Nested Hierarchies and Cladistics- a Primer

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Revisiting Nested Hierarchies:
From A Summary of the Principles of Hierarchy Theory:

Nested and non-nested hierarchies: nested hierarchies involve levels which consist of, and contain, lower levels. Non-nested hierarchies are more general in that the requirement of containment of lower levels is relaxed. For example, an army consists of a collection of soldiers and is made up of them. Thus an army is a nested hierarchy. On the other hand, the general at the top of a military command does not consist of his soldiers and so the military command is a non-nested hierarchy with regard to the soldiers in the army. Pecking orders and a food chains are also non-nested hierarchies.

For example in the nested hierarchy of living organisms we have the animal kingdom.

To be placed in the animal kingdom an organism must have all of the criteria of an animal.

For example:

All members of the Animalia are multicellular (eukaryotes), and all are heterotrophs (that is, they rely directly or indirectly on other organisms for their nourishment). Most ingest food and digest it in an internal cavity.

Animal cells lack the rigid cell walls that characterize plant cells. The bodies of most animals (all except sponges) are made up of cells organized into tissues, each tissue specialized to some degree to perform specific functions.

The next level (after kingdom) contain the phyla. Phyla have all the characteristics of the kingdom PLUS other criteria.

For example one phylum under the Kingdom Animalia, is Chordata.

Chordates have all the characteristics of the Kingdom PLUS the following:

Chordates are defined as organisms that possess a structure called a notochord, at least during some part of their development. The notochord is a rod that extends most of the length of the body when it is fully developed. Lying dorsal to the gut but ventral to the central nervous system, it stiffens the body and acts as support during locomotion. Other characteristics shared by chordates include the following (from Hickman and Roberts, 1994):

bilateral symmetry
segmented body, including segmented muscles
three germ layers and a well-developed coelom.
single, dorsal, hollow nerve cord, usually with an enlarged anterior end (brain)
tail projecting beyond (posterior to) the anus at some stage of development
pharyngeal pouches present at some stage of development
ventral heart, with dorsal and ventral blood vessels and a closed blood system
complete digestive system
bony or cartilaginous endoskeleton usually present.

The next level is the class. All classes have the criteria of the kingdom, plus all the criteria of its phylum PLUS the criteria of its class.

This is important because it shows there is a direction- one of additive characteristics.

Yet evolution does NOT have a direction. Characteristics can be lost as well as gained. And characteristics can remain stable.


Cladistics is a method of categorizing organisms based on shared characteristics. Each clade (allegedly) consists of a common ancestor and all of its (alleged) descendents:
intro to cladistics

The basic idea behind cladistics is that members of a group share a common evolutionary history, and are "closely related," more so to members of the same group than to other organisms. These groups are recognized by sharing unique features which were not present in distant ancestors. These shared derived characteristics are called synapomorphies.

cladistics:

Cladistics can be distinguished from other taxonomic systems, such as phenetics, by its focus on shared derived characters (synapomorphies).

And also what is cladistics?

The clade is not constructed based on ancestor-descendent relationships, those are assumed. And ancestor-descendent relationships form a non-nested hierarchy- see Eric B Knox, "The use of hierarchies as organizational models in systematics", Biological Journal of the Linnean Society (1998), 63: 1–49


Each clade, note- not the entire cladogram, can be a nested hierarchy based on shared characteristics in that each descendent node will consist of and contain, ie share, a set of defined characteristics present in the alleged common ancestor. However each clade is also a non-nested hierarchy in that the alleged common ancestor does not consist of nor contain all descendents.

The point being is that if your basis for clade-construction is to make it conform to a nested hierarchy based on shared characteristics, then yes, you should see that a clade is a nested hierarchy based on shared characteristics, duh.