Lab 3: Cladistics

Use Textbook Chapter 4

Goals of this lab: Understand the principles of making a cladogram: how to select synapomorphic/symplesiomorphic characters?

The Cladogram of Hardware

• You are supplied with 10 objects, which you should treat as fossil organisms.
  • Make a matrix of characters versus individuals, in which you work out presence/absence of specific features.
  • Use this data matrix to select the outgroup, select the character(s) in which the group differs from its presumed ancestor.
  • Use the data matrix to sort out which specimens share most characters, i.e. are most closely related and thus must end up in sister groups.
  • Draw a parsimonious cladogram and Venn diagram
  • Comment on your cladogram: which characters did you treat as 'more important' than others? Are there characters that you must let evolve twice independently?
• Now add the hardware organism from the additional box. What effect does this addition have on your cladogram?
• Let us now try this on real organisms: the bivalves and brachiopods from last week's lab.

Note: this is only one possibility. In this possible solution I have treated 'color' as a feature that rather easily changes, and I did not use the charcater 'ridges present on part of stem only' in classification. I used 'tightness of ridges' only to subdivide groups which were formed by combining individuals with a similarly shaped slot in the head.

The ancestral forms (A, B) as well as the forms with slit in head (C, D) are all grey or silvery, but descendant groups develop colors black and gold. Note that I placed the gold colored type within a dominantly black colored group; I thus used the shape of the head slot as an important character to define sister groups. I imply that a gold and black specimen with the same shape oif slot in their head are more closely related to each other than to anyone else.

Since the gold-colored individual has ridges that cover only part of its stem (as have the two individuals with slotted head, C and D), I chose to consider this character (full length ridges or not) also as variable within a sister group. I also let color grey evolve again (or return to ancestral form) in species J, in the sister group with square depression. Here I thus say that I consider the two specimens with square dents in their head (one of which is grey, the other black) as more closely related to each other than to speciemns with the same color but differently shaped slot in head.

Also note that I treated 'shape of slot in head' as a sub-character of 'presence or absence of depression in head'. If you let gold (E) split off the main stem instead of being attached to the side stem with cross heads, you infer that the square heads are more closely related to the 3 non-gold cross heads than the gold one is to the other three. You can prevent this by splitting off the square heads first, then gold, then the other cross heads, but that lets square head develop from line head, then cross head from square head (and most people like cross head to develop from line head, then square head).

If the additional form (K) is added, we get a form which is grey/silver, has crossed slot in head, and a convex head, with ridges covering full length of the stem, and the additonal shiled below the head, which no other form shares.

The choice is now: do we consider it likely that the character 'convex head' evolved twice (within the sister group with slotted head (B,C) and the sister group with cross-head (E-G), and do we place this individual within the group containing D through G? In that case, the color within that group also is allowed to change back to the ancestral condition of grey, while the head shiled is added. Or do we consider it likely that the character 'cross head' evolved twice independently, and branch the added species K off from the other convex head species (C), with addition of 'cross slit' and 'head shield'? Since I have been using the 'shape of dent in head' as a very important character throughout, I should go for the first choice, and let K branch off from E, F , G or H. I could branch it off the gold one rather easily, since both are short with rather tight ridges, and then color of that form was already different than that of the main group. But I could also branch it off from F; I prefer these two possibilities over branching it off from G or H, which (using the low-density of ridges as a charcater) seems to be a set of sister species.

We do not have an 'outgroup' available for brachiopods and bivalves, and thus can not 'root the cladogram': we do not know which group is the most 'prmitive' and branched off first.

Bivalves:

In bivalves (organisms which filter feed, pumping water over their gills) the character of the hinge between the valves is an important character. We thus split the 5 taxa into a group with curved hinges with sockets and teeth (3 taxa) and a group with a straight hinge, no clear teeth, and a triangular area between the hinge and the umbo of the shell. The latter group has a larger width than height of the shell (w/h>1); it can be split into the ponderous arc (w/h not much >1) and the turkey wing (w/h >> 1).

The group of 3 taxa consists of the two cockles, in which h/w is equal to or larger than 1; the other taxon is the Cardita, in which h/w <1. The two cockles are sister taxa and distinguished by presence/absence of spines.

Brachiopods:

We split the 5 taxa into a group of 3 taxa, characterized by having biconvex shells and a sulcus, and a group of 2 taxa, which have concavo-convex shells. The latter group is then subdivided on the nature of the convex side, which is very rounded in one taxon, flat in the other. In the set of 3 taxa the two genera of Spirifer-like forms (Spirifer and Mediaspirifer) are sister taxa (shell w>h, strongly developed sulcus), with the other taxon (Atrypa) branching off first (about 1, sulcus more weakly developed). In Mediaspirifer w/h is not as large as in Spirifer.