Unexpected behavior in two locus genetic systems: an analysis of marginal underdominance at a stable equilibrium.

The phenomenon of marginal underdominance at a stable equilibrium in a two-locus-two-allele deterministic selection model is studied analytically using bifurcation theory. This technique and additional numerical studies indicate several new aspects of the phenomenon that are of biological importance. Marginal underdominance can occur at both loci simultaneously and can be present in a model with small fitness differentials. This can occur for a fitness scheme that is a one parameter generalization of the classical symmetric fitness model.