David Campbell replied:
> Actually, a complex molecular system can be built gradually if
> the pieces are available.
And "irreducibly complex" systems have been observed to arise.
As expected, many of these systems arose indirectly, involving
changes that were not directly selected for the final function
of the system, and developed sequentially. The fallback position
I've seen recently is that these are only simple systems, that
"very complex" irreducibly complex systems are less likely to
have evolved. But this begs the question of whether "IC-ness"
is a terribly good predictor of design.
> For example, a complex system that makes compound A
> into compound E could be built up starting from something that
> makes D into E in one step, adding something that makes C into
> D... Also, the assembled pieces need not be single-step. The
> citric acid cycle can be split into two complex parts that
> function independently in some bacteria, for example.
E.coli can "switch hit" -- sometimes they split the TCA cycle
and sometimes (such as during growth on two-carbon units and
fatty acids) they engage the complete cycle.
I would point out that the majority of examples described in Behe's
book are of extant systems. Yes, today's blood clotting system in
mammals is irreducibly complex and could not evolve de novo from
a mammal that had none of the clotting system's components. However,
the clotting system did not first arise in mammals. It would be far
better to examine those organisms more similar to the ones in which
we think the systems first arose (This is where comparative
biochemistry can provide some insight). For example, sharks do just
fine with immune systems that are "missing" some components found
in other vertebrates. And other organisms do just fine without
a clotting system as "complex" as that found in mammals. Unfortunately,
comparative biochemistry gets little attention in Behe's book.
A pity since it's really the only thing one can use in such cases,
given the limitations of our current understanding of biochemistry.
One thing I do like about Behe's postion is his general acceptance
of common descent. For me this injects a breath of long awaited
fresh air into a debate that has long been dominated with
anti-evolutionist sentiments. Evolution occurred, now let's
discuss how it happened.