"Irreducible Complexity" or gene duplication?
Hemophilia
A is
an inherited X-linked recessive
disease,
characterized by improper clotting of the blood due to a
deficiency in
Coagulation Factor VIII.
Advocates
of the theory of irreducible
complexity have asserted that
this cascade could not have evolved by natural selection,
and therefore
requires an "Intelligent
Designer" [a resurrection of the
18th-cent. "Argument from Design"].
For a rebuttal, see this excerpt form Darwin's
God
by Kenneth R. Miller. The various coagulation factors turn
out to be a gene family
derived from a serine
protease locus with the
insertion of an EGF domain,
which
allows specific and localized blood clotting. The multiple
factors amplify the small signal from the original cut as a
cascade that promotes a major response, in the
same manner as a photomultiplier tube.