A Pseudomonas strain carrying plasmid-containing genes for toluene degradation was applied to toluene-contaminated soils for the purpose of bioremediation. Two months later, Molly Microbiologist conducted a soil dilution for a viable plate count from this environment and found Streptomyces isolates that could degrade toluene. She analyzed the genes involved and found that they were located on a plasmid and were identical to those in the Pseudomonas species. The most likely explanation is: