FIG 2.

Units of selection as evolutionary units. A bacterial cell and a conjugative plasmid carrying antibiotic resistance genes constitute different evolutionary units, given that they are independent beneficiaries. At the top, a resistance gene that is externally acquired (small red rectangle) by the cell can be integrated either in the chromosome (black string ball) or in a conjugative plasmid (blue ring). In a selective event, the cell with the red gene in the chromosome reaches 4 copies, but the plasmid is independently transferred to a different bacterial cell (green), which is also selected and reaches 4 copies. At the end, the balance for each type of cell is 4 copies, with 8 copies for the plasmid, indicating that under this single selective antibiotic event, the plasmid is a better beneficiary than any of the bacterial cells hosting it; in other words, the plasmid is an independent unit of selection, a different evolutionary unit.