Early genetic studies showed conservation of gene order in the enteric bacteria. Two recent methods using pulsed field gel electrophoresis (PFGE) to determine the physical map of the genome are: (i) partial digestion with the endonuclease I-CeuI, which digests the DNA of bacteria in the rrn operon for rRNA (ribosomal RNA), thus establishing the "rrn genomic skeleton" (the size in kbp of the intervals between rRNA operons); (ii) analysis of XbaI and B1nI sites within Tn10 insertions in the chromosome. The order of I-CeuI fragments, which is ABCDEFG in S. typhimurium LT2 and E. coli K-12, was found to be conserved in most Salmonella species, most of which grow in many hosts (host-generalists). However, in S. typhi, S. paratyphi C, S. gallinarum, and S. pullorum, species which are host-specialized, these fragments are rearranged, due to homologous recombination between the rrn operons, resulting in translocations and inversions. Inversions and translocations not involving the rrn operons are seldom detected except for inversions over the TER (termination of replication) region. Additive genetic changes (due to lateral transfer resulting in insertion of nonhomologous DNA) have resulted in "loops" containing blocks of DNA which provide new genes to specific strains, thus driving rapid evolution of new traits.