How new species emerge in nature is still incompletely understood and difficult to study directly. Self-replicating molecules provide a simple model that allows us to capture the fundamental processes that occur in species formation. We have been able to monitor in real time and at a molecular level the diversification of self-replicating molecules into two distinct sets that compete for two different building blocks ('food') and so capture an important aspect of the process by which species may arise. The results show that the second replicator set is a descendant of the first and that both sets are kinetic products that oppose the thermodynamic preference of the system. The sets occupy related but complementary food niches. As diversification into sets takes place on the timescale of weeks and can be investigated at the molecular level, this work opens up new opportunities for experimentally investigating the process through which species arise both in real time and with enhanced detail.