(A description of Hall's
experiments on the evolved beta-galactosidase system. From pp.
145-147 of Finding Darwin's God)
What happens if we step things up to the next
level, asking evolutionary mechanisms to "design" a
multi-part system, a system that, in Behe's terminology, would
be irreducibly complex? The most direct way to do this would
be with a true acid test - by using the tools of molecular genetics
to wipe out an existing multipart system, and then see if evolution
can come to the rescue with a system to replace it.
The "lac" genes of bacteria are
just such a system. Lactose is a sugar, similar in many ways
to sucrose, ordinary table sugar, that bacteria can use as a
food. In order to do this, they first have to produce an enzyme
that can cut lactose in half, releasing two simple sugars (glucose
and galactose) that the metabolism of the cell can use for energy.
Not surprisingly, bacteria are "smart" enough to "know"
that there is no point in making this enzyme - known as galactosidase
- when there is no lactose in the medium in which they are growing.
In the absence of lactose they automatically shut off the genes
required to utilize lactose for food. Clever biochemistry. Behe
might say, "clever design."
The bacterium pulls this feat off by combining
highly sensitive control genes with the "structural"
genes that actually specify the amino acids in the galactosidase
enzyme. The control gene keeps the enzyme gene switched off except
when it is needed to produce enough of the enzyme to metabolize
lactose. Could evolution have produced such a lovely two-part
system? Barry Hall tested this possibility in 1982 by deleting
the structural gene for galactosidase. He then "challenged"
cells with the deletion to grow on lactose. At first, of course,
they couldn't. Before long, mutant strains appeared that could
handle lactose nearly as well as the originals.
How could this be? How could these cells have
reconstructed the information from the missing gene in such a
short time, using only the random, undirected processes of mutation
and natural selection? The answer, of course, is that these bacteria
didn't make the new galactosidase enzyme from scratch. They made
it by tinkering with another gene, in which a simple mutation
changed an existing enzyme just enough to make it also capable
of cleaving the bond that holds the two parts of lactose together.
Now, you might think that this wouldn't be enough, and you'd
be right. Simply re-engineering an existing protein to replace
galactosidase would make no difference unless the control region
of that gene was also changed to ensure that the gene was expressed
when lactose was present. Significantly, when Hall looked at
the control regions of the mutant replacement gene, he found
that they had been mutated as well - some of them were now switched
on all the time, but a few of them responded directly to lactose,
switching the gene on and off as needed.
That would have been impressive enough, but
Hall's clever germs didn't stop there. When he selected them
further to grow on another sugar (lactulose), he obtained a second
series of mutants with a new enzyme that accidentally (in a sense)
produced allolactose, the very same chemical signal that is normally
used to switch on all of the lac genes. This important development
meant that now the cells could switch on synthesis of a cell
membrane protein, the lac permease, that speeds the entry of
lactose into the cell. Summarizing this work, evolutionary biologist
Douglas Futumya wrote:
Thus an entire system of lactose utilization
had evolved, consisting of changes in enzyme structure enabling
hydrolysis of the substrate; alteration of a regulatory gene
so that the enzyme can be synthesized in response to the substrate;
and the evolution of an enzyme reaction that induces the permease
needed for the entry of the substrate. One could not wish for
a batter demonstration of the neoDarwinian principle that mutation
and natural selection in concert are the source of complex adaptations.
[DJ Futumya, Evolution, ©1986, Sinauer Associates, Sunderland,
MA. pp. 477-478.]
Think for a moment - if we were to happen
upon the interlocking biochemical complexity of the re-evolved
lactose system, wouldn't we be impressed with the "intelligence"
of its design? Lactose triggers a regulatory sequence that switches
on the synthesis of an enzyme that then metabolizes lactose itself.
And the products of that successful lactose metabolism then activate
the gene for the lac permease, which ensures a steady supply
of lactose entering the cell. Irreducible complexity. What good
would the permease be without the galactosidase? And what use
would either of them be without regulatory genes to switch them
on? And what good would lactose- responding regulatory genes
be without lactose-specific enzymes? No good, of course. And
therefore, by very same logic applied by Michael Behe to other
systems, we could conclude that the system had been designed.
Except we know that it was not designed. We know it evolved because
we watched it happen right in the laboratory!
No doubt about it - the evolution of biochemical
systems, even complex multi-part ones, is explicable in terms
of evolution. Behe is wrong.
An excerpt from
Chapter 5 of
Finding Darwin's God
A Scientist's Search
for Common Ground Between God and Evolution.
© 1999 by Kenneth R. Miller
Cliff Street Books
Behe's "Acid Test" Criticisms of this excerpt.
Miller's Analysis of