The tsunami and subsequent backwash are thought to have brought the marine Japan Sea stickleback, Gasterosteus nipponicus, from the sea and a freshwater population of the threespine stickleback, Gasterosteus aculeatus, from upstream into newly formed freshwater habitats in the town center. This brought the two closely related fish species into contact and resulted in interspecific hybridization
[...] genomic regions derived from G. nipponicus rapidly declined across the genome, and by 2020 the population had returned to an almost entirely G. aculeatus genomic composition. Because the generation time of this population is approximately one year, most of the G. nipponicus-derived genome was eliminated within about 10 generations.
Genomic regions containing major reproductive isolation loci associated with freshwater adaptation, migration to the sea, mate choice, and hybrid male sterility showed particularly rapid declines in G. nipponicus ancestry. However, these major loci alone could not explain the genome-wide removal observed over 10 generations. Individual-based simulations suggested that numerous weak genetic incompatibilities distributed throughout the genome may also have contributed to the rapid elimination of G. nipponicus ancestry.
“There were two major surprises in this study,” said Takuya Hosoki and Jun Kitano. “First, we did not expect genomic regions associated with major reproductive barriers to be purged so rapidly. Second, we were surprised that most of the foreign genome continued to disappear over subsequent generations. An important next question is whether this combination of a few strong reproductive barriers and many weak genetic incompatibilities represents a general mechanism by which species boundaries are maintained after hybridization.”
This study is one of the few to track genomic changes in a natural population from immediately after interspecific hybridization through approximately 10 generations. By directly observing the progressive removal of foreign genomic regions, the study reveals how multiple reproductive barriers can work together to maintain species boundaries following extensive hybridization.
Paper titled "Rapid genome-wide purging following tsunami-induced hybridization in a stickleback population" DOI