Hybrid vigour

Hybrid vigour (or hybrid vigor) is the improved activity and survival of hybrid offspring. The technical term in genetics is heterosis.

Inbreeding in a normal population leads to the offspring getting worse, less fit, less fertile and usually not living as long as the parents. The opposite, outbreeding, leads to fit, healthy, fertile offspring. This effect has been known for a long time. Animal breeders have known it since the 18th century, and Darwin investigated it in detail with plants.^[1][2]

On the other hand, when two parents are from widely different populations, such as different subspecies, this usually does not apply. In that case it is more usual for the hybrids to have lower fitness. Mules are usually not fertile, and that automatically makes them of lower fitness, as that term is used in biology. They are very hardy animals, but they leave few offspring. The puzzle for scientists has been to explain these facts.

1. ↑ Ashby E. 1948. Hybrid vigour. In New Biology 4, 9–25.
2. ↑ Darwin C.D. 1876. The effects of cross and self fertilisation in the vegetable kingdom. London: Murray.