Horizontal gene transfer. Genetic material is transferred between organisms, especially bacteria, by several mechanisms. New DNA can be introduced by bacterial viruses (bacteriophages) via transduction. In some cases, genes are passed purposefully from one bacterium to another by a kind of bacterial sexual exchange called conjugation. Segments of DNA (released from broken cells) may be taken up from the environment in a process called transformation. In all cases, the new DNA may be incorporated into the chromosome by recombination. The movement of genetic elements called transposons (sometimes referred to colloquially as “hopping genes”) can augment the effects of all these processes, if transposons are part of the introduced DNA. These processes are described in more detail in Chapter 14.