If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

3a-b, Phylogeny

Thinking about evolutionary distance between species. Constructing phylogenetic trees (cladograms).

Want to join the conversation?

  • starky tree style avatar for user Joseph Li
    At , does the placement of the organisms matter? Because the answer in the video doesn't correspond with the answer given by the college board.
    (12 votes)
    Default Khan Academy avatar avatar for user
  • piceratops seedling style avatar for user Sarina Edwards
    I know he wrote simple answers for the sake of time, but if you were to use the basic answers he used, would that be more than enough for AP graders? I mean, they answered the questions and were direct, but sometimes I feel like I have to explain myself in specific detail, especially with these phylogenetic questions.
    (4 votes)
    Default Khan Academy avatar avatar for user
    • aqualine ultimate style avatar for user Natasha G.
      Obviously I can't say for sure since I'm not an AP grader, but I've taken some practice AP tests, and used the actual AP key to grade my own tests, and the answers on that key were generally very simple and to-the-point. There was no floweryness at all. I think you really just need to state the answer and call it good. But be sure that you are actually answering the question. And even the things that seem really obvious are still important to write down. Hope that helps!
      (5 votes)
  • blobby green style avatar for user cassandra villalobos
    why is it bad to have a large amino acids sequence? can there be any harm if you have a lot of amino acid sequence?
    (3 votes)
    Default Khan Academy avatar avatar for user
    • female robot grace style avatar for user tyersome
      Proteins vary widely in size from <100 aa to >33,000 aa, so there doesn't appear to be a size that is too big.

      But, maybe I'm not understanding your question ...
      Why do you think it is "bad to have a large amino acids sequence"?
      What do you mean by bad?
      What do you mean by "large amino acids sequence"? — I was assuming you meant a long polypeptide ...
      (1 vote)

Video transcript

- [Voiceover] 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. And the way to read this is for 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. All right, 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. And 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. 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. And I'm just scanning this chart here and 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 in E. ferus 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. ferus and E. africanus. So, I'll do it here. E. E. ferus. And E. E. africanus. I'd do that there. Now where else do we see some low numbers. Well, I see this three right over here and so that's the number of sequence differences or the amino acid differences between G. gallus and A. forsteri. 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. And you can see when you compare D. polylepis to any of the other ones, you see a pretty big difference. D. polylepis to E. ferus is a 21 difference. 18 difference with G. gallus. 17 difference with A. forsteri and 20 amino acids are different 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. So, D. polylepis is most, is least related, I should say, is least related because it has a large amino acid, amino acid sequence difference from the other four. From the other, other four. Or you can say, 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. ferus to G. gallus, E. ferus to G. gallus, you still don't get close to 20, while D. polylepis is pretty close to 20 difference with all of them. And 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 as you do to get to D., the common ancestor with D. polylepis. All right, 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, what's the shape of their backbone or their different bones, or the shape of different parts of their body, while amino acid sequence, you're looking at, well, how are their proteins actually made up. I, personally, would go with the amino acid data. So, I, I believe. I believe, I believe amino acid sequence data are, data's plural, are more likely, are more likely to accurately represent the true evolutionary relationship, are more acc, are more likely to represent the true evolutionary relationship. True evolutionary relationship. And let me provide my reasoning. You could have convergent morphology. You can have convergent morphology and what that means is, for example, you could look at a bat's wings and a bird's wings and say, okay, look, they each 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 you 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 a convergent parts, I guess you could say. Had convergent morphology. You can have convergent morphology, morphology, while having very, while being far apart on the evolutionary tree. While being far apart on evolutionary, on evolutionary tree. Now there's arguments for morphology as well, 'cause 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 gel 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 could look at what's going on with the DNA, I like looking at that, because that doesn't, that allows you to not be tricked by the convergent morphology or far apart things, like bats and birds or dolphins and fish.