1.5 The Meaning of a Hierarchy 23
Figure 1.12 Some gene families. The first row belowGenein this classification con-
sists of superfamilies. The row below that contains families. Below the families are
some individual genes. See (Cooper 1999), Chapter 4.
individual genes shown in the diagram are members (also calledinstances)
of their respective families rather than being subsets. However, the nuclear
receptor gene diagram in figure 1.13 illustrates that the distinction between
subclass and instance is not very clear-cut, as the entire superfamily evolved
from a single ancestral gene. In any case, the relationships in this last dia-
gram are neither subclass nor instance relationships but rather more complex
relationships such as:evolves by mutation,duplicates,andtranslocates.
Although hierarchical classification is an important method for organiz-
ing complex information, it is not the only one in common use. Two other
techniques are partitioning and self-organizing maps. Both of these can be re-
garded as classification using attribute values rather than hierarchical struc-
tures. Inpartitioning, a set of entities is split into a specified number of subsets
(MacQueen 1967). Aself-organizing mapis mainly used when a large num-