighboring major branch (Metathuria). In this analogy, a clade is what falls would off after you cut through one of the branches (an ancestral twig and all of its descendents). This is known as the snip rule.

What is a cladogram?

A cladogram is a branching diagram that depicts a particular hypothesis of phylogenetic relationship. Ideally, it should correspond as closely as possible to the hierarchical classification system used, that is, one should use a phylogenetic classification system. Here is an example that is based on Animal WWW Links.


Here is the "indented classification" that corresponds to the figured cladogram above, and is the same as the indentation pattern in the Animal WWW Links.

Metazoa (1)
Porifera (Sponges)
Eumetazoa (2)
Bilateria (3)
Platyhelminthes (Flatworms)
Coelomata (4)
Eutrochozoa (5)
Deuterostomia (6)
Note the numbers correspond to ancestral taxa: clades that include two or more terminal taxa. Terminal taxa don't have any number after them. Moreover, notice how terminals do not have any children, that is, they do not have any taxa listed immediately following them that are indented further to the right. For example, the ancestral taxon "Eutrochozoa" (5) includes the terminal taxa (or "twigs") Annelida and Mollusca, according to this cladogram hypothesis. Annelida and Mollusca are sister taxa. The least inclusive clade that includes both Annelida and Mollusca is Eutrochozoa. Imagine the ancestor for clade Eutrochozoa at "node" 5. Eutrochozoa is the clade that includes this ancestor and all of its descendents.

Completing the following exercise should make you comfortable with the relationship between a cladogram and its corresponding phylogenetic classification. Part "a" of this exercise is to construct a cladogram from the following indented phylogenetic classification:

Tetrapoda (tetrapods)
Amniota (amniotes)
Mammalia (mammals)
Sauropsida (sauropsids)
Sauria (diapsids)
Testudines (turtles)
Amphibia (amphibians)
Anura (anurans)

To complete this exercise, follow these steps:

1) Copy the indented classification to the top third of a blank page.
2) Put an asterisk after each terminal taxon (see above).
3) List the terminal taxa only in a single vertical column along the right margin of the bottom two thirds of your page;
4) Connect up terminal taxa that are sister taxa. There can be more than two sister taxa that join at the same ancestral node, for example, in the case of ancestral taxon Gymnophiona. (This could be thought of as the hypothesis that each of the children of Gymnophiona was derived separately and simultaneously from the same ancestral species. More normally, such a polytomy indicates that we are uncertain about the branching pattern within the clade.)
5) Connect the ancestral nodes together so that you are joining clades that are sister taxa, according to the phylogenetic classification. For example, Batrachia and Gymnophiona are sister taxa within the ancestral taxon, Amphibia (or "Lissamphibia" in many systems).
6) Finally, add a "dangling root" to the most inclusive (deepest) node to indicate that these organisms are assumed to share a common history with other organisms outside this clade. If you have trouble with any of the steps above, use the Metazoa example to compare between the cladogram and the classification.

Part "b" of the exercise is to draw a circle neatly around each clade in your diagram. Remember the snip rule (see above). The point of this is to emphasize that cladograms are a way to arrange clades as nested hierarchies.

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