Wednesday, March 12, 2003

How do Complex Systems Evolve? The March 4, 2003 issue of the Proceedings of the National Academy of Sciences contains this article addressing the evolution of circadian clock genes in prokaryotes. This refers to a cluster of three genes that regulate certain physiological processes that occur at regular, often daily intervals. Prokaryotes are cells that lack internal organelles, such as a nucleus. Bacteria are the most common examples of such cells. They are contrasted with the more complex eukaryotic cells, which contain nuclei and other organelles. Clock genes are ubiquitous in eukaryotes, but until recently they were found only in one small group of prokaryotes, the cyanobacteria.

The article presents compelling evidence that homologs of the cyanobacteria clock genes exist in other prokaryotes, namely the Archaea and Proteobacteria. Their research enabled them to reconstruct a plausible scenario for how these genes evolved from simpler precursors via familiar processes such as natural selection, gene duplication and lateral gene transfer.

Another Blow for Intelligent Design The latest wave of creationism known as "Intelligent-Design" Theory (ID) offers two main scientific arguments in defense of their views. One is that certain biological structures exhibit "irreducible complexity", meaning that they are built from several parts in such a way that the removal of any one part causes the structure to cease functioning. This is said to pose an insurmountable challenge to evolution by natural selection, because such a structure allegedly could not form by gradual improvements in smaller, precursor systems. The second argument is that natural processes can not substantially increase the information content of the genome, a feat that would be required for evolution to be a viable theory.

Critics have pointed out numerous flaws in both arguments, and the present paper gives further empirical support to these critiques. The researchers studied a particular, three-gene complex, the kai genes, that regulates circadian rhythms in prokaryotes. They observe that "Comprehensive study of the kaiABC cluster expression in Synechococcus sp. PCC7942 showed that all three kai genes are essential for circadian rhythmicity, and inactivation of any of them completely abolishes it." Thus, this system satisfies the definition of irreducible complexity. Nonetheless, they go on to observe that the present indispensability of the three parts in modern prokaryotes does not imply that simpler systems could not have existed in earlier organisms, and they provide evidence to back up this assertion.

As for how information grows, the researchers argue forcefully that gene duplication with subsequent divergence accounts for part of the formation of the three-gene system from precursors possessing one or two parts. An impressive increase in information by anyone's standards. This is just one more example of why the arguments offered by ID proponents are flawed both in theory and in practice. It also illustrates how evolutionary theory continues to produce results in quotidian scientific work.


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