TOWARDS A MOLECULAR-GENETICS OF EVOLUTION OF SPECIES

Inquiring about the genetic barriers between different species amounts to asking how populations become genetically isolated. The genetic ba rrier which separates Salmonella typhimurium and Escherichia coli is p rimarily recombinational. The structural component of this barrier is genomic sequence divergence. The mismatch repair enzymes act as inhibi tors of interspecies recombination, whereas the SOS system acts as an inducible positive regulator. These genetic systems control also the g enomic stability in bacterial populations, The mismatch repair maintai ns genetic stability, while the SOS system generates genetic variabili ty. These opposing activities allow mismatch repair and SOS systems to determine both the rate of accumulation of sequence divergence and th e extent of genetic isolation, which are the key components of the spe ciation process, Recent results suggest that these bacterial paradigms could be extended to eukaryotes.