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Developmental Diversity

The great morphologic disparity of Cambrian and Ordovician trilobites might appear to be a paradigmatic example where we can infer the loss of great developmental diversity. Comparative studies of the genes involved in development have now demonstrated that many developmental processes are highly conserved across all bilaterian animals and some originated even deeper in time, as shown by genes with the same developmental role in cnidarians and vertebrates [summarized in Erwin (2006b); see also Raff (2007)]. This pattern of extreme conservation of developmental patterning suggests that the loss of developmental diversity caused by extinction may be less extensive than it might otherwise appear. Studies of other arthropods, coupled with detailed studies of the patterns of morphologic evolution of trilobites (including developmental information retrieved from fossilized representatives of larval stages), have demonstrated that information on developmental patterning can be recovered (Hughes, 2007). Although patterns of gene expression, much less the network of gene regulatory interactions, cannot be identified, Hughes (2007) has compared the repatterning of the cephalic and trunk regions during the Cambrian and Ordovician diversifications of trilobites. His analysis shows that the Cambrian radiation of the group involved fundamental changes in various parts of the body plan: the number of body segments, how they were formed, and in the articulations between them. In contrast, the Ordovician radiation involved morphological “embellishments” of trilobite subclades whose architectures had already stabilized.

The deep conservation of developmental processes across many clades is consistent with recent comparative studies of the evolution of gene regulatory networks, suggesting that the evolution of regional patterning systems during the initial diversification of animal body plans generated a hierarchical structure (Davidson, 2006; Davidson and Erwin, 2006). Studies from echinoderm endomesoderm formation and arthropod and vertebrate heart development have revealed a network of highly conserved regulatory genes at the core of these systems whose interaction is required for development of the relevant body parts. Surrounding this kernel of conserved regulatory interactions, however, is a network of other interactions, and downstream a set of structural genes whose activity is controlled by the network. Elements of this surrounding network are less refractory (to varying degrees) to evolutionary modification, and of course the structural genes are the locus of adaptive evolution.

If this result is generally true of metazoan developmental evolution, one implication is that although the loss of biodiversity will result in the loss of downstream elements of the regulatory hierarchy, these elements are also the most labile to evolutionary change. In contrast, kernels appear to be broadly conserved within major body plans, and in some cases



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