The Modern Molecular Clock

 

The first molecular phylogenetic trees were constructed from the amino acid sequences of small proteins. One of those proteins was cytochrome c and it turned out to be very useful because homologues could be found in all species, including bacteria.

The original trees were published by Emanual Margoliash but I'm showing a later version here from Fitch and Margoliash (1967). This is a very famous tree that's found in many textbooks. (The version shown here is from Mulligan (2008).)

From the very beginning, the authors of these molecular phylogenetic trees noted that the rate of change in each lineage was approximately constant. You can see that in the tree shown here. The number of changes in the lineage leading to yeast (Saccharomyces) is 17+10+2=31 from the common root. The number of changes in the lineage leading to insects is 31 or 28, depending on the species. The number leading to humans and monkeys is 32.

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