Selection intensity for codon bias and the effective population size of Escherichia coli.

The selection intensity for codon bias and the synonymous diversity have been used in the recent literature to estimate the effective population size of Escherichia coli. The results have varied between (10)5 and (10)8. It is suggested here that most of this disparity can be explained by a model that accounts for the population structure of the species. Thus it is assumed that weakly selected characters, like synonymous substitutions, are selectively fixed within individual lines or colonies but spread throughout the population in an essentially neutral way when colonies replace one another. In this way, the effective population size that enters expressions for the codon bias will be that of an individual colony, which, if hitchhiking effects are considered, can be a very small number. The effective population size that appears together with the mutation rate in expressions for the synonymous diversity, on the other hand, will be related to the total number of colonies that make up the species and can be a very large number.