Developmental variation, homology, and the pharyngula stage.

Understanding how development varies both inter- and intraspecifically can be important for systematic and evolutionary studies. This review will explore three different ways such understanding can be applied to evolutionary analyses. First, developmental data can be useful for homology determination. Interspecific variation in development has been thought to make developmental data poor candidates for determining homology. However, an updated developmental criterion that is more broadly comparative and mechanistic augments the available criteria used in homology determination. Second, modern cell and molecular biology are providing a better understanding of the many developmental processes involved in a structure's formation and will augment the number of characters available for phylogenetic analyses. Recent work has revealed that what had been thought to be a highly conserved developmental stage, the pharyngula (the phylotypic and zootypic stage of craniates) is highly variable. This variation can be seen in the development of such tissues as neural crest and placodes. These tissues are particularly interesting from a phylogenetic standpoint because they and the structures they form contribute to key synapomorphies of craniates. Finally, understanding developmental processes and how they form the variety of morphologies seen in nature will help in constructing the transformations that occurred during evolution. One such example involves descriptions of how lateral line development is affected in different mutant lines of zebrafish. The many species of teleost fishes express great variation in the patterns of their lateral lines, and this is often an important systematic character. Understanding the genetic basis of lateral line development would help not only in hypothesizing possible transformational series but also in determining how many genes may have been required for these transformations.