Population fitness over time. A) Hybrids perform better on average than the separated parent populations for some generations. After a variable number of generations, the fitness of newly formed hybrids collapses and does not recover. B) If hybrid individuals that exhibit heterosis are crossed to each other, the fitness advantage does not persist. Instead, the fitness of the offspring of hybrids collapses back to the same level as the parent populations. All fitness values are population averages of 40 (parent populations) or 80 (hybrids) individuals.