ShuiEro ... from vague up to sometimes ;) crystal clear drop by drop ...

Understanding the processes that generate or breakdown reproductive isolation between species is essential to understanding evolution. Assortative mating mediates reproductive isolation between species, but its dynamics in natural populations are poorly understood. Here we show that strong assortative mating maintains reproductive isolation in a natural hybrid population following an initial breakdown when the hybrid population formed, and strongly shaped the genetic structure of this population over \~{}25 generations. Intriguingly, although in the wild these mate preferences result in nearly 100% of matings occurring between similar genotypes, this barrier breaks down in the laboratory. Our results highlight the importance of assortative mating in shaping hybrid population evolution and imply that short-term breakdown in assortative mating can have long-term evolutionary consequences.The emergence of new species is driven by the establishment of mechanisms that limit gene flow between populations. A major challenge is reconciling the theoretical and empirical importance of assortative mating in speciation with the ease with which it can fail. Swordtail fish have an evolutionary history of hybridization and fragile prezygotic isolating mechanisms. Hybridization between two swordtail species likely arose via pollution-mediated breakdown of assortative mating in the 1990s. Here we track unusual genetic patterns in one hybrid population over the past decade using whole-genome sequencing. Hybrids in this population formed separate genetic clusters by 2003, and maintained near-perfect isolation over 25 generations through strong ancestry-assortative mating. However, we also find that assortative mating was plastic, varying in strength over time and disappearing under manipulated conditions. In addition, a nearby population did not show evidence of assortative mating. Thus, our findings suggest that assortative mating may constitute an intermittent and unpredictable barrier to gene flow, but that variation in its strength can have a major effect on how hybrid populations evolve. Understanding how reproductive isolation varies across populations and through time is critical to understanding speciation and hybridization, as well as their dependence on disturbance.