Molecular evidence for evolutionary relationships: Examples

0:00- [Instructor] An investigation was carried out on four different plant species to determine which of three species was most closely related to an unknown plant species. The results of the investigation are shown in the data table below. Which plant species appears to be most closely related to the unknown species?

0:19Support your answer. So pause this video and see if you can figure that out. Alright, so here in this data the first row is for our unknown species. We know that it tests positive for Enzyme M. This is blanked out because this says, how many differences in amino acid sequences are there between the species in that row and the unknown species, and of course the unknown species would have zero differences 'cause it is itself.

0:44And then this is the gel electrophoresis banding pattern. So what we wanna do is look for as many similarities as possible to our unknown plant species. So first, if we looked at the enzyme, whether we test positive for it, species one does not test positive for it, species two and three do.

0:59So species one is not looking good. If we look at differences in amino acid sequences, species one, once again, has the most differences in amino acid sequences, so that confirms our belief that it might be the most different from the unknown plant species. But then if we look between plant species two and three, plant species two has fewer differences.

1:21So right now, plant species two is looking like the leading candidate. Plant species three has more differences. And then when we look at the gel electrophoresis banding pattern, plant species two has the exact same pattern as our unknown plant, and it's the only one with the exact same pattern.

1:37And so because of that, I'm feeling really good saying that two is most closely related to the unknown species. Two, because same banding pattern, and we could say, and least different amino acid sequences, and tests positive for Enzyme M. Let's do another question like this.

2:27Scientists attempted to determine the evolutionary relationships between three different finch species, A, B, and C. In order to do this, they examined the physical characteristics and DNA of these species. DNA was extracted from all three species and analyzed using gel electrophoresis.

2:47The results are shown in the diagram. And then they ask us, which statement best describes the method used above to determine the evolutionary relationships between three species of finches? So the first choice is, examine the structure of the beaks and compare them.

3:07Statement two, observe behavioral and physical characteristics of all the finches and group them by similarities. C, obtain molecular evidence from all three species and identify similarities. Four, compare common ancestors of all three of the species and see if they are the same.

3:24So pause this video and see which choice you like. Alright, well clearly they're looking at molecular evidence. They're taking the DNA of the different species, they're amplifying them, which means they're making a bunch of copies of certain sequences of that DNA, and then they're running a gel, which means they allow those different segments to separate on this gel.

3:47The smaller segments are going to migrate down further, the larger segments are going to migrate less far. And the more similarities you have on this gel electrophoresis right over here, this banding pattern, the more similar that DNA probably is. And so they're definitely using molecular evidence, and three is the only one that talks about molecular evidence, so I will go with choice three.

4:13They did say in the beginning that they examined physical characteristics and DNA of these species, and so you could maybe say examine the structure of the beaks and compare them, but that's very specific, and it didn't say for sure that they're doing that, that may be one of the physical characteristics that they're looking at, but we know for sure that they're obtaining molecular evidence from all three species, and they're identifying the similarities.

4:41So now, this next part, it says, based on the data they collected using gel electrophoresis, label the branching tree diagram below, writing the letters A, B, and C to represent the possible evolutionary relationships between species A, B, and C. So pause this video and see if you can figure this out, and I'll show you the banding pattern again.

5:04Alright, so down there, and actually I'll redraw it up here, they draw a tree that looks like this, and they want us to put A's, B's, and C's in these blanks here. And one way to think about it is, these two are more closely related. They have a more recent common ancestor than from this one over here.

5:26So if we look at the banding patterns, it's pretty clear we have this similar band on A and B, we have this similar band on A and B, and they have this band right over here in common, although C here has that one as well. The only other band that A shares in common with C is this one right over here, and B doesn't share anything else in common with C.

5:51So it's pretty clear that A and B share the most common banding patterns. So I would write that A and B have the most recent common ancestor, and that C is more distant.