Inbreeding depression is a well-known problem in the small, captive populations of endangered species that are propagated in zoos in the hope of reestablishing wild popu- lations. Special breeding designs are required to minimize inbreeding in these popula- tions (Frankham et al. 2002). Inbreeding may also increase the risk of extinction of small populations in nature. Thomas Madsen and colleagues (1995, 1999) studied an isolated Swedish population of a small poisonous snake, the adder Vipera berus, that consisted of fewer than 40 individuals. The snakes were found to be highly homozygous (we will see shortly how this can be determined), the females had small litter sizes (compared with outbred adders in other populations), and many of the offspring were deformed or stillborn. The authors introduced 20 adult male adders from other populations, left them there for four mating seasons, and then removed them. Soon thereafter, the population increased dramatically (Figure 9.15), owing to the improved survival of the outbred offspring.
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