Figure 10.15 Numerical Taxonomy Seven strains of bacteria were tested for 100 different traits, each test producing a positive or negative result. Each strain was then compared with the other strains by determining the similarity coefficient, the percentage of the total positive characteristics tested held in common in each of the strains. Note that each strain holds all characteristics (100%) in common with itself. In (a), the strains are arranged randomly; in (b), they are arranged so that the organisms with similar properties are grouped together. Note that the seven organisms fall into two unrelated groups, designated here as A and B, each of which is probably composed of a single species.

The greater the number of positive characteristics that two organisms have in common, the more closely they are related. After numerous characteristics are examined in a variety of strains, a computer program is used to calculate a similarity coefficient for each pair of strains and to construct a similarity chart. On the basis of this chart, the strains can be ordered in such a way that those that have more than a 70% to 80% similarity coefficient are classified as a single species, while other more distinct groups are classified as separate species and perhaps even different genera.

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