Inbreeding depression is of major concern in the management and conservatio
n of endangered species. Inbreeding appears universally to reduce fitness,
but its magnitude and specific effects are highly variable because they dep
end on the genetic constitution of the species or populations and on how th
ese genotypes interact with the environment. Recent natural experiments are
consistent with greater inbreeding depression in more stressful environmen
ts. In small populations of randomly mating individuals, such as are charac
teristic of many endangered species, all individuals may suffer from inbree
ding depression because of the cumulative effects of genetic drift that dec
rease the fitness of all individuals in the population. In three recent cas
es, introductions into populations with low fitness appeared to restore fit
ness to levels similar to those before the effects of genetic drift. Inbree
ding depression may potentially be reduced, or purged, by breeding related
individuals. However, the Speke's gazelle example, often cited as a demonst
ration of reduction of inbreeding depression, appears to be the result of a
temporal change in fitness in inbred individuals and not a reduction in in
breeding depression.