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...