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I understand the difference between cladograms and phylograms; the latter can show the evolutionary linkage over time. However, I am having trouble finding out which two organisms are most closely related.

enter image description here

In the image above for example:

Referring to the cladogram, would these following statements be correct

-Bacteria 1 is least related to Eukaryote 4 (the furthest branch is always the least related as there common node is furthest away, correct?)

-The most related organisms are Bacteria 2 and 3, and Eukaryote 2 and 3

Referring to the phylogram

-Is Eukaryote 2 the least related to Bacteria 1 as its branch is the furthest away? Or is it Eukaryote 4 as it is the furthest away (bottom of list)?

-Is Eukaryote 1 most closely related to Eukaryote 3?

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  • $\begingroup$ I hate to be pedantic, but it should be bacterium (singular) rather than bacteria (plural). I know few people know any Latin any more (including the originator of the diagram), but just so you know next time. After all science is about precision. $\endgroup$ – David Apr 21 '16 at 19:10
  • $\begingroup$ @David I didn't make the image, but doesn't it work either way since there are many organisms of each species... $\endgroup$ – city7lights Apr 22 '16 at 14:24
  • $\begingroup$ Yes, I know you didn't make the image, and appreciate your difficulty. (I would have modified it myself, but I do that sort of thing.) However I think you would always use the singular (as in eukaryote). For example if you substituted 'animal' or 'microorganism' where the plurals are English, 'animals 1' or 'microorganisms 2' would strike one as odd. You certainly wouldn't want to mix them. $\endgroup$ – David Apr 23 '16 at 7:59
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-Bacteria 1 is least related to Eukaryote 4 (the furthest branch is always the least related as there common node is furthest away, correct?)

In terms of relative relationships, eukaryote 4's least-related relative is Bacteria 1. Bacteria 1's least-related relative is the entire rest of the tree

-The most related organisms are Bacteria 2 and 3, and Eukaryote 2 and 3

You cannot actually answer this question because a cladogram only shows relative relationships. To know which species pair is the most related (has the least total distance between them), you would need to know the branch lengths.

-Is Eukaryote 2 the least related to Bacteria 1 as its branch is the furthest away?

To determine relationships, you want to look at the total branch length between two species. Therefore, this is correct

Or is it Eukaryote 4 as it is the furthest away (bottom of list)?

Avoid this way of thinking. A phylogeny is not a 'list'. Pay no attention to the order the species labels appear: you only want to pay attention to how the species are connected. The same phylogenetic tree could be drawn differently, placing Eukaryote 3 at the bottom, for example.

-Is Eukaryote 1 most closely related to Eukaryote 3?

Eukaryote 1's closest relative is Eukaryote 4: the distance from Eukaryote 1 to Eukaryote 4 is the shortest out of all possible pairs with Eukaryote 1.

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