Section IIntroduction

Nematodes, and Caenorhabditis elegans in particular, provide excellent material for the study of evolution for two main reasons. First, because of the wealth of information from C. elegans on the genetic hierarchies underlying the development of the organism's features (characters), candidate loci can be targeted for studying their possible roles in character evolution. Although these studies (reviewed in the second half of this chapter) are just beginning, many evolutionary changes are similar to mutant phenotypes, suggesting that much of evolution may proceed by changes at the kinds of regulatory loci defined by genetic studies.

Second, because nematodes diverged early in metazoan evolution (possibly before the divergence of deuterostomes from protostomes), the primitive features that they share with other metazoan groups must have resembled those features in the ancient ancestor common to all these groups. Because C. elegans uses the same mechanisms and molecules as other Metazoa for pattern formation, cell signaling, cell fate determination, etc., the common ancestor also had these features. Besides maintaining this important position within the Metazoa, there is enormous diversity within the phylum Nematoda, and even within the family Rhabditidae, to which C. elegans belongs. This diversity (the greater portion of which must yet be explored; Sudhaus 1991) can be exploited to address fundamental questions about patterns and processes of evolution if the relationships of taxa within these groups can be established. The phylogenetic context of C. elegans is therefore reviewed in the first half of this chapter.

Placing the C. elegans model in a phylogenetic and evolutionary context can help identify which biological questions are relevant over a phylogenetically broad range of taxa. Because different pairs of organisms shared ancestors at different times, similarities between organisms that are due to shared ancestry (homologies) may be shared at distinct hierarchical levels of organization. But similarities may also arise from convergence. A phylogenetic framework is thus essential for drawing the historical inference about whether the form of a feature (character state) is primitive or derived and is ultimately required for assessing the explanatory power of the model system (Kellogg and Shaffer 1993; Sidow and Thomas 1994).