The authors measured intracellular (within-cell) competition between plasmids in Escherichia coli and showed that this competition influences plasmid population dynamics along with intercellular selection[2]. In an experiment, they created synthetic plasmid dimers that could be split in a controlled manner and measured drift and selection dynamics between competing plasmids in a single cell[2][3]. The dissimilar (incompatible) plasmids co-resolved in the cell for a longer time due to methylation-driven replication[2][3]. Less transcriptionally active plasmids had a competitive advantage within the cell and preferentially fixed, promoting gene loss rather than gain[2][3]. The fixation of new plasmid variants depends nonlinearly on the interaction between transcription and translation of plasmid genes[2]. Higher plasmid copy number decreased the probability of fixation of new variants and slowed the rate of plasmid evolution[1][2]. The authors therefore concluded that plasmid evolution is governed by two levels of dynamics - intracellular and intercellular - whereby short-term intracellular competition may determine which new variants spread population-wide[1][2].