An Introduction to Cladistics
This module provides a brief introduction to the philosophy, methodology, and implications of cladistic analysis. Many of the terms that you will see along the way are high-lighted, e.g., evolution, which means that they are included in the UCMP Glossary of Phylogenetic Terms. Each of these terms can be clicked on for a brief definition.
After you've read through the pages on the implications and methodologies, you will be armed in the ways of cladistics. Therefore, if you dare, you can take a journey into the real world of cladistics. Should you choose to venture on this journey, pray you are well-armed with good luck and wits!
Synapomorphies are the basis for 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.
Note that it is not enough for organisms to share characteristics, in fact two organisms may share a great many characteristics and not be considered members of the same group. For example, consider a jellyfish, starfish, and a human; which two are most closely related? The jellyfish and starfish both live in the water, have radial symmetry, and are invertebrates, so you might suppose that they belong together in a group. This would not reflect evolutionary relationships, however, since the starfish and human are actually more closely related. It is not just the presence of shared characteristics which is important, but the presence of shared derived characteristics. In the example above, all three characteristics are believed to have been present in the common ancestor of all animals, and so are trivial for determining relationships, since all three organisms in question belong to the group "animals." While humans are different from the other two organisms, they differ only in characteristics which arose newly in an ancestor which is not shared with the other two. As you shall see on the next page, chosing the right characters is one of the most important steps in a cladistic analysis.
What assumptions do cladists make?
There are three basic assumptions in cladistics:
The first assumption is a general assumption made for all evolutionary biology. It essentially means that life arose on earth only once, and therefore all organisms are related in some way or other. Because of this, we can take any collection of organisms and determine a meaningful pattern of relationships, provided we have the right kind of information. Again, the assumption states that all the diversity of life on earth has been produced through the reproduction of existing organisms.
The second assumption is perhaps the most controversial; that is, that new kinds of organisms may arise when existing species or populations divide into exactly two groups. There are many biologists who hold that multiple new lineages can arise from a single originating population at the same time, or near enough in time to be indistinguishable from such an event. While this model could conceivably occur, it is not currently known how often this has actually happened. The other objection raised against this assumption is the possibility of interbreeding between distinct groups. This, however, is a general problem of reconstructing evolutionary history, and although it cannot currently be handled well by cladistic methods, no other system has yet been devised which accounts for it.
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 convention is to call the "original" state of the characteristic plesiomorphic and the "changed" state apomorphic. The terms "primitive" and "derived" have also been used for these states, but they are often avoided by cladists, since those terms have been much abused in the past.
Continue your journey by selecting one of the topics below.
Introduction to Cladistics
For additional reading:
More information concerning cladistics on the internet can be found in UCMP's list of phylogenetics resources.Still confused? Try our illustrated introduction to cladistics in the educational module, "What Did T. rex Taste Like?"