Measuring epistatic fitness

There is an empirical method of measuring how common epistatic fitness interactions are in nature.

Linkage disequilibrium is produced by epistatic selection, and the degree of linkage disequilibrium in a population can be measured. There are several possible causes of linkage disequilibrium and its existence does not demonstrate epistatic selection; however, if linkage disequilibrium is absent or low, we can infer that epistatic selection is unimportant in nature.

A few general surveys of the extent of linkage disequilibrium in natural populations have been made. One for bacteria found high levels of linkage disequilibrium in some species, such as Escherichia coli (opposite) but low levels in other species, such as Neisseria gonorrhoeae. The reason why many bacteria show linkage disequilibrium is that they reproduce asexually, and there is no recombination to break the linkage disequilibrium down. N. gonorrhoeae presumably has enough sexual exchange of genes to produce linkage equilibrium.