Migration Thresholds Govern Allele Collapse Under Repeated Gene Flooding

A recent study proposed "gene flooding" as a strategy for controlling invasive populations through repeated introduction of inbred individuals. Their simulations demonstrate that sustained demographic influx can drive collapse of resident alleles under certain conditions. However, the broader dynamical structure governing when such collapse occurs remains unclear. Here, forward-time population genetic simulations were used to examine allele replacement under repeated asymmetric migration in a density-regulated population. Systematic variation of migration intensity revealed that allele replacement does not proceed gradually as migration increases. Instead, the system exhibits a sharp transition separating regimes of resident allele persistence from regimes of rapid allele loss. This threshold behavior remains robust across variation in mating structure and migrant reproductive disadvantage, indicating that demographic influx itself plays a dominant role in governing evolutionary outcomes. These results clarify the conditions under which migration-driven allele replacement may occur for gene flooding to be useful and highlight the potential for nonlinear tipping points in population genetic systems experiencing sustained demographic introduction.