Tuesday, May 23, 2006

An Introduction to Cladistics

Cladistics is a particular method of hypothesizing relationships among organisms.   Like other methods, it has its own set of assumptions, procedures, and limitations.   Cladistics is now accepted as the best method available for phylogenetic analysis, for it provides an explicit and testable hypothesis of organismal relationships.

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.

There are three basic assumptions in cladistics:
  1. Any group of organisms are related by descent from a common ancestor.
  2. There is a bifurcating pattern of cladogenesis.
  3. Change in characteristics occurs in lineages over time.
The final assumption, that characteristics of organisms change over time, is the most important assumption in cladistics.   It is only when characteristics change that we are able to recognize different lineages or groups.

The output from a phylogenetic analysis is a hypothesis of relationship of different taxa.   This hypothesis can be represented as a cladogram, a branching diagram.

The building of a cladogram includes these rules:
  • All taxa go on the endpoints of the cladogram, never at nodes.
  • All cladogram nodes must have a list of synapomorphies which are common to all taxa above the node (unless the character is later modified).
  • All synapomorphies appear on the cladogram only once unless the character state was derived separately by evolutionary parallelism.


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