The principle of parsimony is commonly used to determine the minimum number of genetic changes required to account for the amino or nucleotide sequence differences between organisms sharing a common ancestor. The evolutionary distances separating organisms in a phylogenetic tree are usually expressed in units of nucleotide mutations or amino acid substitutions along each arm of the tree between branch points (see Figure 12-1).
Some evolution rates (point mutations per 100 million years):
Triose phosphate isomerase = 3
Hemoglobin = 21
Nonfunctional pseudogenes = 400
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