Trilobite Lab by Amina Nur, Julio Gonzalez, Lydia Mikhail, and Katie Rupp

BIOL& 212 : Systematics Lab – Trilobite tree questions

Group Members: Amina Nur, Julio Gonzalez, Lydia Mikhail, and Katie Rupp   

1.   Post a Picture of your tree.

     On your tree, which species is the outgroup?

     Why did you choose this species?  Explain.

Figure 1. This photo of a phylogenetic tree depicts the evolutionary relationships amongst fifteen different species of trilobites. Our tree was constructed by making a similarity matrix based on observations of each of the fifteen trilobites.

Figure 2: This is our similarity matrix, which we used to compare the presence and absence of traits in different species of trilobites to construct a phylogeny.

Our group choose Peronopsis interstricta (species 3) as an outgroup. P. interstricta doesn’t have many of the characters or traits that developed and evolved later on in the tree, at it had other traits that were not the focus of the study and it was drastically different from the other species. The outgroup (species 3) was used to determine the timing of evolution of each character. Based on P. interstricta, the facial features with the eyes developed first then it was followed by the development of the axial ring, the genal spine, spikes, tail, and lastly the change in the texture as it shown in figure 1.

2.   According to your tree, what is one basal or ancestral characteristic?  One derived characteristic?

One basal or ancestral characteristic in the trilobite is the body parts. All the Trilobite species including the out group have the following body parts: cephalon, thorax, pygidium which indicates that it is ancestral trait. One derived characteristic is the development of a spinal tail. As some of species didn’t have the spinal tail, and other some have double spines instead of one. Furthermore, the out group doesn’t have these traits which concludes that the spinal tail is not ancestral trait and it is derived.

3.   According to your tree, is the rear ‘spine’ of species 6 homologous or analogous (homoplastic) to that of species 14?  Explain.

The spine in species 6 is analogous or homoplastic to the spine in species 14. Because both species have the spine as a result from convergent evolution, and they are not sharing a recent common ancestor. The single spine (tail) developed due to environmental factors or due to any other reasons, and it doesn’t exist in a common ancestor.

4.   Are there any traits that were lost but then evolved again independently?

     If so, what are they and where do they occur?

No, there were no traits that were lost then evolved again independently because using our phylogeny tree that we constructed in the experiment, we constructed it based on the fact that the structures on the trilobite got more and more “complex” as the organism evolved. With that, our tree was constructed on the idea (or principle) of parsimony, which is a rule that is used to choose among possible cladograms, which states that the cladogram makes the least amount of changes in characteristics.   

5.   Describe one important difference between your tree and a tree estimated by a different lab group (identify which group’s tree you used).  Upon reflection, which tree seems better?  Why?

    Our group used Hannah Zaini’s groups phylogenetic tree, as shown in Figure 2, to compare and contrast our thought processes after constructing our own phylogenetic tree of fifteen different species of trilobites. One major difference between our phylogenetic trees is that our group placed a lot of emphasis on the development of one or two tail-like structures. We placed the trilobites with tails all very close to each other, with the trilobites with one tail evolving prior to those with two. Our group intentionally made the presence of a tail a key trait in our phylogeny to eliminate any homoplasy that may occur if they were not closely related on the tree. In other words, because we founded our tree on the principle of parsimony, assuming the trilobites with tails were closely related was the simplest solution. In contrast, Hannah’s group did not indicate the presence of a tail as a developed trait on their phylogeny. Because of this, the trilobites with tail-like structures were not as closely related in their phylogenetic tree. As a result, the development of a tail occurred multiple times in the tree, independently from each other. This is an example of homoplasy, the independent development of a similar trait, without being derived from a common ancestor. This means that over time, evolutionary pressures created a demand for a trait  that caused two different trilobites to have similar structures. More specifically, in our tree, we placed Dalamnites verrucosus as one of the most recent trilobite species to branch off from a common ancestor. On the other hand, Hannah’s group branched off Dalamnites verrucosus from a common ancestor about halfway through their tree. The approach taken by his group was slightly less parsimonious because the trait for a tail-like structure independently appeared multiple times throughout their phylogeny. In respect to the trait for a tail being an important aspect of the evolution of trilobites throughout time, our group’s phylogeny is probably a better representation. Our trilobite phylogeny was more parsimonious than the other group’s tree, meaning we made the least assumptions about the evolutionary relationships amongst the fifteen trilobites we were given. This is only because the other group did not perceive the development of a tail-like structure to be biologically informative enough to be noted within their phylogenetic tree.  In a way, it is difficult to decide which tree is ‘better,’ because it all depends on what specific element of the tree is being taken into consideration. There are many different ways to construct a phylogenetic tree, and if the evidence and logic that is being used to argue the structure of the tree is strong enough, any phylogenetic tree can be valid.

Figure 2: This is a different groups phylogenetic tree, depicting the evolutionary relationships between the same fifteen trilobites. By referencing this tree, we were able to identify key differences in the foundations of our phylogenies. Through analysis, we discussed one of the most substantial differences between the two phylogenetic trees.