Possible Explanation

Can Molecular Machines Be Built Step by Step?

Molecular Machines and Integrated Biological Systems • Comparative Assessment

The central issue is whether known evolutionary mechanisms provide an adequate account of the origin of complex molecular machinery or whether some systems provide positive evidence of intelligent organization.

The Explanation

The debate cannot be resolved merely by pointing to the complexity of a molecular machine, nor by showing that some of its components have evolutionary relatives. Both observations matter, but they address different parts of the question.

A satisfactory evolutionary explanation should identify plausible precursors, functional intermediate stages, mechanisms of change, and evidence connecting those stages. A satisfactory design argument should identify positive characteristics of intelligent causation rather than depend only upon gaps in an evolutionary reconstruction.

Supporting Evidence

Studies of the bacterial flagellum provide a useful test case. Comparative genomics identifies relationships among flagellar components and other systems, and researchers have proposed histories involving duplication, modification, and co-option. At the same time, the flagellum remains a highly integrated system whose assembly, regulation, energy use, and function require extensive coordination.

Limitations

Ancient evolutionary histories cannot normally be replayed directly. Proposed pathways vary in evidential support, and incomplete reconstruction does not by itself establish design. Conversely, identifying homologous components does not by itself provide a complete historical account of how an integrated machine arose.

Questions That Remain

What evidence would allow us to distinguish an evolutionary pathway that is merely possible from one that is historically well supported, and what positive evidence would justify adding intelligent causation to the explanation?

Intelligent Design Response

A Visit With Jesus considers molecular machinery an important part of the design question because cells contain extraordinary functional organization. We should acknowledge demonstrated evolutionary mechanisms and proposed historical pathways while still asking whether those mechanisms provide an adequate explanation for the origin of the coordinated systems themselves.

Research Sources

Behe — Darwin's Black Box and Irreducible Complexity

Michael J. Behe • Free Press • 1996

States the Intelligent Design challenge concerning whether complex molecular systems can arise through selectable intermediate stages.

Behe supplies the design-side question that a stepwise evolutionary account must address: whether the relevant intermediate arrangements could perform useful functions while the modern system was being assembled.

Design Challenge

Liu and Ochman — Stepwise Formation of the Bacterial Flagellar System

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

Provides a specific reconstruction of stepwise flagellar evolution using comparative genomic evidence.

This study is valuable because it moves beyond a general assertion that evolution could produce a complex system and proposes an actual historical sequence inferred from relationships among flagellar proteins.

Evolutionary Reconstruction

Abby and Rocha — Evolution of Type III Secretion from the Flagellum

Sophie S. Abby; Eduardo P. C. Rocha • PLoS Genetics • 2012

Provides additional comparative evidence concerning the evolutionary history of flagellar and secretion machinery.

The study supplies evidence relevant to component reuse, system relationships, and the reconstruction of evolutionary histories for complex bacterial molecular machinery.

Comparative Evidence