Does inbreeding affect the extinction risk of small populations? predictions from Drosophila

A fundamental assumption underlying the importance of genetic risks within conservation biology is that inbreeding increases the extinction probabilit y of populations. Although inbreeding has been shown to have a detrimental impact on individual fitness, its contribution to extinction is still poorl y understood. We have studied the consequences of different levels of prior inbreeding for the persistence of small populations using Drosophila melan ogaster as a model organism. To this end, we determined the extinction rate of small vial populations differing in the level of inbreeding under both optimal and stress conditions, i.e. high temperature stress and ethanol str ess. We show that inbred populations have a significantly higher short-term probability of extinction than non-inbred populations, even for low levels of inbreeding, and that the extinction probability increases with increasi ng inbreeding levels. In addition, we observed that the effects of inbreedi ng become greatly enhanced under stressful environmental conditions. More i mportantly, our results show that the impact of environmental stress become s significantly greater for higher inbreeding levels, demonstrating explici tly that inbreeding and environmental stress are not independent but can ac t synergistically. These effects seem long lasting as the impact of prior i nbreeding was still qualitatively the same after the inbred populations had been expanded to appreciable numbers and maintained as such for approximat ely 50 generations. Our observations have significant consequences for cons ervation biology.