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transcription, the detailed structural organization of enhancer/silencer modules clearly establishes their activity as on/off switches (Davidson, 2001). In effect, mutational events at both the protein domain and transcription module levels must exert influences “upward” in the sequence of molecular interactions while selection must influence which ones are preserved and then amplified within the population in which those mutational events first occur. The relationships between these modular levels and the screening activity of natural selection is diagrammed in Fig. 4.2. If this point of view is validated by further analysis and experimental findings, it must be taken into account in the continuing debate about “levels of selection.” Thus, below the level of the individual organism (the primary Darwinian “unit of selection”) there are not just genes (as in the traditional levels-of-selection argument) but additional levels of genetic–molecular organization, namely genetic networks and their modules, that natural selection actively screens.

To sum up: if one accepts this view of a molecular sequence of upward interactions, with the effects of mutational events feeding through from the DNA sequence level, at the lowest modular levels, to that of network modules and networks, and selection screening downward through this

FIGURE 4.2 A diagrammatic representation of the effects of mutations at each modular level and the ways these transmit upward, within the developing organism, to affect the next modular level of organization.

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