2015 AP Biology free response 3

The amino acid sequence of cytochrome c was determined for five different species of vertebrates. The table below shows the number of differences in the sequences between each pair of species.

So just to give us some context for what we're talking about, we're talking about cytochrome c. This is a protein; you might remember it when we studied the electron transport chain. You don't really have to know that for this question, but it's a protein that's found in a lot of different species, and we're going to compare the difference in the sequence of amino acids for that protein in the different species.

So even though you have the same or similar protein, we call them all cytochrome c, the sequence might be slightly different when you go from one species of vertebrate to another. The way to read this is, in each of these rows, you could say, "All right, this row describes D. polylepis," and we can see the number of sequence differences between D. polylepis and G. gallus. There's 18 differences. There's clearly going to be zero differences between that species and itself.

Now let's answer their question. Using the data in the table, create a phylogenetic tree on the template provided to reflect the evolutionary relationships of the organisms. Provide reasoning for the placement on the tree of the species that is least related to the others.

So looking at these differences, it's reasonable to say, well, the things that have the fewest differences in the sequence of cytochrome c, well, those are probably most closely related. They already gave us a tree here, so whatever goes here and here are probably pretty closely related; they shouldn't have too many differences in the sequence of cytochrome c, and these two should be pretty similar and shouldn't have many differences, while this one should have a good number of differences from any of the other four.

So let's first look at which ones have very little difference. I'm just looking, I'm just scanning this chart here looking for small numbers. So you see here there's only one amino acid difference, or one sequence, well, yeah, one amino acid difference between cytochrome c and E. ferrous and E. africanus. So I would say that these are fairly closely related; they probably share an ancestor not too far in the distant past. So let me write E. ferrous and E. africanus. I'll do it here: E. ferrous and E. africanus.

Now where else do we see some low numbers? Well, I see this three right over here, and that's the number of sequence differences or the amino acid differences between G. gallus and A. forsteri, however you pronounce that. So I would say those are probably pretty closely related, and so let me write this here: G. gallus and A. forsteri.

And who have we not dealt with yet? Well, we haven't dealt with D. polylepis yet; we've dealt with the other four, so I'll put that here: D. polylepis. You can see when you compare D. polylepis to any of the other ones. You see a pretty big difference. D. polylepis to E. ferrous, it's a 21 difference; 18 difference with G. gallus; 17 difference with A. forsteri; and a 20 amino acid difference in the sequence for cytochrome c between D. polylepis and E. africanus.

So this has the most differences, and they say provide reasoning for the placement on the tree of the species that is least related to the others. So I'll provide the reasoning. D. polylepis is least related; I should say is least related because it has a large amino acid sequence difference from the other four. You can say it has the largest, maybe I'll write that: has the largest amino acid sequence difference from the other four. Because even if you were to compare E. ferrous to G. gallus, you still don't get close to 20, while D. polylepis is pretty close to 20 difference with all of them.

So that makes sense that even these four share a common ancestor that you don't have to go as far back in time to get to the common ancestor that you do to get to D. polylepis.

Alright, let's do part B now. So that was part A; let me label that. That was part A. Now part B: Identify whether morphological data or amino acid sequence data are more likely to accurately represent the true evolutionary relationships among the species and provide reasoning for your answer.

So morphological data, this is looking at the morphology of the different species, and you could say like, "Okay, what's the shape of their backbone or their different bones, or the shape of different parts of their body?" While amino acid sequence, well, you're looking at, "What are their proteins actually made up of?"

I personally would go with the amino acid data. So I believe, I believe, I believe amino acid sequence data are, data’s plural, are more likely to accurately represent the true evolutionary relationships among the species.

Let me provide my reasoning: You could have convergent morphology. You can have convergent morphology, and that all that means is, for example, you could look at a bat's wings and a bird's wings and say, "Okay, look, they have similar morphology, but they aren't related just because they both have wings." Or you could look at a dolphin and a fish and say, "Okay, they both have flippers; maybe they're more closely related if you just look at the body type." But that's convergent; they actually came from different ancestors, but then their ancestors, because they had similar environments, had convergent parts, I guess you could say, had convergent morphology.

You can have convergent morphology while being far apart on the evolutionary tree. Now there's arguments for morphology as well because you might be looking at, you know, we're only looking at cytochrome c here; that might be some type of an anomaly or maybe you have some convergence or divergence for that particular protein that does not actually fit with what's actually happened in evolutionary history.

But in general, if I can look at the molecular sequences, if I can look at sequences of proteins, if I can look at what's going on with the DNA, I like looking at that because that allows you to not be tricked by the convergent morphology of far-apart things like bats and birds or dolphins and fish.