Various levels of redundancy in developmental gene function appear common in complex metazoans. There might be no apparent phenotype at many, or even any, of a gene's specific expression sites in homozygous null mutant embryos. Here we ask what underlies the origin of such arrangements. The generation of families of genes by duplication has clearly been important. Additionally, however, selection might have driven molecularly unrelated genes, which encode proteins of similar physiological function, to become expressed during the same sets of developmental events (times and places), even though each such gene might initially have evolved in connection with just one of these events.