An introduction to biological systematics
An introduction to biological systematics

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An introduction to biological systematics

2.10 Chimps, gorillas and humans

Activity 9

Timing: 0 hours 10 minutes

Dr. Patterson uses a diagram showing alternative cladograms for humans, chimpanzees and gorillas (Figure 10) to summarise evidence supporting the hypothesis that chimps are our closest relatives. He also provides two reasons why this theory should be accepted.

Figure 10 Alternative cladograms for humans, chimpanzees and gorillas
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Dr. Colin Patterson
I've summarised the information on the relationships between us, chimps and gorillas. On each of the three possible cladograms, I've entered the characters supporting it, using the numbers down the right side of the table. You can check it through in detail later but, as you can see, there are twelve possible human / chimp synapomorphies, three possibles for the chimp / gorilla pairing, and four possibles for the human / gorilla pair. Now if we chuck out all the dubious ones, where outgroup comparison is ambiguous, there are nine unambiguous characters favouring the human / chimp relationship, and three each for the other two possibilities.
Given that evidence, we should accept the hypothesis that chimps are our closest relatives. And I'll suggest two different reasons why we should accept it.
The first brings in the principle of parsimony, or economy of explanation. There are nineteen characters in the table, and the hypothesis that chimps are our nearest relative explains twelve of them, as the result of common ancestry. The other two alternative hypotheses, the chimp / gorilla and human / gorilla pairings, each explain only three or four of the nineteen characters. Now, in using the principle of parsimony to choose between hypotheses, we aren't implying that evolution is parsimonious - that it goes by the shortest route. We don't know whether it does or not. Parsimony says nothing about evolution. It's simply a principle of rational explanation.
The second reason why we should choose the human / chimp hypothesis concerns probability. Given the three alternative trees, one of them supported by twelve characters, one by three, and one by four, you could do statistical tests to find out what chance there is that a wrong tree should be supported by so many characters. The test and the result will depend on the assumptions you make, but the people who published this table reckoned that the result is significant at about the 3% level.
So we can accept the human/chimp pairing. This doesn't mean we accept it unconditionally, but that, like every other hypothesis in science, it's subject to test. And specifically, the hypothesis predicts that other samples of characters will show the same distribution, with the majority of them favouring the human / chimp pairing.
We can get one test from another set of DNA sequences. They're from an immunoglobulin pseudogene and, again, they favour the chimp / human pairing by 3:1, and they bring the significance of the result below 1%, meaning odds of over 100:1 in its favour.
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