The key to speciation is the evolution of genetic differences between the incipient species. For a
lineage to split once and for all, the two incipient species must have genetic differences that are
expressed in some way that causes matings between them to either not happen or to be unsuccessful.
These need not be huge genetic differences. A small change in the timing, location, or rituals of
mating could be enough. But still, some difference is necessary. This change might evolve by natural
selection or genetic drift.
Reduced gene flow probably plays a critical role in speciation. Modes
of speciation are often classified according to how much the geographic
separation of incipient species can contribute to
reduced gene flow. The following table compares some of these speciation