Scientists have long speculated as to the extent that inbreeding contributes to the decline and eventual extinction of large assimilated populations, most agreeing that in natural populations the impact of inbreeding depression on Line population survival is insignificant in comparison to that of demographic and environmental variables. While there is ample evidence of fitness reduction due to inbreeding depression in captivity, no field studies on natural populations have been conducted to test the proposed effect on extinction.However, Saccheri recently found in his study of a large metapopulation of the Glanville .fritillary butterfly that extinction risk increased significantly with decreasing heterozygosity, an indication of inbreeding, even after accounting for the effects of the relevant ecological factors. Rate of maturation,larval survival, adult longevity and egg-hatching rate were found to be adversely affected by inbreeding, and appear to be the fitness components underlying the relationship between inbreeding and extinction. One wonders, nevertheless,how applicable these results would be to the increasing number of species with small isolated populations due to habitat loss and fragmentation.

If the phenomenon "extinction risk increased significantly with decreasing heterozygosity" agrees with the proposed effect on extinction, the passage assumes（）