Indirect measures of gene flow and migration: F-ST not equal 1/(4Nm+1)

The difficulty of directly measuring gene flow has lead to the common use o f indirect measures extrapolated from genetic frequency data. These measure s are variants of F-ST, a standardized measure of the genetic variance amon g populations, and are used to solve for Nm, the number of migrants success fully entering a population per generation. Unfortunately, the mathematical model underlying this translation makes many biologically unrealistic assu mptions; real populations are very likely to violate these assumptions, suc h that there is often limited quantitative information to be gained about d ispersal from using gene frequency data. While studies of genetic structure per se are often worthwhile, and F-ST is an excellent measure of the exten t of this population structure, it is rare that F-ST can be translated into an accurate estimate of Nm.