BIO 280: Evolutionary Biology

Activity: Introduction to Cladistics

Systematics is the field of inquiry by which biologists seek to organize what they think they know about patterns of diversity. Systematics includes: (1) taxonomy, the identification and naming of organisms, (2) phylogenetic reconstruction, the formation of hypotheses about organisms' evolutionary relationships, and (3) classification, the ranking or grouping of organisms according to their supposed evolutionary relationships. Ideally, all members of such a group should descend from a single ancestral form. In other words, each group should be monophyletic. Thus, constructing a good phylogeny depends on recognizing homologies, features that have common ancestry.

In the last thirty years or so, biologists have accepted cladistics, a.k.a. phylogenetic systematics, as one approach to phylogenetic recostruction. Cladistics uses recency of common ancestry as the only criterion of evolutionary relatedness. A cladogram is constructed according to the occurrence of shared derived characters within a lineage. Derived characters are evolutionary innovations or novelties-- features that appear for the first time within a lineage and are called synapomorphies. This sharing of derived characters (synapomorphies) defines the nodes (= branch points) of a cladogram. A clade includes all organisms sharing the derived character/s that define/s a node. In other words, a clade consists of the first organism to have the derived feature/s plus all of its descendants.

Occasionally it happens that a particular group possesses a derived character or a set of derived characters uniquely. That is, the group does not share the derived characters with other groups. In that case, the unique derived character/s serve/s to set the group apart, but cannot be used to infer relatedness to other groups. These traits are known as unshared derived features or autapomorphies.

Characters inherited from distant ancestors are said to be primitive, and are not used in constructing a cladogram. In the cladistic view of the world, shared primitive characters provide no information about the degree of evolutionary relatedness. A shared primitive character is a sympleisiomorphy.

Figure 1 below includes 5 taxa (groups of organisms-- could be species, populations, genera, etc.) designated by the letters A, B, C, D, and E. The numbers 1,2,3, and 4, represent occurrence of derived characters. In this scheme, 4 is an unshared derived feature of taxon E. 3 is a shared derived feature of D+E (a synapomorphy). 2 is a shared derived feature for C plus D plus E.

Here are some points to be aware of:

1. There are two general methods a cladist uses to decide whether a feature is primitive or derived within a monophyletic group.

* IN-GROUP COMPARISON: If a feature occurs in all members of a group, that feature will have been present in the most common ancestor of that group-- a situation vastly more probable than the feature evolving independently in all the different members of the group. In Figure 1, for example, if a particular character occurs in C, D, and E, but not in A and B, the character is considered to be a derived feature that can be used to recognize the clade made up of C, D, and E.

* OUT-GROUP COMPARISON: Homologous characters shared by a particular group with organisms known (or thought) to be distantly related to that group are primitive for the group under comparison. In figure 1, features shared by A with the other groups must be considered primitive for groups B,C,D,and E. A group that has fewest derived characters serves as a standard according to which more-derived groups can be recognized. That most primitive group is known as an outgroup.

It follows that characters have polarity. A single character may be primitive or derived depending on what part of the cladogram is under consideration. In figure 1, the derived character 2 that distinguishes the clade made up of C, D, and E is a primitive character for the purpose of defining the clad composed of D and E. Therefore, it is useless for defining the clade composed of D + E.

2. A cladogram does not give information about ancestors ("who came from whom"), but rather indicates, for example, that E is more closely related to D than to C. That is, the common ancestor of D and E was more recent than the common ancestor of C, D, and E. In the same way, C, D, and E are more closely related to each other than they are to B.

3. The higher up a node (branch) occurs in a cladogram, the more derived are the organisms above that point. E is the most derived group in figure 1. The branching sequence of a cladogram indicates only relative timing of divergence of groups--- it does not show absolute timing of evolutionary events!

4. Whenever a lineage splits, the resulting branches are "sisters" of each other, or sister groups. In figure 1, D and E are sister groups of each other and C is a primitive sister group of the clade made up of D and E. Sister groups are united by shared, derived characters. D and E share the derived character '3'. C, D, and E share the derived feature '2', but since C lacks '3', C is primitive with respect to D and E.

5. The branching pattern of a cladogram must always be dichotomous (splitting two ways at branch points). That is true, because for any three groups, two must always be more closely related to each other than either is to the third, in the sense of sharing a more recent common ancestor. But the dichotomous branching can be more complex than that of figure 1. Figure 2 (below) shows a cladogram closer to what a real world cladist is likely to deal with.

6. A cladist tries to find the most parsimonious ("stingy") phylogeny. In other words, the best phylogeny requires the smallest number of evolutionary innovations to match the derived character states s/he observes. For example, if the same derived character state occurs in 2 groups, a cladist will make a cladogram that shows one origin of that character with two descendent groups rather than two different origins of the character inherited through two separate lineages. Similarly, if a feature is widespread in a lineage, with only a few exceptions, a cladist will treat that pattern as the result of a single origin followed by some secondary losses of the feature (thus invoking only a few changes), rather than postulating independent origins of the feature in most but not all of the members of a lineage. This is an assumption about nature-- there is no guarantee that nature is parsimonious, but it is conventional for scientists to start from the simplest hypotheses they can imagine. And cladograms are hypotheses-- they are suggestions of possible relationships that may be supported or refuted as additional data become available.

Reinforce the foregoing ideas by answering the following questions with reference to figure 2:

1. Which is the most primitive group(s)? The most derived group(s)? How did you reach your decisions?

2. How many clades are represented? List by letter the members of each clade.

3. Which is the outgroup?

4. a Which is the sister group of H? (remember sister groups may contain more than one letter!)

b. Which is the sister group of E?

c. Which is the sister group of B?

d. Which is the sister group of E+F?

5. Which feature/s is/are:

a. Shared, derived features (synapomorphies) of the clade made up of G and H?

b. Shared, derived features of the clade made up of B,C, and D?

c. Primitive or ancestral features (sympleisiomorphy) of the clade made of E and F?

d. Primitive or ancestral features (sympleisiomorphy) of the clade made of B,C,,and D?

6. Which features are unique derived features (autapomorphies ) of which groups?