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Failure of establishment of reproductive isolation upon meeting of otherwise divergent populations.
Rather than selection for reproductive isolation when two formerly allopatric populations meet, instead gene flow can be sufficiently high that the populations in a sense merge, retaining their status as a single gene pool/species rather that diverging into two distinct species. Basically even given postzygotic barriers, if matings are sufficiently rampant then a substantial fraction of total progeny will be hybrids before mechanisms can evolve that prevent hybridization prezygotically.
In particular, if the a smaller population should meet up with a similar larger population then a race will ensue between assimilation of the smaller population into the larger one and evolution of prezygotic barriers in the smaller population that will serve to prevent that assimilation from occurring. If prezygotic mechanisms win, then speciation will have occurred. If assimilation wins, then speciation will not have occurred, and indeed the smaller population may be lost in terms of at least certain aspects of its distinctiveness, though the smaller population's alleles may nonetheless be retained within the now-merged population.
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