Taxonomy and phylogeny  (Page 4/6)

To generate these tree structures, taxonomists use multiple characteristics to compare organisms, including external morphology, internal anatomy, behaviors, biochemical pathways, etc. Aristotle and Linneaus relied primarily on morphological characters (size and shape). However, as noted above, taxonomic schemes derived from comparing only a few characteristic can be very flawed. So modern taxonomists rely, if possible, on many characters, and taxonomies are constantly revised if additional characters are measured and included in the analysis. In addition, the development of a method called cladistics has revolutionized taxonomic thinking, and cladistics also depends on an understanding of evolutionary relationships. A clade is a group of organisms that consists of a common ancestor and all of its descendants. So another name for clade is a monophyletic group. For example, birds (including the bald eagle and your parakeet AND their ancestral organisms including one class of dinosaurs) are a clade. Clades can contain any number of species, but that number must include all of the descendants and the common ancestor. In the example above, A, B, C, D and E, as well as the ancestors indicated by the branch points, are a clade. A and B, plus their common ancestor, are also considered to be a clade.

What's the difference between a phylogeny, a phylogenetic tree, and a cladogram? For our purposes, there really isn't much of a difference. In this class we will use these terms interchangeably — they all describe a tree structure that represents the hypothesized evolutionary relationships within a group of organisms, based on data derived from various sources. The important things to remember are 1) organisms are related, and 2) that we can represent our hypotheses about these relationships with tree structures.

Cladistics relies on classifying characteristics of organisms as either ancestral or derived (another term that is sometimes used for ancestral characters is primitive ). Ancestral characters are those inherited attributes that resemble those of an ancestor to the group. Derived characters are those features that are different from features found in the ancestor. The assumption of ancestral relationships in this approach is important, and signifies yet another way that evolutionary theory is the framework for understanding much of biology.

But how do you know which features are ancestral, and which are derived? There are several ways to do that, but for your purposes, you can assume that characters shared by all the organisms in the group are probably ancestral. Other characters, not shared by all the organisms in the group, are assumed to be derived. Lists of characters are generated, and used to prepare a tree-like structure known as a cladogram .

The best way to illustrate this process is with an example or two. Here is a table with some characters in rows and some organisms in columns. If there is an “X” in the column for an organism, the organism has that particular characteristic.