The mechanism of the evolution of the complex blood clotting cascade is understandable when it is observed that different components did not appear all at once but developed over a period of 400 million years and that most of the proteins involved are closely related serine proteases whose similarities are accounted for by gene duplication and whose differences are accounted for by the shuffling around of a small number of different functional domains. [p126]
Is the clotting cascade irreducibly complex?
How did the cascades evolve? In attempting to defend himself against the criticisms of his use of the blood clotting system as an example of irreducible complexity, Behe criticized the invoking of gene duplication and exon shuffling as "so much hand waving." He claimed that gene duplication was irrelevant since it was not evidence per se for natural selection. This claim ignores the fact that the gene duplication and natural selection are extremely interwoven concepts. As described previously, the power of gene duplication is that since there are now two copies of a gene, one copy can continue to carry out its original function. Natural selection insures that any mutations of the original gene would not be tolerated since they would likely reduce the fitness of the organism. At the same time, freedom from elimination by natural selection allows the duplicated gene to experiment with a number of random mutations. If, by chance, those changes provide a new function that is advantageous to the organism, natural selection now increases the fitness of the organism. Thus, gene duplication has natural selection written all over it. [p126]
