Final answer:
A flood is NOT a potential agent of selection because it does not directly select for specific traits, in contrast to active selection pressures like human fishing or predation. A flood is a random event that may lead to genetic drift (option 2).
Step-by-step explanation:
The question asks to identify which option is NOT a potential agent of selection. It is important to first understand that agents of selection are factors that can cause changes in allele frequencies within a population and contribute to evolutionary processes. The conditions for a population to remain at Hardy-Weinberg Equilibrium include: no mutation, random mating, no gene flow, infinite population size, and no selection. If any of these conditions are not met, then the population may evolve.
The options given are 1) human fishing, 2) genetic drift, 3) a flood, 4) a predator. Human fishing and predation are forms of selection pressure because they tend to non-randomly remove certain individuals based on specific traits, potentially changing allele frequencies. Genetic drift is a random process that can also lead to changes in allele frequencies over time, especially in small populations or after bottleneck events where a large portion of a population is wiped out. Despite causing significant change in environment and potentially impacting a population, a flood is not an active selection pressure on its own; it's a random event that can lead to genetic drift but does not select for specific traits. Therefore, the correct answer is that a flood is NOT a potential agent of selection when considering the evolutionary forces that directly alter allele frequencies in a population's gene pool. Instead, it can cause a situation where genetic drift may occur if a random subset of the population survives.