BOOK REVIEW
Have We Solved Darwin's Dilemma?
Massimo Pigliucci
The Plausibility of Life: Resolving Darwin's Dilemma. Marc
W. Kirschner and John C. Gerhart. xvi + 314 pp. Yale University
Press, 2005. $30.
Is life plausible? Well, it's more than plausible, it has actually
happened! What we need to ask, rather, is whether our explanations
for how life came about and diversified are plausible. So the title
of Marc W. Kirschner and John C. Gerhart's book implies the wrong
question. Despite that, The Plausibility of Life makes for
informative and enjoyable reading, and the issues the authors raise
are worthy of attention.
The book takes as its starting point the position that the currently
accepted version of evolutionary theory, the so-called Modern
Synthesis, although not wrong, is incomplete. Similar rumors have
been circulated in other recent books, including Mary Jane
West-Eberhard's Developmental Plasticity and Evolution, Eva
Jablonka and Marion Lamb's Evolution in Four Dimensions,
and Phenotypic Evolution, a book by Carl Schlichting and
myself. But those rumors have been strenuously denied by researchers
who have situated their careers squarely within the Synthesis, which
came of age in the 1930s and 1940s.
The two key questions concerning The Plausibility of Life,
then, are, Do we actually need to add more components to the
structure of evolutionary theory? And if so, is Kirschner and
Gerhart's book the contribution some of us have sought for years?
The answer to the first question is a definite "yes," and
to the second one, a qualified "partially so." Let me explain.
The Modern Synthesis itself built on Darwin's two major
realizations: first, that all living organisms are related to one
another by common descent; second, that a primary explanation for
the pattern of diversity of life—and especially for the
obvious "fit" of organisms to their environments—is
the process that he called natural selection. It took about seven
decades for biologists to add the next round of important building
blocks to the Darwinian view of life. Modern Synthesists such as
Theodosius Dobzhansky, Ernst Mayr, George Gaylord Simpson and G.
Ledyard Stebbins reconciled disparate fields of biology, from
population genetics to paleontology, by expanding the array of
evolutionary processes to include migration, mutations, assortative
(nonrandom) mating and (random) genetic drift.
What the Modern Synthesis glaringly left out was the field of
developmental biology and its subdiscipline, embryology, which
wandered almost on their own for a few additional decades, until
"evo-devo" (evolutionary developmental biology) came into
vogue in the 1990s. The incompleteness of the Synthesis consisted
not only in leaving out that entire area of research (which would be
bad enough) but also in failing to address (or worse, sweeping under
the carpet) important questions, chief among them the problem of the
origin of so-called evolutionary novelties.
Evolutionary novelties are those (usually) complex structures that
allow the organisms that evolve them to exploit new aspects of their
environment or to adapt to new niches, often in a spectacularly
successful fashion. They also happen to be the sort of thing Darwin
struggled with most in On the Origin of Species: eyes,
wings, hearts and brains. Not coincidentally, the difficulty of
evolving such complex biological structures has also fueled
evolution deniers, from the Reverend William Paley (whose
"watch found on the heath" argument evoked an extensive
response from Darwin) to the modern supporters of so-called
"intelligent design."
Now, it has never been clear to me why the existence of natural
phenomena that are at the moment difficult to explain seems to fuel
both the reaction that "there is a fundamental flaw in the
theory" (intelligent design) and the opposite response that
"there is nothing wrong, everything has been explained"
(the Modern Synthesis). Science, by its very nature, deals with
things for which we lack explanations—things that might
otherwise compel us to turn to supernatural notions. By the same
token, if evolutionary biologists really had already explained
everything, what on earth would justify my job as an active researcher?
Assuming, then, that there really are problems that have not been
satisfactorily solved within the Modern Synthesis, do Kirschner and
Gerhart provide us with answers? Well, first of all, the book is
aimed at a general audience. Thus it is hard to evaluate it as an
original contribution to science, because many of the arguments are
not sufficiently developed or are not substantiated by a thorough
analysis of the literature. This is an interesting trend that seems
to have taken hold in recent years (Evolution in Four
Dimensions is another example), and it is hard to know what to
make of it. On the one hand, we are witnessing a welcome return to
the idea that cutting-edge science is for everybody. Darwin's On
the Origin of Species, after all, sold out on its first day
and was purchased by laypeople as well as academics (of which there
were few at the time anyway). On the other hand, with trade books
one can get away with speculations and generalizations that would be
harder to pass through the rigorous process of peer review that
technical books usually undergo.
Be that as it may, Kirschner and Gerhart's main idea is that the
missing piece in the edifice of the Modern Synthesis is what they
call "facilitated variation." The basic idea is sound:
Organisms are not analogous to human-engineered machines (contrary
to what creationists and proponents of intelligent design keep
insisting); rather, they are characterized by developmental systems
that are capable of accommodating quite a bit of disruption—be
that from changes in the external environment (phenotypic
plasticity) or from mutations in their genetic makeup (genetic
homeostasis). This ability to accommodate is in turn made possible
by the modular structure of the genetic-developmental system itself,
which allows organisms to evolve new phenotypes by rearranging
existing components. A splendid example of this evolutionary and
developmental flexibility is provided by the homeobox genes that
preside over the spatial differentiation of embryos, from fruit
flies to humans.
Facilitated variation is indeed one of the empirical as well as
conceptual developments that have marked evolutionary biology during
the past decade or so. However, it is not the stunningly new insight
that Kirschner and Gerhart imply it to be. Plenty of other authors
have talked about concepts such as the modularity of organisms,
genetic and developmental homeostasis, evolution by alteration of
regulatory sequences, and even evolution as a continuous process of
"tinkering" with preexisting elements (as opposed to
creating new complex characters from scratch)—an idea put
forth by François Jacob as long ago as 1977. Kirschner and
Gerhart's effort is a valuable update of some of these ideas, but it
hardly constitutes "a new theory" to complement the Modern Synthesis.
Kirschner and Gerhart also peculiarly ignore or downplay at least
two other major pieces of the evolutionary puzzle that have been
brought forth recently as crucial to the development of a new
synthesis. One is the inherent capacity of organisms to accommodate
phenotypic plasticity—environmentally or genetically induced
changes. Plasticity has been studied for more than a century, but it
has come of age in the past two decades as a central characteristic
of living organisms, one that has been implicated in speciation,
adaptation and the origin of phenotypic novelties. Although
Kirschner and Gerhart do mention phenotypic plasticity, they confine
it to a secondary role (the term doesn't even make it into the
index). They apparently do not realize that it is a crucial
component of what they call facilitated variation.
The second crucial piece, entirely missing from The Plausibility
of Life, is the (very) recent resurgence of the concept of
inherited epigenetic variation. A significant amount of empirical
research and at least one book tackling its conceptual implications
(Evolution in Four Dimensions) are forcing biologists
to face the existence of several layers of heritable variation above
and beyond the classical genetic one. Inherited epigenetic variants
can interact with their genetic counterparts to multiply by orders
of magnitude the phenotypic variation available to natural
selection, thereby expanding the mechanistic bases of evolutionary
theoretical explanations and greatly increasing their plausibility
as an account of life's diversity.
The Plausibility of Life ends with a brief critique of
intelligent design, suggesting that the concept of facilitated
variation will provide a solid argument to rebut creationists. I
applaud the authors' intention, as it seems to me that more
scientists ought to face the realities of public misunderstanding of
science. But their presentation is too brief, and a bit too
simplistic. The "controversy" about evolution has nothing
to do with the soundness of scientific explanations of the history
of life: It's not a scientific controversy, but a social, cultural
and political one. Creationism is the result of centuries of
anti-intellectualism in the United States, coupled with sometimes
cynical exploitation of the issue for political gain. In addition,
many scientists have no interest in getting out of the ivory tower
to talk to the very same public that pays their salaries and funds
their precious research grants. The recent defeat of intelligent
design at the trial in Dover, Pennsylvania—at the hand of a
conservative judge appointed by George W. Bush—will do much
more to promote sanity in public education than any theory about
facilitated variation, as scientifically sound as the latter may be.