Question & Objection

Does the Bacterial Flagellum Have an Evolutionary History?

Molecular Machines and Integrated Biological Systems

The bacterial flagellum has often been presented as a major example of irreducible complexity. However, comparative genomics has identified relationships among flagellar proteins, evidence of gene duplication, and connections with other bacterial molecular systems.

Researchers have proposed stepwise histories in which an ancestral core was modified, duplicated, and expanded over evolutionary time.

Why This Matters

If the major example used to illustrate irreducible complexity has a well-supported evolutionary history, that would weaken the claim that its organization requires intelligent causation.

Response

The evolutionary evidence should be taken seriously. Comparative studies support common ancestry among flagellar components and provide evidence for duplication, divergence, loss, transfer, and modification. This makes a simple claim that "the flagellum could not have evolved" too strong.

There is nevertheless a distinction between demonstrating evolutionary relationships and recovering every stage in the origin of the complete system. Ancient events must largely be reconstructed from surviving organisms and genomes.

Question That Remains

How much of flagellar evolution is strongly reconstructed from evidence, and which stages in the origin of its integrated machinery remain uncertain?

Research Sources

Liu and Ochman — Stepwise Formation of the Bacterial Flagellar System

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

Use: Stepwise Reconstruction

Relevance: Provides a specific phylogenetic reconstruction of stepwise flagellar evolution.

Source note: Liu and Ochman infer an ancient core and propose that core components arose through successive duplication and modification of precursor genes.

Pallen and Matzke — Evolutionary Origin of Bacterial Flagella

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

Use: Evolutionary Model

Relevance: Presents the case for viewing bacterial flagella as evolved rather than independently designed systems.

Source note: The paper brings together comparative and mechanistic evidence relevant to constructing evolutionary models of flagellar origin.

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

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

Use: Comparative Genomics

Relevance: Provides comparative genomic evidence concerning the history of flagellar-related secretion machinery.

Source note: The study helps establish historical relationships among complex bacterial systems and provides evidence relevant to their order of origin and diversification.