Modification of Existing Biological Systems
The Explanation
Biological innovation does not always require an entirely new component to appear at once. Existing genes and proteins can be modified. Genes can be duplicated and their copies can diverge. Proteins sometimes possess weak secondary activities that can be enhanced by mutation and selection. Regulatory changes can alter when, where, and how strongly existing genes are expressed.
Developmental systems also reuse many conserved genes and signaling pathways. Changes in their regulation and interaction can therefore contribute to anatomical differences without requiring a completely new collection of genes for every new structure.
Supporting Evidence
Research on duplicated genes documents several pathways by which copies can be retained and subsequently acquire specialized or altered functions. Laboratory studies also demonstrate changes in protein specificity and activity, while comparative developmental studies show that related regulatory components can be deployed differently in different organisms.
Limitations
Modification of an existing component is not the same question as explaining the original appearance of that component. Likewise, demonstrating changes in individual genes or regulatory elements does not automatically reconstruct the origin of an entire integrated biological system.
Questions That Remain
How much biological novelty can arise through modification and redeployment of existing components? When major innovations require changes to several interacting systems, how were those changes historically coordinated?
Intelligent Design Response
From an Intelligent Design perspective, reuse and modification of existing biological components are compatible with substantial evolutionary change. The further question is whether modification of existing systems adequately explains their origin and the emergence of new levels of organization.