Skeletons – Prediction based on Hypotheses - Bison

Just to refresh your memory, here are the three groups of hypotheses in the class.

Group 1: A single ancestor. This gives a “tree” shaped phylogeny – The one on the right is an example, but they vary -- be sure to look at all of them in order to make predictions, etc. You can't tell by looking at it what the common ancestor down at the bottom was.

Group 2: Several ancestors with some speciation (splitting of groups into more than one species) This gives a row of bushes – a “hedge” shaped phylogeny. These vary a lot. Some are close to the "lawn" shown below, others, like the hypothesis shown here, have only a few "bushes".

Group 3: No speciation at all – every species starts independently. This gives a whole bunch of straight lines – a blades of grass or “lawn” shaped phylogeny.

Now, consider the radius and ulna, as discussed in the previous assignment. If you look at various skeletons, you'll note that the presence of two bones in the second part of the forelimb is a similarity that occurs in most. You've already given some thought to the function of these bones, which is to allow the wrist to rotate.

Now, consider the bison. Much like cattle, bison have hooves, and they support a great deal of weight on them. Their front hooves do not rotate with respect to the rest of the limb (in fact, you can imagine that it might be pretty dangerous for the "wrist" of a bison to turn, considering the weight it supports).

Finally, consider the three hypotheses. Think about the organisms that each one predicts could show homologies.

Based on what you know of bison, and of the radius and ulna, what would each hypothesis predict about the forelimb of the bison? Would each hypothesis predict that the bison would more likely have two bones in the second part of the forelimb, or only one? Give your answer and explain why that answer is the one predicted by each hypothesis. If the answer depends on the particular version of the hypothesis within one of the three groups, explain that too.

(OVER) The TREE hypothesis predicts:

The HEDGE hypothesis predicts:

The LAWN hypothesis predicts: Now go down to the end of the hallway near MG 138 and have a look at the bison skeleton in the case. Note that there are two bones in the second section of the forelimb of the bison. Look carefully at the "wrist" joint; the two bones (radius and ulna) are fused together there (the black vertical line is a wire holding the skeleton together). In fact, the bones are at least partially fused all along their length. They start out as two separate bones in the embryo, but fuse during development. By the time the bison is born, they can't move separately from each other at all.

Why do you think the bison has two bones in the lower part of its forelimb? (Before you say that it's for additional strength, consider how much weight the single bone above them (humerus) supports). Considering this as a similarity with humans, would this be a homology or an analogy? Explain.

What can you conclude about each of the three class hypotheses based on this evidence? Tell in each case whether the hypothesis is supported, falsified, weakened, or if there is no conclusive evidence provided, and explain your answer. If it depends on which particular version of the hypothesis is being considered, explain. If the evidence requires some modification in the hypothesis, suggest how it should be modified.

Tree

Hedge

Lawn