Earlier, we looked at different parts of the genome (Y-chromosome vs. the rest), and compare their rates and divergences to test of common ancestry. That comparative work removes a great deal of uncertainty and the unknowns, and makes the results much more accurate. It is a very well controlled experiment.
Very closely related to this conversation is some work that Stephen Shaffner did. His angle uses the same formula (T = MR), but focuses on different types of mutations, and the relative differences in rates. He looks at the very tight correlation between the variation among humans, and the differences between chimps/humans, and other species too.
The key point about this analysis is that it does a much better job controlling for uncertainties in T and R. We know T is hard to tell from the fossil record. We know that R varies by type of mutation (we can measure this in the lab). So instead of trying to infer T, we can just see if the relative rates ® match up with the relative differences (D) for different classes of mutations. That is why the fit is so tight on these graphs. It is a much better controlled experiment.
From here, we can see that there are several lines of converging evidence, that are all correlated with each other.
- Mutation rate (the rate of change)
- Variation (difference between genomes of the same species)
- Divergence (difference between genomes of different species)
All these things are tightly correlated. Without common ancestry, there is no reason divergence should be correlated with these things.
Human variation, which is the cumulation of a lot of mutations, looks just like divergence (human with other species) as if it is the cumulation of a lot of mutations too. That is the key point. It just looks like the differences are the result of the cumulation of a lot of mutations by the same process.
See the data below…
This carries over to other species divergences too.
