Symbiosis is the phenomenon in which organisms of different species live together in close association. Symbiogenesis is the name given to the process by which symbiotic partners combine and unify. This letter reconsiders previous work using the NKCS model of coevolution to explore symbiogenesis. In particular, the role of different replication rates between the coevolving partners is considered. This is shown to provide a broader scope for the emergence of endosymbioses and subsequent horizontal gene transfers.