Prolonged Transient Persistence of Polygenic Polymorphism under Stabilizing Selection Alone

In evolutionary theories of quantitative traits, a mutation-selection balance is essential to preserve polymorphism, where stabilizing selection is widely considered to eliminate genetic variation at underlying loci. To isolate the role of selection, we re-examine the effect of stabilizing selection on genetic polymorphism in a mathematically rigorous manner. Following the classical stabilizing-selection model, we explicitly decompose the population mean μ across all n contributing loci, transforming the model within a n-dimensional dynamical system. The fully polymorphic stationarity equations have a unique algebraic candidate, which is a biologically feasible interior equilibrium if and only if all its components lie in (0,1). Linearization at the equilibrium point reveals a saddle-point structure: under equal allelic effects, exactly one eigenvalue is negative, and n-1 eigenvalues are positive. Under equal allelic effects, the linear stable and unstable eigenspaces are E^s=span{1} and E^u=ker(1^T), respectively; the corresponding local nonlinear invariant manifolds are tangent to these spaces at p*. The stable mode restores the population mean toward the equilibrium value μ*, which need not equal the optimum θ, whereas the unstable modes are mean-neutral to first order. The relative rate ratio |λ_s|/λ_u=2n-1 shows that collective relaxation is faster than locus differentiation, while the absolute local residence time is controlled by λ_u and the initial unstable displacement. A nonzero unstable component causes local departure from the fully polymorphic saddle, but the local analysis does not classify the global limiting state. In the full nonlinear system, the non-trivial equilibrium is not asymptotically stable. Simulations show that escape from the polymorphic neighborhood is substantially delayed, producing long finite-time persistence of polymorphism. Consequently, the saddle geometry can generate potentially long deterministic finite-time persistence of polygenic polymorphism, but it does not provide asymptotically stable or finite-population maintenance in the absence of mutation.