A model to explain the origin in bacteria of clusters of genes that have similar functions and are conditionally expressed. The idea is that genes whose products are used only during conditions that happen infrequently are continually mutating to inactive alleles. When conditions that require the missing gene products reoccur, the bacteria die off unless horizontal transfer of active alleles can take place. But only small segments of DNA are transferred by episomes. Therefore, when two or more genes are needed simultaneously, selection will favor genes that are near one another. Also, cotranscribed genes are more likely to function adaptively in a new host, since the host needs to be able to recognize only one promoter. Thus the transmission of DNA by horizontal mobile elements (q.v.) tends to select the stepwise formation of gene clusters that function as operons. See Chronology, 1996, Lawrence and Roth; sympatric speciation.
Subjects: Genetics and Genomics.