Long term effects of outbreeding: experimental founding of island population eliminates malformations and improves hatching success in sand lizards

Loss of genetic variation is an increasing problem in many natural populations as a result of population fragmentation, inbreeding, and genetic drift, which may lead to inbreeding depression and subsequent “extinction vortices”. In such cases, outbreeding offers a potential population saviour from extinction. Here we compare offspring viability between an experimentally founded outbred island population of sand lizards Lacerta agilis, and an inbred mainland source population on the Swedish West coast. We have studied the mainland population for over a decade during which >4000 offspring from >500 parents were monitored. We conducted an outbreeding experiment in which lizards from the mainland population with relatively low genetic variation were crossbred with lizards from distant populations that lack gene flow. The resulting 454 offspring were introduced to an otherwise uninhabited island with ideal sand lizard habitat. A survey of the island two decades later showed that offspring produced by females from the experimentally founded population had 13% higher hatching success (99.3% versus 86.4%) and elimination of the malformations occurring in 21% of clutches in the mainland source population. These results co-occur with higher genetic diversity. We conclude that outbreeding improved offspring viability in our island population ca 5–6 generations after the founding event, that is, with sustained viability effects at a time when heterotic effects are expected to have subsided.