Final answer:
Conjugation is the mechanism of horizontal gene transfer that facilitates the spread of antibiotic resistance genes within a bacterial population.
Step-by-step explanation:
The mechanism of horizontal gene transfer responsible for the spread of antibiotic resistance genes within a bacterial population is conjugation. During conjugation, DNA is transferred from one bacterial cell to another via a structure known as a pilus or mating bridge. This process can result in the transfer of genes, including those conferring antibiotic resistance, between pathogenic and nonpathogenic bacteria, thereby spreading resistance within a bacterial community.