Possible Explanation

Irreducible Complexity and the Design Argument

Molecular Machines and Integrated Biological Systems • Irreducible Complexity

The argument from irreducible complexity claims that some systems require several interacting parts before their present function can operate, raising questions about how they could arise through successive selectable stages.

The Explanation

Michael Behe introduced the term "irreducible complexity" for systems composed of interacting parts in which removal of a necessary component causes the system to lose its basic function. The bacterial flagellum became one of the best-known examples in the Intelligent Design debate.

The argument is not simply that a system is complicated. It asks whether a functional evolutionary pathway exists in which earlier stages can perform useful functions before the complete modern system is present.

Supporting Evidence

Some molecular systems do depend upon numerous coordinated components. The flagellum requires structural, regulatory, assembly, and motor functions. Intelligent Design advocates argue that such interdependence creates a significant explanatory problem for gradual evolutionary accounts.

Limitations

Loss of the present function when a component is removed does not prove that earlier forms could not have possessed different functions. Co-option, redundancy, scaffolding, gene duplication, and changes in function can potentially produce pathways that are not revealed by simply removing parts from the modern system.

Questions That Remain

Can proposed evolutionary histories demonstrate plausible and sufficiently supported functional stages for systems claimed to be irreducibly complex?

Intelligent Design Response

Irreducible complexity is most useful as an investigative question rather than a shortcut to a design conclusion. It directs attention to the origin of coordinated function and asks whether proposed natural pathways adequately explain the system rather than merely asserting that a pathway must have existed.

Research Sources

Behe — Darwin's Black Box and Irreducible Complexity

Michael J. Behe • Free Press • 1996

Provides the principal Intelligent Design formulation of irreducible complexity.

Behe argues that some molecular systems depend upon multiple interacting components and challenges gradual evolutionary accounts to provide functional pathways to such systems.

Design Argument

Pallen and Matzke — Evolutionary Origin of Bacterial Flagella

Mark J. Pallen; Nicholas J. Matzke • Nature Reviews Microbiology • 2006

Provides a detailed evolutionary response to the use of the bacterial flagellum as an example of irreducible complexity.

Pallen and Matzke argue that comparative evidence, component homologies, co-option, and evolutionary mechanisms provide a framework for reconstructing flagellar evolution.

Evolutionary Response

Liu and Ochman — Stepwise Formation of the Bacterial Flagellar System

Renyi Liu; Howard Ochman • Proceedings of the National Academy of Sciences • 2007

Provides a proposed stepwise evolutionary reconstruction of bacterial flagellar development.

Liu and Ochman use comparative genomic evidence to infer a history involving duplication and modification of flagellar components, providing a concrete evolutionary proposal relevant to the irreducible-complexity argument.

Stepwise Reconstruction