One of the Hardy-Weinberg conditions that is often easiest to meet in a controlled lab setting involves the absence of mutation. In an isolated lab environment, the biologist can ensure that the genetic makeup of the fish population remains stable by controlling environmental factors and minimizing exposure to mutagens.
When studying small populations, mutations can introduce new alleles into the gene pool, potentially disrupting genetic equilibrium. By maintaining a stable environment with no introduction of new genetic material, researchers can more effectively observe natural genetic distributions and test various hypotheses without the complication of ongoing mutations.
Therefore, while the Hardy-Weinberg principle outlines five conditions for a population to be in genetic equilibrium (large population size, no mutation, no migration, random mating, and no selection), controlling the factor of mutation stands out as particularly feasible in laboratory conditions.