One of the newer claims of creationists has been the so called "Irreducible Complexity". The argument is that there are certain biological systems which feature several necessary parts. That is, some structures will not function if just one part is removed. This poses a problem for evolution because it must work in small steps: Things like the blood clotting system and the bacterial flagellum can't evolve all at once.
How must this be answered? Let's begin with the bacterial flagellum. Scientists have discovered something called the Type Three Secretory System which has a lot in common with the flagellum, but is much simpler. It is not used to propel through water, rather it is used to inject proteins into cells. Why is this important? Well, scientists are aware of a process called co-option. Co-option is when a system or part evolves for one function, and then evolves to take on another. For instance, cats have evolved a scratchier tongue for cleaning. This is the key to understanding Irreducible Complexity: The entire complex system cannot be generated in one go, but the individual parts can be. These parts serve a useful function, and natural selection will favor the individuals with these parts. Later on, they can be combined to become the flagellum. Nick Matze has written a step by step scenario for the evolution of the flagellum. Here is how it evolved:
What about the other so called "Irreducibly Complex" Structures? Let's take a look at the Blood Clotting Cascade. Biologist Kenneth Miller describes the evolution of Blood clotting:
"A primitive clotting system, adequate for an animal with low blood pressure and minimal blood flow, doesn't have the clotting capacity to present this kind of a threat. But just as soon as the occasional clot becomes large enough to present health risks, natural selection would favor the evolution of systems to keep clot formation in check. And where would these systems come from? From pre-existing proteins, of course, duplicated and modified. The tissues of the body produce a protein known as alpha-1-antitrypsin which binds to the active site of serine proteases found in tissues and keeps them in check. So, just as soon as clotting systems became strong enough, gene duplication would have presented natural selection with a working protease inhibitor that could then evolve into antithrombin, a similar inhibitor that today blocks the action of the primary fibrinogen-cleaving protease, thrombin."
Interestingly enough, dolphins and pufferfish lack some of the factors that Michael Behe claims are essential to the blood clotting cascade, yet their blood clots just fine (if slightly less efficient than ours).
Other cases of irreducible complexity have been claimed; yet they have all failed to be unevolvable on close examination:
For more information, see "Finding Darwin's God" By Ken Miller.
Addendum:
Here is another way "irreducible complexity" can evolve. Consider an arch of stones:
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How could this be built one stone at a time? It seems that you would not get more than one two stones in place before the entire thing would topple. But consider if another structure were in place while the arch was built, and then later was removed:
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We now can see how the "Irreducibly complex" arch could be built one step at a time.
