Gene swamping is most likely to occur when:

Gene swamping occurs when a population that is experiencing gene flow from a different population effectively loses its genetic identity due to the overwhelming influx of alleles from the incoming population. This is particularly likely when the migration rate is larger than the rate of selection acting on the resident population.

In situations where the migration rate exceeds the selection rate, the new alleles introduced by migrants can quickly spread through the resident population. The selection pressure, which would typically favor certain traits or alleles over others, is not strong enough to effectively counterbalance the incoming genetic material. As a result, the local adaptations may be diminished or lost entirely, leading to a homogenization of genetic traits across populations.

This dynamic explains why the migration rate being larger than the selection rate is the most critical factor in gene swamping, highlighting the importance of gene flow in shaping genetic diversity and evolutionary trajectories in populations.