The tree shown depicts phylogenetic relationships among...

The tree shown depicts phylogenetic relationships among known strains of HIV and the strains sampled from infected children at Benghazi Hospital in Libya. This phylogeny indicates that the Benghazi cluster is a ___ since all of the strains from the infected children share a more recent common ancestor with each other than any share with other strains in the tree. The known strain that is most closely related to the Benghazi cluster is from __, the country of origin of many migrants to Libya. This result suggests that the infection was transmitted by ___

Transcript

00:01 Okay, so we're trying to be detectives by using phylogenetic trees for this question.

00:09 So we're looking at how different strains of hiv related to each other in this phylogenetic tree to determine if a nurse got infected from a patient or not.

00:19 So when we're looking at a phylogenetic tree, it's really important to know how to read them.

00:25 So the ends of the branches represent the present, the bottom, and then as you go away from the branches that represents the past.

00:33 So when we're looking at geophiligenetic trees, what's really important are the connections between those branches.

00:40 So things that are really closely related are going to have nodes closer to the present or in this case the top of the tree.

00:47 Whereas if you start at the end of the branches and trace down, things that are more distantly related are going to have connections further down or further in the past.

00:57 So we're going to use this information to figure out where she got it.

01:01 Hiv from.

01:02 So, all the p's represent the patient strains.

01:07 Hiv evolves really quickly.

01:09 So even within a person, there can be different versions or strains of that virus.

01:14 N represents the nurses, and l represents the population of the law of it, louisiana.

01:19 So we should tell them that the hiv came from the patient because the population of hiv is much more diverse.

01:27 No, we do not care about how diverse it is.

01:31 It might be.

01:31 Might just mean that patient has had hiv for longer, which is why we've had more of this divergence.

01:37 That doesn't tell us about relatedness, right? if the patient gave the nurse hiv, we'd expect their strains to be closely related to each other.

01:46 So that's not helpful.

01:48 B, it appears unlikely the nurse got hiv from patient x because each is monophyletic and thus has an inferred ancestor different from the other.

01:56 So monophily means, right, all of the descendants came from the same ancestor.

02:05 So i think of it as a snip test.

02:08 So if i, for the patients, it is monophyletic.

02:11 If we cut right here, all of the branches would fall off together...

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