Complexity is a problematic term that begs definition. Four distinct categories of complexity describe structure and function at different levels of biological organization, from the molecular to the ecological (McShea 1996):

1 The number of different physical parts (for example, genes, cells, organs, or organisms) in a system.

2 The number of different interactions among these parts.

3 The number of levels in a causal specification hierarchy.

4 The number of parts or interactions at a given spatial or temporal scale.

In evolving, life has climbed increasing levels of complexity in each of these categories. This is most obvious from simple measures of cell number and type (Table 8.1). From unicellular ancestors, multicellular forms have evolved many times in different lineages (Figure 8.1). Thus, there have been increases in the number of cell types both global (for example, bacteria to vertebrates) and within lineages (for example, animals and the green algae and green plant clade). Interestingly, the maximum number of cell types in general plateaus in bacteria at three, in protists at about four, in protostomes at about 50, and perhaps in vertebrates, too (Carroll 2001). The number of genes has also increased during evolution of macroscopic forms from unicellular ancestors, but the quantitative relationship between cell-type number and gene number is unclear at present.

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