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Miracles and Molecules
Douglas J. Futuyma
Allen Orr has incisively revealed
the profound flaws in Darwin's Black Box. My comments
will only expand on a few of Orr's points.
Unfortunately for Michael Behe's
argument, molecular biology has only strengthened neo-Darwinian
evolutionary theory by providing abundant evidence not only
on the history of evolution, but on the mechanisms of evolutionary
change. DNA sequences, a rich source of data on phylogenetic
relationships among organisms, have in almost all cases confirmed
the broad relationships that had earlier been inferred from
comparative anatomy. The common origin of all living things
has been affirmed by similar DNA sequences, such as those encoding
histone proteins in plants, fungi, and animals, and those encoding
ribosomal RNA in both those "higher" organisms and bacteria.
Although Behe evidently accepts the common ancestry of diverse
forms of life, this increasingly indisputable fact greatly narrows
the scope for supernatural meddling in life's history.
Behe would find a role for a
divine designer in the origin of complex biochemical systems.
But molecular evolutionary biology increasingly provides insights
into the natural mechanisms by which such systems evolve. One
such mechanism, as Orr notes, is duplication of genes, or parts
of genes, followed by functional divergence. Gene duplication
is the consequence of unequal crossing-over, a process well
studied by geneticists, that can both increase and decrease
the number of copies of a gene on a chromosome. Such variation
in number has been observed, for instance, in human hemoglobins:
some individuals have more, and others fewer, than the normal
number of hemoglobin genes. (Thalassemias are disorders resulting
from such deficiencies.) Over the course of vertebrate evolution,
gene duplication has given rise to a family of hemoglobin genes
that have diverged in function. The hemoglobin of the lamprey,
a primitive jawless vertebrate, consists of a single protein
chain (a monomer), encoded by a single gene. In jawed vertebrates
such as fishes and mammals, hemoglobin is a tetramer: an aggregate
of four chains of two types (alpha and beta), encoded by two
genes with related sequences. This tetramer has a cooperative
oxygen-binding capacity not available to the lamprey. In salmon,
quadruple copies of the beta gene, differing slightly in sequence,
yield four types of hemoglobin with different, adaptive oxygen-loading
properties.1 In mammals, successive duplications
of the beta gene gave rise to the gamma and epsilon chains,
which characterize the hemoglobin of the fetus and early embryo
respectively, and enhance uptake of oxygen from the mother.
Thus a succession of gene duplications, widely spaced through
evolutionary time, has led to the "irreducibly complex" system
of respiratory proteins in mammals. In addition, some duplicate
hemoglobin genes have become pseudogenes: sequences similar
to functional hemoglobin genes but bearing mutations that abolish
their function. These sequences show that superfluous genes
rapidly degenerate.
DNA sequencing has also shown that with slight
alteration, or sometimes none at all, gene products acquire
very different functions. The crystalline lens of the eyes of
vertebrates is composed of one or another protein, such as lactate
dehydrogenase, that performs an entirely different, enzymatic
function elsewhere in the body. Both lactalbumin, a component
of milk, and part of the lactose synthetase enzyme are encoded
by genes that differ only slightly in sequence from the gene
for lysozyme, which retards infection by breaking down bacterial
cell walls, As the Nobel Prize-winning molecular geneticist
Francois Jacob said, evolution consists largely of molecular
tinkering--producing new objects from old odds and ends.2
Molecular tinkering also includes mixing and
matching--combining duplicated pieces of genes into new genes.
For instance, about five different modules, in different combinations,
compose each of the many proteins involved in blood clotting--and
these modules further are constituents of proteins with quite
different functions, such as the digestive enzyme trypsin.
Complex biochemical systems then, bear the molecular
stamp of their evolutionary origins. Often, these systems can
be found, in one or another organism, in a primitive, less complex
state--a state that functions adequately, even if not as efficiently
as the more complex state that evolved in other lineages. The
eye of a mammal is wondrously, perhaps "irreducibly," complex,
but an eye without a lens, capable at least of distinguishing
light from dark, is better than no eye at all. Likewise, a lamprey's
hemoglobin, even if less efficient than that of a jawed vertebrate,
suffices to keep lampreys alive. Yet it is doubtful that a mammal
could survive with a lamprey-like hemoglobin, for the physiological
functions that have evolved in mammals, such as maintaining
high body temperature, demand oxygen at a rate that can be supplied
only by more efficient, tetrameric hemoglobin. Likewise, it
is unlikely that a mammalian fetus could survive without its
special hemoglobin. What was once merely an advantage has become
a necessity. As Orr emphasizes, irreducible complexity is acquired--it
evolves.
Among vertebrates, only a subset--the jawed
vertebrates that first evolved about 430 million years ago--have
tetrameric hemoglobin, and of these only a subset--the mammals
whose ancestors became differentiated from other reptiles about
320 million years ago--have fetal hemoglobin. These facts permit
two possible explanations. One--Behe's explanation--is that
the common ancestor of all vertebrates, or of all life, was
equipped with all the molecular machinery any of its descendants
would ever use, and that most of the machinery was lost in most
lineages. This hypothesis is not only ludicrous, but also, as
Orr points out, makes predictions that are contradicted by evidence.
The alternative hypothesis is that new molecular complexities
cane into existence in various lineages of organisms at different
points in time.
If this is true, and if we were to follow Behe
in denying a natural, evolutionary origin of each such instance,
then each origin of a divergent, duplicate hemoglobin requires
us to postulate a special intervention by the omnipotent designer.
Bear in mind that the several new hemoglobins I have described
are only a few of the many, slightly different hemoglobins that,
like those of the salmon, contribute to the complex, fine-tuned
adaptation of diverse organisms to their environments. And these
are but a tiny fraction of the "irreducibly complex" molecular
adaptations to be found among vertebrates, insects, plants,
and other forms of life. Behe, then, must be forced to see the
designer's handiwork everywhere. Life must present him with
countless instances of supernatural intervention--of miracles.
When scientists invoke miracles, they cease
to practice science. Were a geologist to cite plate tectonics,
a chemist hydrogen bonds, or a physicist gravity as an instance
of the miraculous, he or she would be laughed out of the profession.
Moreover, they would not be doing their job, which is to seek
answers by posing and testing explanatory hypotheses. Faced
with the unknown, as all scientists are, the scientist who invokes
a miracle in effect says "this is unknowable" and admits defeat.
It is only through confidence that the unknown is knowable that
physical scientists have achieved explanation, and that biologists
have advanced understanding of heredity, development, and evolution
to heights scarcely hoped for just a few decades ago. Yet Behe,
claiming a miracle in every molecule, would urge us to admit
the defeat of reason, to despair of understanding, to rest content
in ignorance. Even as biology daily grows in knowledge and insight,
Behe counsels us to just give up.
1 Peter W.
Hochachka and George N. Somero, Biochemical Adaptation
(Princeton: Princeton University Press, 1984), pp. 279-303.
2 Francois Jacob, "Molecular
Tinkering and Evolution," in D. S. Bendall, ed., Evolution
from Molecules to Men, (Cambridge: Cambridge University
Press, 1983), pp. 131-44.
Originally published in the February/
March 1997 issue of Boston Review
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