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Hey guys can you help me with this one!?

What happens between two nodes (in an evolutionary tree) or a between a node and a tip?

A) at least one character changes

B) all organisms die

C) Nothing can happen between two nodes

D) Monophyly, Paraphyly, Polyphyly

E) None of the above

I came down to E) and C). Since B) doesnt even make sense, organisms don't die among two nodes- they evolve. D) it is irrelevant to the question asked A) I think that probably is wrong as well, since among a node and a tip, the organism stays the same and finally I thought C) could be the answer, but at the same time a new trait could be formed,

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  • $\begingroup$ Thank you, I did remove them. And yes I did try to solve it my self before I post the question on the internet. I wasn't aware that I have to mention what are my thoughts. $\endgroup$
    – ANNA MARIA
    Commented Dec 12, 2018 at 20:37
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    $\begingroup$ The organism most definitely does not stay the same between internal node and tip(terminal node). the only way that can happen is if no evolution has occurred (ancestor and descendant identical) in which case you are not looking at an evolutionary tree. $\endgroup$
    – John
    Commented Dec 15, 2018 at 5:30

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Your answer is A. for one simple reason, two different nodes represent at least one notable/measurable difference between groups, so between any two nodes there must be at least one difference.

Look at this example, whether A and B represents Mammals and Birds or two subspecies of chimp, or two strains of virus the fact they are two separate nodes means there is some difference between them. When constructing a tree there has to be at least one character difference to have more than one node. A tree with no differences is just a dot by itself.

Also an organisms does not stay the same between a node an the tip (the tip is a node) to use the below example the starred node between A and B is the most recent common ancestor of A and B which is not the same thing as A or B.

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  • $\begingroup$ Yes I understand that. But on occasion the node it could be a result of a population immigrating on a different location. That doesn't mean that a new characteristic arose... It does?! $\endgroup$
    – ANNA MARIA
    Commented Dec 13, 2018 at 19:59
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    $\begingroup$ New and arose are not part of the question nor the definition of a character. Answer A is"at least one character changes" A character change be a loss, a different numerical measurement or color, a different location or time almost anything. A character is part of the data used to describe the specimens and that has to be a att least one character difference between nodes, nodes have to have a definable difference. without a difference the specimens will be grouped into the same node. $\endgroup$
    – John
    Commented Dec 13, 2018 at 21:47
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The question does not make much sense and I think you've gone about as far as anyone can go.

A) at least one character changes

It depends what it means by character and it depends the detail semantic of the tree. In general, an evolutionary tree, usually called a phylogenetic tree attempts at representing time by the length of the edges in between two nodes. The goal is, in general, not to represent anything else.

In the comments, @John has argued that time can be consider as a character. Personally, when I read the term "character" I think of a phenotypic character. While it will pretty much always be true that among two nodes some difference will exist in the phenotypic character, it is not a fundamental consequence of the concept of evolutionary tree. This is why I argued that A is false. @John takes the opposite stance in his answer. Note also, that sometimes (although quite rarely today) phenotypic states are used to build a tree and hence, in such case, it would be a direct consequence of the methodology that two nodes must necessarily differ by the value of at least one phenotypic character.

This disagreement between @John and I highlights that the question is poorly phrased and actually makes little sense.

B) all organisms die

Well, most likely enough time separate the two nodes that all organisms that were represented in the ancestral node are dead but of course they have reproduced, so that we actually have another node that descend from it.

C) Nothing can happen between two nodes

As you said, they evolve. So, things happen!

D) Monophyly, Paraphyly, Polyphyly

As you said, those terms make no sense here!

E) None of the above

If there is any intuitive answer to the question what has happened between two nodes is time has passed. I think the answer your teacher was expecting here is E but it is also possible (s)he was expecting A. Really that's a pretty poorly written question.

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  • $\begingroup$ I know, all the given questions, are like that!! Thank you so much for responding, Ecology and evolution is somewhere very far back in my undergrad education :) $\endgroup$
    – ANNA MARIA
    Commented Dec 12, 2018 at 23:02
  • $\begingroup$ @ANNA FYI, if you consider that the answer has answered your question you can click on the checkmark next to the answer (just below the downvote/upvote buttons). That being said, I would recommend waiting a little bit to see if other users seem to approve with this answer. $\endgroup$
    – Remi.b
    Commented Dec 12, 2018 at 23:06
  • $\begingroup$ A) has to be true, because for there to be two nodes the two different nodes must represent some difference. $\endgroup$
    – John
    Commented Dec 13, 2018 at 17:54
  • $\begingroup$ @John While it will pretty much always be true, it does not have to be. The only thing that must separate two nodes is time. It is also very much possible that the only change in between two nodes relate to synonymous mutations (and other mutations that has no effect on the phenotype), hence not causing a change in any character fo interest. I have clarified that in my answer. $\endgroup$
    – Remi.b
    Commented Dec 13, 2018 at 18:09
  • $\begingroup$ @Remi.b Time is a character a character is part of data for a specimen, It can also be location. Characters do not need to be genetic the best ones are but they do not have to be. You are confusing a character with a characteristic. $\endgroup$
    – John
    Commented Dec 13, 2018 at 21:52

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