2.9 Conflicting morphological characters
Activity 8
Timing: 0 hours 10 minutes
This clip looks at conflicting morphological characters and at how it is possible to resolve them, with the aid of a table of molecular characters (Figure 9).
Dr Patterson also explains how to determine whether a nucleotide shared by two or more species is derived or primitive.
Figure 9 Nucleotides at selectived positions (left column) in sequences of non-coding DNA in the region of the beta haemoglobin family of genes in various higher primates. Asterisks denote gaps in the sequences of the species concerned. Based on Williams, S. A. and Goodman, M. (1989) A statistical test that supports a human/chimpanzee clade based on non-coding sequence data, Mol. Biol. Evol., 6, 325–330
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Dr. Colin Patterson
So here we've got conflicting morphological characters, and all of them seem to be derived. They can't both be true, so how do we resolve the conflict?
I’ve given you a table of molecular characters that help to resolve it. These are selected positions in an alignment of over ten thousand nucleotides, from non-coding DNA in the region of the beta haemoglobin genes. We don't yet know the DNA sequence for this region in gibbons, but the table includes the other four apes, and also a couple of monkeys - the Rhesus monkey, Macaca Mulatta from the Old World, and the Spider monkey, Ateles, one of the New World monkeys from South America.
I want to use this table as an example in tackling the particular question of the conflict on chimpanzee relationships, but also in tackling the more general question of deciding on primitive and advanced characters. The method I've just been describing, using ontogeny or development to resolve general and special features in morphology, obviously won't work with DNA, because DNA has no development. Barring accidents, we're born and we die with the same DNA sequences in our chromosomes.
So how can we determine whether a nucleotide shared by two or more species is derived or primitive? Take the first row in the table, the one numbered 34. You'll see that five of the animals have G, Guanine, at that position, but human and gorilla share A, Adenine. This implies that human and gorilla are related, and the criterion we're using to make that guess is called outgroup comparison. We've already found, or others have found, a whole series of characters saying that apes form a group and that, within the apes, humans, chimpanzees and gorillas form a subgroup. Then there's another series of characters showing that the closest relatives of apes are the Old World monkeys, with Rhesus monkey as an example in this table. And then there's another set of characters showing that the next group out is the New World monkeys, with Spider monkey as the example here. So outgroup comparison tells us that for the first character in the table, Guanine is primitive and Adenine is derived.
So that shared adenine, in the first row of the table, implies that gorilla is our nearest relative. Are there any other shared nucleotides supporting that idea? Yes, there are two more. There's one at position 6368, where human and gorilla share Cytosine, C, and all the others have Thymine, T. And then another near the end of the table at 9441, where human and gorilla have Guanine, G, and all the others have Cytosine, C. There's also an Adenine shared by human and gorilla at position 9324, but here outgroup comparison shows that this could well be primitive, because both the monkeys have Adenine too.
Now look at the second row in the table, position 560. Here human and chimp share Cytosine, and all the others have Adenine. So this is a site at which outgroup comparison says that chimps are our nearest relatives. Are there any other positions supporting that idea? Yes, there are lots. The third row, position 1287 is an example. Here, human and chimpanzee share an asterisk, which means they have a gap in the alignment - a gap that has to be put there to preserve matching in the neighbouring parts of the sequence.
The evolutionary interpretation of that gap is that it's a deletion of one nucleotide. Now this position is ambiguous by outgroup comparison, because although gorilla and orang have Thymine there, the Rhesus monkey also has a gap, and the Spider monkey has a hyphen, meaning "missing". The Spider monkey lacks this whole part of the sequence, which is an inserted repeat.
But never mind that one, because there are two more substantial deletions showing that chimps are our nearest relatives, the four-base deletion at position 3057, and the six-base deletion at 7227.
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