Literature

By studying the formation of embryonic molar teeth, researchers developed a model linking dentition patterns of rodents to the influence of signalling molecules secreted by the surrounding tissues. They identified both inhibitor (i) and activator (a) molecules and found that the balance between them determines when and if an additional molar will form. This model was examined to see if it was generic. "If this developmental system is shared by all mammals, different dental phenotypes could be generated simply by varying the a/i ratio. Kavanagh et al. argue that the system has influenced the evolution of functional diversity in mammalian dentition. To test that possibility, they compile data on the proportional area of the molars of 29 species of murine rodents - close relatives of the mice in which the authors discovered the regulatory system." Did the test prove positive? "The predictive mathematical model they derive from the developmental experiments explains nearly 75% of the diversity in molar proportions in these rodents."
In a News & Views article, Polly writes: "The predictive power of their model is impressive, but will it hold for all mammals? From my further analyses, the answer is a qualified 'yes'. The results are shown in Figure 3, which depicts the 'morphological space' (morphospace) for different combinations of relative molar size. Nearly 70% of the variation from 35 additional species, representing 13 mammalian orders, is explained by Kavanagh and colleagues' model."
This research is described as establishing "a remarkable connection between developmental and evolutionary biology." It is this claimed link with evolutionary biology that we now need to consider. If evolutionary biology is described as "variation", then we are all evolutionary biologists. But that definition is naive and unproductive. The interesting questions come when we ask whether there are limits to variation. It is significant that the proposed model does not involve variations via mutations, but variations stimulated and controlled by environmental factors. These variations are not neodarwinian, but emerge via a dynamic developmental system. The different dental structures do not imply different genetic programs, nor do they imply the emergence of new biological information. The developmental model is more suited to a design framework because the same fundamental program yields numerous different dentition patterns that are adapted to the lifestyle of the relevant organisms. The real evolutionary challenge is to explain the origin of the developmental system, the activator-inhibitor mechanism and the program logic.
Within the constraints of the developmental system, adaptive variation is possible. This research can be used to illustrate the concept of 'limits to variation' because the variation is pre-programmed and not open-ended. Development creates a limited range of options for potential variation. If evolutionary biologists were more familiar with thinking like this, they would not be so disturbed by Behe's new book, which is concerned with the limits of what neodarwinism can achieve.

Predicting evolutionary patterns of mammalian teeth from development
Kathryn D. Kavanagh, Alistair R. Evans & Jukka Jernvall
Nature 449, 427-432 (27 September 2007) | doi:10.1038/nature06153

Abstract: One motivation in the study of development is the discovery of mechanisms that may guide evolutionary change. Here we report how development governs relative size and number of cheek teeth, or molars, in the mouse. We constructed an inhibitory cascade model by experimentally uncovering the activator-inhibitor logic of sequential tooth development. The inhibitory cascade acts as a ratchet that determines molar size differences along the jaw, one effect being that the second molar always makes up one-third of total molar area. By using a macroevolutionary test, we demonstrate the success of the model in predicting dentition patterns found among murine rodent species with various diets, thereby providing an example of ecologically driven evolution along a developmentally favoured trajectory. In general, our work demonstrates how to construct and test developmental rules with evolutionary predictability in natural systems.

See also:

Polly, P.D., Development with a bite, Nature 449, 413-415, (27 September 2007) | doi:10.1038/449413a