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(Figure 8.4). That this is not just an academic exercise is shown by the comparative studies in nematodes by Schierenberg and co-workers. In Caenorhabditis elegans and representatives of the Plectidae, the EMS blas-tomere cannot be replaced by other blastomeres when it is artificially eliminated (Lahl et al. 2003); as a consequence, a gut is not formed. In another nematode Acrobeloides nanus, however, the ablated EMS cell is replaced by its neighbour cell C which instead gives rise to gut structures (Wiegner and Schierenberg 1999). This shows that the causal connection between the EMS blastomere and the gut is not in every case as strict as in Caenorhabditis elegans despite a great similarity in the initial cleavage patterns of these nematode species.

Through a comparative experimental approach combined with a phylogenetic analysis and reasoning it is possible to reconstruct the temporal sequence of the evolutionary independence (Figure 8.4). Depending on the evolutionary sequence (polarisation) this could mean either the establishment of a new causal relationship or the interruption of a pre-existing one. Clearly, this interpretation is only possible in a phylogenetic context.

differences and contradictions between experimental and comparative approaches to development

According to what has been discussed above, experiments and comparisons can lead to contradictory conclusions about causal links between developmental steps. Contradiction occurs, for instance, if in the experimental approach C is the cause of D while, in contrast, the comparative analysis suggests that C is not necessarily the cause of D (see above). Both statements are correct but each one holds true only within its specific reference systems. The experimental approach deals with the normal development of living species or group of species; specifically, it deals with what have been called proximate or functional causes (Mayr 1997). As said above, one can interpret existing organisms as balanced, but a disturbance of this balance is only buffered to a certain degree and then the system collapses. This need not be the case in an evolutionary perspective under which seemingly closely interacting and dependent developmental aspects of one species can be decoupled in another species. The comparative approach is clearly evolutionary because it relates causal relationships to changes in time. Hence it deals with the evolutionary or ultimate causes (Mayr 1997). However, the evolutionary causes in this case are twofold: they are the reason for the transformation of the developmental step itself but, in addition, the changes in the causal connection which allows the transformation in the first place are caused by evolution. In particular, the latter aspect touches the domain of experimental developmental biology since it relates to causal connections between developmental steps. Hence, the experimental model organism approach leads to functional statements whereas the comparative approach leads to evolutionary as well as functional statements. The general principle of comparison is the potential falsification of a causal nexus - claiming that a step is not necessarily caused by another step. Furthermore, this falsification of causal relationships explains (as a precondition) the possibility of evolutionary change or transformation of developmental steps.

conclusions

The comparative approach to developmental biology is a direct means to study evolution because it deals with evolutionary change. The comparison of developmental sequences reveals differences in developmental steps which falsify a generalisation of causal relationships between subsequent developmental steps. With this approach independent evolutionary developmental steps are identified. They also show what is allowed within the developmental system without destroying it. Accordingly, the evolutionary framework is set to address further questions. Comparative experiments may reveal a functional dissociation of subsequent developmental steps which could be a starting point for evolutionary dissociation. The phylogenetic framework allows the polarisation of the stated differences, thus indicating the direction of evolutionary changes, but it does not address the general notion of evolutionary independence. However, all this increases the precision of the analysis of what has happened and how evolutionary changes have occurred. This is the prerequisite for addressing important questions: why development changes, and whether the inclusion of development adds new aspects to evolutionary mechanisms.

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