English: Four outcomes of secondary contact of previously allopatrically separated populations of a species. 1) An extrinsic barrier separates a species population into two but they come into contact before reproductive isolation is sufficient to result in speciation. The two populations fuse back into one species. 2) Two incipient species evolve complete reproductive isolation due to selection rejecting unfit hybrids. Mate discrimination acts to reinforce the speciation process. This is speciation by reinforcement. 3) Two separated populations stay genetically distinct while hybrid swarms form in the zone of contact between the two species populations. 4) Genome recombination results in speciation of the two populations, with an additional hybrid species. All three species are separated by intrinsic reproductive barriers.^[1]