Structuralists tend to emphasise that organisms are wholes, and therefore that change in one part must necessarily take into account the inter-connected nature of the entire organism. Whilst structuralists are not necessarily anti-Darwinian, the laws of biological structure are viewed as independent and ahistorical accounts that are not necessarily tied to any particular mechanism of change. A structuralist might thus hold that Darwinian natural selection might be the driving force behind how structures change, but nevertheless be committed to an extra layer of explanation of how particular structures come into being and are maintained.
Typical structuralist concerns might be self-organisation, the idea that complex structure emerges from the dynamic interaction of molecules, without the resultant structure having necessarily been selected for in all its details. For example, the patterning of fingerprints or the stripes of zebras might emerge through simple rules of diffusion, and the resulting unique structure need not have been selected for in its finest details. Structuralists look for very general rules that govern organisms as a whole, and not just particular narratives that explain the origin or maintenance of particular structures. The interplay between structural laws and adaptation thus govern the degree to which an adaptationist account can fully explain why a particular organism looks as it does.
Structuralism and boundary conditions
Given the above account, a fruitful way of thinking about structuralism is as an attempt to provide a set of boundary conditions, governed by the physical, mechanical and chemical nature of matter, within which all types of biological change, including Darwinian evolution, must take place. These boundary conditions exist at a variety of levels of organisation. For example, dynamic molecular interactions might provide boundary conditions to various types of pattern formation; or the analysis of functional morphology might explain why certain structures (e.g. the vertebrate backbone) have remained relatively stable for tens or hundreds of millions of years. Typically, structuralism is anti-reductionist, in that these different levels of explanation cannot always be reduced to lower ones. For example, a structuralist might agree with the classical view of the Neodarwinian synthesis that changes in gene fequencies in populations lie behind all evolutionary change, but would argue that, in addition, other rules governing how structures change are required to come to a full account of how diversity comes about.
Structuralism and genes
Extreme structuralists tend to downplay the importance of the gene in understanding biological structure. For example, Muller and Newman have proposed that physical laws of structure govern the genesis of major diversifications (such as in the Cambrian explosion) with successful structures co-opting genetic generative mechanisms later. Such views can be construed as anti-adaptionist as they insist that selection and adaptation can only work on structure that is presented to them, and not generate the structure in the first place. Again, the incompleteness of natural selection in providing a complete account of biological form is implied. As a result, structuralism has come under assault from classical Neodarwinists such as Maynard-Smith for, in effect, introducing a metaphysical level of biological explanation, arguing that the locus of the extra laws is far from clear.
Constraint and structuralism
Günter Wagner and others have argued that without at least some sort of structuralist background in the form of the idea of constraint there would not even be the possibility of Darwinian evolution. In effect, without the solidity given to morphological characters that the laws governing their evolution impart to them, they would dissolve and not be available for subsequent modification. Such views would probably be highly controversial if it were not for the fact that there have been rather limited contacts between mainstream evolutionary theorists and the more esoteric ideas represented by various structuralists. However, the field of Evo-devo is providing an arena within which these ideas are being examined more carefully by a variety of evolutionary thinkers. It is thus likely that more intense examination of structuralist views will lead to them being either more forcibly rejected by mainstream evolutionary biology, or to a more fruitful engagement and integration of at least some of them into future general accounts of structure and evolution.
While it is convenient to think of structuralists as emphasising "form" over "function", and thus being inherently anti-adaptionist, this simplicity hardly does justice to the complex interactions between the study of form per se and Neo-darwinian evolution. Some structuralists undoubtedly see the primacy of the laws of structure as being so important that the effects of selection and adaptation must be restricted to rather minor "fine-tuning" with little real relevance to the origin of form. Nevertheless, the true importance of structuralism - if any - must await the further examination and integration of this rather heterodox approach to biology before a full assessment is possible.
- For an overview of biological structuralism see Brian Goodwin, ”Beyond the Darwinian Paradigm: Understanding Biological Forms,” in Evolution: The First Four Billion Years, eds. Michael Ruse and Joseph Travis (Harvard University Press, 2009)