Introduction Pathways of change

The molecular mechanisms that bring about biological form in modern-day embryos . . . should not be confused with the causes that led to the appearance of these forms in the first place . . . selection can only work on what already exists.

The evolution of form is . . . descent with modification (of development).

S. B. Carroll 2005: 294-295

Combining words into new formulas is an all too easy exercise. But in our case it could turn into a dangerous trick, if evolutionary developmental biology (evo-devo) does not prove to be a fruitful new adventure in science. The question is increasingly acute, as a rapidly rising number of researchers are lured by the new flag, more and more resources are put into experimental and theoretical efforts under this banner, and evo-devo is finally getting public acknowledgment in the form of dedicated university chairs, specialised journals, workshops, and the launch of new professional societies.

Over the past few years, the nature, or the identity, of evo-devo has been passionately debated. However, it would be unwise to attempt to crystallise this discipline's content in a brief formula. Taking a historical perspective, hardly any facet of contemporary science would recognise itself in what in the past would have seemed an adequate definition of a research field under the same name. Scientific disciplines change along the years. In biological parlance, one could say that scientific disciplines develop, or that they evolve. So it would be ironic to attempt to fix the meaning of a discipline whose field is growing at the frontier between the two traditional fields of investigation of change - ontogeny and phylogeny - in biological systems.

Exactly at this frontier is the place where we want to focus here. What is really going on at this cutting edge? Is there only a peaceful coexistence of two distinct research agendas, or is there, on the contrary, evidence of a symbiotic relationship growing? In other words, is evo-devo, with respect to its parental disciplines - developmental biology and evolutionary biology - just a multidisciplinary collective enterprise, or is it instead an effective interdisciplinary venture?

We are convinced that a cross-fertilisation is really occurring between the two parent disciplines, and that the hybrid is already showing its higher fitness in a disciplinary environment where either developmental biology or evolutionary biology alone would not fare too well.

This inspection of the dynamics of evo-devo's growing tip will probably suggest that little of what is currently advertised under the new discipline's name really belongs unequivocally to it. That is, quite a lot of fashionable evo-devo is in fact the product of repeated backcross within either one or the other of the parent disciplines, which is developmental biology more often than evolutionary biology. This backcross progeny has its established place and, with its undeniable success within the province of one of the two parent disciplines, is steadily providing evo-devo with wonderful tools to be used in pursuing its more specific aims on which we actually want to focus attention here.

The first need for cross-fertilisation between formerly separate fields is the development of a common language. Difficulties to be overcome are not so much in the existence of terms specific to either field, and completely ignored in the other, but rather in the different meaning that the same word may take in either field. Are we sure we are asking the same question, in evolutionary biology and in developmental biology, when we ask what is gastrulation, or a carpel? Again, is a larva, or a metamorphosis, in any reasonable sense the same thing in both fields?

Clearly, recognising this problem and addressing it in a search for common ground is a basic, constructive way towards the identification of shared, and possibly overlooked questions. This is actually what is already going on.

To be sure, this is not necessarily done for free, as it may require re-thinking many of our cherished terms and concepts. This does not simply affect those concepts or categories that perhaps belong more to philosophy than to everyday science, such as cause, or change. Sooner or later, a cultural revolution will also affect more technical concepts such as developmental stage, segment or gene. This happens because all these things are not 'given', as we quite often take for granted in many traditional biological disciplines.

Taking an evolutionary perspective, we are forced to acknowledge that all these interesting things we are speaking about are the products of evolutionary history. As soon as we acknowledge the historically contingent nature of all these objects, be they body axes, or meristems, we find ourselves right within the field of evolutionary developmental biology.

We cannot systematically address questions in developmental biology as if our study objects were something independent of time, independent of evolutionary change. Everything we deal with is the product of history. This nearly trivial truth casts a deep shadow over the traditional, uncritical use of 'model systems' as satisfactory ways to discover 'the rules' of developmental pathways.

By refining or fine-tuning our historical sensibility, we discover that most of our questions in developmental biology have been framed until now in terms of end products (the adult, the organ, the gross trait of body structure) to be eventually obtained, and offered to the action of natural selection, rather than in terms of existing kinds of organisation, from which the system can move towards alternative states within a range only defined by the starting conditions and by the rules of change.

This is one side of the coin, that is, the effect of injecting an evolutionary dimension, or perspective, into developmental biology. But there is also the other side of the coin, that is, the effect of the awareness that all traits offered to selection are the products of developmental processes.

The weakest point in the standard theory of evolution is, indeed, understanding the origin of variation. We can hardly content ourselves with explaining it only in terms of mutation and sexuality. We may well dispute whether the long neck of the giraffe evolved under the selective pressure of critically important food items only available in the canopy of the acacias during the dry season in the savannah or, as some researchers suggest, as an effect of sexual selection. But what neither of these Darwinian scenarios will eventually offer is an explanation of why the giraffe's extra-long neck is supported by no more than seven cervical vertebrae, exactly the same number we find nearly universally in mammals. This is exactly the point where developmental biology can reciprocate in shedding light on an evolutionary problem by revealing what developmental processes can, or cannot, make available to selection. By understanding the rules of development, we may come closer to understanding the origins of variation; and so, in the end, closer to understanding evolution.

Thus, following a few years of lively and yet somewhat chaotic cross-fertilization, it would be very difficult to maintain that evolutionary biology and developmental biology are still the same as they were before the beginning of the dialogue. This ongoing intercourse has probably blurred many traditional distinctions between research agendas, but this seems unavoidable, in a science of change as biology largely is. The title of this book, Evolving Pathways, alludes to this cross-fertilising dialogue, as it can be read either as 'pathways that evolve', or as 'to evolve pathways', moving from development or evolution, respectively.

Scientists looking for general principles should become aware of the historically determined nature of the kinds of systems they investigate. There is probably no universal recipe for disentangling the search for 'laws', or general principles, from the study of historically contingent and unique events.

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