Forms of Becoming:THE EVOLUTIONARY BIOLOGY OF DEVELOPMENT(EVO-DEVO)

Form and Function is the title of a classic book published in 1 9 1 6, in
which Edward Stuart Russell reconstructed the history of animal
morphology from Aristotle to the early 1 900s-a long period of time
in which the most important and revolutionary event in the history
of biology took place, including the advent of a world incorporating
evolutionary change. And yet for Russell the most radical choice between
alternative views in the study of animal form was not preDarwinian
conceptions versus those dominated by the methods and
priorities of evolutionary biology. It was instead the more ancient
and perhaps never fully resolved opposition which, at the beginning
of the nineteenth century, saw the two giants of comparative anatomy,
Georges Cuvier and Etienne Geoffroy Saint-Hilaire, facing one
another. The first championed a view that Russell called teleological
and which consists in claiming the primacy of function over form,
while the second defended, with equal conviction and authority, the
view that Russell called morphological, which claims the primacy of
form over function.

And today? Which are the most important questions concerning
the multiplicity of forms of living organisms? Much water has of
course since passed under the bridges, and not only those on the
Seine, whose banks are a few feet from the Museum d'Histoire
Naturelle where, for almost forty years, Cuvier and Geoffroy SaintHilaire
worked side by side-and it has certainly not invalidated
the dichotomy that set the two French men of science against each
other two hundred years ago. Something naturally has changed
since then. Today we are researching both the mechanisms by
means of which organisms are constructed, and the dialectical relationships
they develop with the forever mutable environment in
which they live. In other words the understanding of living forms
today involves two quite distinct branches of biology: developmental
and evolutionary.


In regard to the first branch what matters is form. The only functional
aspects that deserve attention are those that pertain to the
mechanisms that are the basis for the construction of forms, without
regard for the achievements that these will be able to realize in
either the short or the long term. Developmental biology also
accommodates hopeless «monsters:' such as calves with two heads
or fruit flies with four wings, insofar as they are forms to which it is
possible to give life.
For evolutionary biology, however, what matters is survival and
reproduction. This presupposes an efficient use of the resources that
the environment offers and, therefore, an appropriate level of efficiency
on the part of the animals' organs. The forms that exist (or,
better, those that survive the process of natural selection) satisfy the
functional criteria established by the environmental milieu.
Paradoxically, therefore, it is precisely in evolutionary biology that
primary attention to the function of organs survives. Cuvier, consigned
to us by history as the staunch defender of fixed characteristics,
was the fiercest proponent of this cause. A science like developmental
biology, on the other hand, for which only the modest
time-scale in which the individual is constructed seems relevant, even
today champions the primacy of form over function central to Geoffroy
Saint-Hilaire's vision. While it would not be historically accurate
to attribute truly evolutionary ideas to Geoffroy Saint-Hilaire, he certainly
was far removed from sharing his rival's firm belief in the unchanging
nature of species.
It is also true that few researchers in evolutionary biology today
read the works of Cuvier to find inspiration for their own work.
Similarly it would be difficult for researchers in developmental biology
to find inspiration in the works of Geoffroy Saint-Hilaire, although,
as we shall soon see, the latter statement needs some further
clarification.
For Cuvier, comparing the anatomical structure of a cat, a sparrow,
and a lizard was a legitimate (and interesting) activity, since the three
animals have many organs and apparatuses in common. On the other
hand, the comparison of a cat to a butterfly, or a sparrow to an oyster,
would have been pointless, given the great distance separating their

respective organizational designs. Today, however, it is possible to
attempt such daring comparisons, above all because modern developmental
genetics has shown that many important stages in the construction
of an animal take place, in species as different as an insect
and a mammal, under the direction of the same genes. An extremely
interesting discovery, but one that gives rise to new and more difficult
questions.
The greatest problem at this point is no longer understanding
what animals as different as a mouse and a fruit fly have in common,
but, on the contrary, finding the causes of their diversity. And it is
not sufficient, either, to hand off the problem to evolutionary biology,
only to be faced with the reply that the mouse and the fruit fly
are different because in the course of generations their ancestors
have had to confront different environmental conditions, which, little
by little, have selected the two different animal forms which we
see today. The fact is that the problem of the differences between different
living things remains also, at least in part, a problem of developmental
biology. A problem made more complex by the fact that
nature seems incapable of producing many forms that, in theory,
would seem to represent only very modest variations compared to
other forms that, instead, are actually produced. Development seems
to have its obligatory points of passage, in which natural selection
has no way of intervening. It is limited to choosing between the different
variants that are available to it.
At this point we have to ask ourselves if it will ever be possible to
reach an adequate comprehension of living forms and their evolution,
as long as we remain tied to the traditional separation between
developmental and evolutionary biology. The negative replies to this
question are becoming increasingly insistent. In response to this, a
discipline is taking shape that aims at integrating the concepts, the
problem sets, and the methods of inquiry that pertain to the two
traditions, and it is called evolutionary developmental biology, or
evo-devo.
It is in terms of evo-devo that I present an interpretation of living
forms in these pages. I should perhaps more correctly state: of animal
forms. I shall write of plants and other living things only very briefly,

due not only to my own background in zoology, but also to the fact
that to date evolutionary developmental biology has concerned itself
only marginally with plants, and not at all with fungi. There is, however,
a treasure in the pages of the scientific literature devoted to botanical
subjects and it is waiting to be re-read and re-interpreted from
an evo-devo perspective. If the reading of these pages results in the
recruitment to the new discipline of even only a couple of people interested
in plants and fungi, this small book will have already fulfilled
its purpose.
The book is divided into four sections. In the first I introduce some
fundamental concepts of the comparative method, in the context of
the history of biology. I then proceed to illustrate the unequal distribution
of animal forms in the hypothetical space of expected forms.
We shall thus see that the already existing forms are clustered in several
privileged areas, leaving some unsuspected voids corresponding
to forms that, for some reason, do not exist in nature. In the second
section I discuss the limits of the gene's role in producing the forms
of living organisms, with a critique of the current concept of the
genetic program. And finally I clarify how to understand today's notion
of "development of organisms." In the third section I invite the
reader to explore some consequences of an interpretation of living
organisms in terms of an evolutionary developmental biology-a
choice that obliges us to assume a flexible, perhaps a pluralist, attitude
toward the many traditional concepts of comparative morphologyfrom
the abstract (what is homology?) to those that refer to real objects
(what is a larva?) . The fourth section, with the epilogue that
concludes the book, is devoted to origins. The origin of legs, for instance,
or the origin of the subdivision of the body into segments,
such as we observe in an earthworm or a millipede. More generally, it
is devoted to the origin of evolutionary innovation, the final link between
the problems of developmental biology, which must tell us how
it is possible to build these forms, and evolutionary biology, which
must tell us how they changed over the course of time.
A few sincere and necessary thanks conclude these introductory
pages. To do justice to all the people who sparked my interest in the
topics treated in this book, I would need an extremely long list that
PREFACE xiii
would include scholars I have never met except through their writings.
I will lirnit myself, therefore, to only three names that represent three
generations. First, Pietro Omodeo, who since the time I wrote my doctoral
dissertation under his guidance, made me appreciate both the
value of theoretical reflection in biology and the importance (and fascination)
of a frequent revisiting of past authors. Then Wallace Arthur,
the first of the biologists of my generation with whom, starting at the
end of the 1 980s, I could engage in a dialogue on the subject of evolutionary
developmental biology. And finally Giuseppe Fusco, the first of
my students to courageously accompany me on this new and fascinating
adventure.
Sincere thanks, also to Michele Luzzatto, for his constant encouragement,
during these last few years, to write (and conclude!) this
small book. To him, as to Lucio Bonato and Giuseppe Fusco, I also
owe many precious comments on a first draft of the book.
For this U.S. edition I am deeply indebted to Robert Kirk, who was
first responsible for getting my book included in the editorial program
of Princeton University Press and afterward followed it carefully
through the production process; and to Mark Epstein, who in a
very accurate and sensible way selected the English words through
which my thoughts are being conveyed in these pages.

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