Cytochrome-C

Cytochrome-c is a protein that is found in most organisms; it is involved in electron transport in mitochondria.

About 70% of its 104 amino acid sequence is set in stone - if it changes, the protein does not work. A mutation that leads to such a change will lead to an organism that does not survive, will not reproduce and so the mutation is quickly lost.

The other 30% does not matter so much, and a change in one position can lead to a functional protein (though I would guess it is still restricted to a subset of amino acids), and so to an organism that survives, produces and passes on the mutation. Nevertheless, such mutations are very rare events for cytochrome-c, which means the variations between species are minimal, and this gives us a great insight into how they evolved, confirming the nested hierarchy indicated by morphology and genetics.

Here is a list of species (from here) indicating the number of differences in the amino acid sequence compared to that of humans.

  • Chimpanzee 0
  • Rhesus monkey 1
  • Rabbit 9
  • Pig 10
  • Dog 10
  • Horse 12
  • Penguin 11
  • Moth 24
  • Yeast 38

What this means is that since chimp and human lineages diverged, they has not been a single change in the cytochrome-c amino acid sequence. However, go back a bit further, 40 million years, when monkeys and primates split. At some point in one lineage a mutation caused a change, and we see one difference between us and Rhesus monkeys. At the other end of the scale, looking at yeast, we can see there are 38 difference, because there has been so much longer since humans yeast diverged. Those difference will be split down the two lineages; there are perhaps 25-30 mutations between us and the common ancestor.

The penguin result is interesting as it is so low, given how long ago we split from penguins? There is a consistent pattern across all birds that the rate of mutation is slowed, not just in cytochrome-c. The reasons for this are not clear, but this paper gives one view.

Look at table 3 in this paper, which goes beyond the simple list above, and shows difference between all the species, not just compared to humans. We can see that the three birds in the table have very few differences between them, and are clustered together.

In fact, we can see that for any group, animals in the group have small difference, and have large differences with animals outside it.

It exactly matches what we would expect from the nested hierarchy of evolution.


See also here:

https://www.biology-pages.info/T/Taxonomy.html

https://content.csbs.utah.edu/~rogers/evidevolcrs/lectures/molevol-2x3.pdf


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