Which mechanism is primarily responsible for genetic variation in bacteria?

Genetic variation in bacteria is primarily driven by horizontal gene transfer. This process allows for the movement of genetic material between bacteria, enabling them to acquire new traits such as antibiotic resistance or metabolic capabilities that they did not have before. Horizontal gene transfer occurs through mechanisms like transformation, transduction, and conjugation, which facilitate the exchange of DNA between individuals, even those that are not directly related.

In contrast, binary fission is the primary method by which bacteria reproduce asexually, resulting in two identical daughter cells. While this process allows for population growth, it does not create genetic diversity among the offspring. Mitosis is a process used for cell division in eukaryotes, not bacteria, and asexual reproduction does not involve genetic variation since it typically results in clones of the parent organism. Therefore, horizontal gene transfer is essential for introducing and spreading new genetic information across bacterial populations.