Final answer:
The HW principle helps population geneticists estimate allele frequencies in a population using molecular data, which is derived from genotyping or DNA sequencing, rather than phenotypic variations, genetic drift, or genetic recombination.
Step-by-step explanation:
The HW (Hardy-Weinberg) principle is commonly used as a starting point in investigations on natural populations by population geneticists. When they genotype individuals in a population, to estimate allele frequencies, they primarily rely on molecular data.
This data typically comes from DNA sequencing or genotyping technologies that can identify specific alleles present in individuals. Options like phenotypic variations, genetic drift, and genetic recombination are also important in population genetics, but they're not the primary source for initial allele frequency estimations according to the HW principle.