We Can Use Genetic Differences among Populations or Species to Reconstruct Evolutionary History Early in this chapter, we noted that the course of evolutionary history involves the transformation of one species into another and the splitting of species into two or more new species. Our examples have demonstrated that speciation is associated with changes in the genetic structure of populations and with genetic divergence. Therefore, we should be able to use genetic differences among species to reconstruct their evolutionary histories. In an important early example of phylogeny reconstruction, W. M. Fitch and E. Margoliash assembled data on the amino acid sequence for cytochrome c in a variety of organisms. Cytochrome c is a respiratory pigment found in the mitochondria of eukaryotes, and its amino acid sequence has evolved very slowly. For example, the amino acid sequence in humans and chimpanzees is identical, and humans and rhesus monkeys show only one amino acid difference. We should expect little evolution in a protein whose proper function is essential for fitness, but such utter similarity is remarkable, considering that the fossil record indicates that the lines leading to humans and monkeys diverged from a common ancestral species approximately 20 million years ago. Column (a) of Table 26.4 shows the number of amino acid differences between cytochrome c in humans and a variety of other species. The table is broadly consistent with our intuitions about how closely related we are to these other species. For example, we are more closely related to other mammals than we are to insects, and we are more closely related to insects than we are to fungi. Likewise, our cytochrome c differs in 10 amino acids from that of dogs, in 24 amino acids from that of moths, and in 38 amino acids from that of yeast. Table 26.4. Amino Acid Differences and the Minimal Mutational Distances between Cytochrome c in Humans and Other Organisms (a) (b) Organism Amino Acid Differences Minimal Mutational Distance Human 0 0 Chimpanzee 0 0 Rhesus monkey 1 1 Rabbit 9 12 Pig 10 13 Dog 10 13 Horse 12 17 Penguin 11 18 Moth 24 36 Yeast 38 56 Source: From W.M. Fitch and E. Margoliash, Construction of phylogenetic trees, Science 155:279-84, 20 January 1967. Copyright 1967 by the American Association for the Advancement of Science. However, more than one nucleotide change may be required to change a given amino acid. When the nucleotide changes necessary for all amino acid differences observed in a protein are totaled, the minimal mutational distance between the genes of any two species is established. Column (b) in Table 26.4 shows such an analysis of the genes encoding cytochrome c. As expected, these values are larger than the corresponding number of amino acids separating humans from the other nine organisms listed.