Final answer:
Horizontal gene transfer is the incorporation of genetic material from one organism to another within the same generation, not involving direct descent. Conjugation is a natural mechanism of horizontal gene transfer that spreads antibiotic resistance genes among bacteria. It allows gene sharing between different species, contributing to genetic diversity and evolution.
Step-by-step explanation:
When an organism incorporates genetic material from another organism that is not its parent, this is known as horizontal gene transfer. This mechanism of gene transfer contrasts with vertical gene transfer, which is the passing of genetic material from parent to offspring. In the context of bacteria and the spread of antibiotic resistance, the horizontal gene transfer mechanism that is naturally responsible for this spread is known as conjugation.
The other mechanisms by which horizontal gene transfer can occur include transformation, where DNA is taken up directly from the environment, and transduction, where genetic material is transferred via a bacteriophage. While often associated with prokaryotes, horizontal gene transfer can occur in eukaryotes as well, although it is less prevalent. Horizontal gene transfer is a source of genetic variation and is vital in the process of natural selection, allowing even distantly related species to share genes which can significantly influence their phenotypes.