The Hardy-Weinberg equilibrium assumes no change in allele frequencies in a population, meaning no evolution occurs. The factors that can affect this equilibrium include:
Genetic drift, which causes random changes in allele frequencies, especially in small populations.
Gene migration or gene flow, which introduces new alleles into a population.
Genetic recombination, which creates new allele combinations but does not affect allele frequencies directly.
A constant gene pool is the assumption that the genetic makeup remains stable, and therefore does not affect the equilibrium.
Thus, the correct answer is (4) Constant gene pool.