Figure 1. Two mechanisms for the functional divergence of orthologous genes in Caenorhabiditis.

Left: The site of expression of orthologous genes (depicted by colored shapes) has diverged in Species A and Species B and is essential for the normal development and/or function of their respective tissues. A promoter swap is necessary and sufficient for cross-species rescue of a null mutation. Right: The expression of the orthologs has not changed, yet the consequence of inactivation produces a distinct phenotype in each. Cross-species rescue is effective without alteration of the ortholog, indicating that the locus of functional evolution is not the gene itself, but in the context for the gene's activity.