Final answer:
The classical hypothesis on genetic variation in natural populations is based on the Hardy-Weinberg Principle, which predicts stable genetic frequencies in an idealized, non-evolving population. Yet, actual populations evolve due to genetic drift, mutation, and selection.
Step-by-step explanation:
The classical hypothesis of how much genetic variation exists in natural populations is majorly drawn from the Hardy-Weinberg Principle of Equilibrium. This principle provides a mathematical baseline, suggesting that under certain theoretical conditions (no mutations, migration, selective pressure, and an infinite population), a population's allele and genotype frequencies would remain stable, meaning the population is not evolving. However, in real scenarios, natural populations are subject to evolutionary forces such as genetic drift, mutation, migration, and selection, leading to changes in genetic makeup over time. This constant genetic variation is necessary for natural populations, as it contributes to the adaptive potential and overall fitness of a species, influencing the population carry and genetic structure.