Final answer:
The classical hypothesis of genetic variation in natural populations is high levels of genetic diversity. Genetic diversity is important for adaptation and evolution, providing the raw material for natural selection to act upon.
Step-by-step explanation:
The classical hypothesis of genetic variation in natural populations is option A) High levels of genetic diversity. Genetic diversity, or the diversity of alleles and genotypes within a population, is important for adaptation and evolution. It provides the raw material for natural selection to act upon and allows populations to respond to changing environments. An example of genetic variation would be the different fur colors and patterns found in domestic dogs.