Co-option and Evolutionary Pathways
Evolutionary explanations for molecular machines frequently involve co-option: existing components or systems acquire new functions or become incorporated into new functional arrangements.
The Investigative Question
Can complex molecular systems develop by recruiting and modifying components that previously performed other functions?
What We Observe
Proteins used in one biological system sometimes resemble proteins performing different functions elsewhere. Such relationships suggest that evolution can reuse existing molecular components rather than requiring every part of a new system to originate from nothing.
Scientific Background
Mechanisms relevant to the evolution of complex systems include gene duplication, divergence, changes in regulation, protein recruitment, recombination, loss of previously necessary components, and co-option of existing molecular structures.
Why It Matters
Co-option is one of the most important responses to arguments that every component of a modern machine would have needed to arise simultaneously. Earlier components need not have performed exactly the same functions they perform today.
What Is Known
Homologous proteins and related molecular systems provide evidence that biological components have repeatedly been modified and reused during evolutionary history.
What Is Proposed
For systems such as the bacterial flagellum, researchers have proposed evolutionary histories involving ancestral secretion systems, simpler motility structures, duplication, divergence, and recruitment of proteins with earlier functions.
What Remains Uncertain
Homology establishes historical relationship more readily than it establishes the exact order in which a complex system arose. Proposed pathways may therefore differ in detail even when common ancestry of components is well supported.
Key Numbers
Gene duplication, divergence, recruitment, recombination, and functional change are documented mechanisms by which existing biological components can enter new systems.
Design Relevance
Co-option is highly relevant to Intelligent Design because it directly addresses claims that coordinated systems cannot arise incrementally. A serious design investigation must evaluate co-option rather than assume that modern components always had their present roles.
Assessment
Evolutionary reuse of components is well established. For any particular molecular machine, the remaining historical question is what evidence connects known mechanisms of reuse and modification to the proposed sequence by which that integrated system arose.