Epistatic fitness

Evolutionary biologists are interested in whether fitness interactions between loci are generally epistatic, and generate strong linkage disequilibrium, or whether they are generally independent and generate linkage equilibrium.

No general answer is yet available, but it is possible to make some observations:

• Different loci will tend to interact multiplicatively when they have independent effects on an individuals' survival and reproduction. This could be for loci that influence events at different times in an organism's life.

• Epistatic interactions are more likely for loci controlling closely inter-dependent parts of an organism. The extent to which we expect loci to interact epistatically or not will depend on how atomistic or holistic a view we have of the organism

Notice that epistatic fitness interaction is not the same as mere physiological or embryological interaction:

Imagine a case in which the A-locus controls, say, muscle strength and the B-locus controls metabolic rate. Muscles and metabolism interact in a physiological sense: when muscles are put to work, the metabolic rate goes up. However, if the population is fixed for homozygotes at both loci (all individuals are A1B1/A1B1) then there cannot be any fitness epistasis. Epistatic fitness requires heterozygosity at both loci, and the kind of fitness relations we saw in the Papilio memnon example.