If this is correct, the cytochrome c of all mammals should be equally different from the cytochrome c of all birds.
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Tip calibration makes use of all relevant fossil taxa during clock calibration, rather than relying on only the oldest fossil of each clade.
This method does not rely on the interpretation of negative evidence to infer maximum clade ages.
Determining the maximum clade age is challenging because it relies on negative evidence—the absence of older fossils in that clade.
There are a number of methods for deriving the maximum clade age using birth-death models, fossil stratigraphic distribution analyses, or taphonomic controls.
By allowing all aspects of tree reconstruction to occur simultaneously, the risk of biased results is decreased.
This approach has been improved upon by pairing it with different models.
If most changes seen during molecular evolution are neutral, then fixations in a population will accumulate at a clock-rate that is equal to the rate of neutral mutations in an individual.
The molecular clock alone can only say that one time period is twice as long as another: it cannot assign concrete dates.
Unlike node calibration, this method reconstructs the tree topology and places the fossils simultaneously.