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SOULI:'/CHAPTER 9 THRESHOLDS FOR SURVIVAL Inbreeding Depression in Captive Populations The literature on inbreeding is replete with examples of the deleteri- ous effects of homozygosity. One example reproduced in Table II is from Wright (1977a). He recounts an experiment on Poland China swine (McPhee et aI., 1931) that may typify the scenario for other species which lack a history of intense inbreeding and selection. The experiment was designed to study the effects of sib (brother-sister) mating. With sib mat- ing, a line loses 25 percent of the remaining heterozygosity in each gener- ation, at least in theory. At such a rate of inbreeding, many deleterious (usually recessive) genes in the line will be fixed by chance, because under these conditions the rate of fixation is greater than the rate at which selection against homozygotes can eliminate them. The swine experiment was discontinued after two generations following a precipitous drop in fitness. Table II shows that the mean number of pigs per litter went from 7.15 in the noninbred herd to 4.26 in the second generation of inbreeding. In addition, the survival of newborn pigs dropped by over 50 percent. Multiplying the size of the litter by the percent of young raised to 70 days gives the number of surviving offspring per litter; this value dropped from four to one. Accompanying this decrease in fecundity and viability went a change in sex ratio favoring males (Chapter 12 explains why this effect is expected), thus producing a shortage of sows. The symbol fin Table II is Wright's inbreeding coefficient, and can be thought of as the amount that heterozygosity has decreased compared with the base popu- lation (general herd). TABLE II. Vital statistics of a herd of Poland China swine and the progeny of two generations of sib mating. Percent f Size of born No. Dam Litter litter alive Percent raised to Sex 70 days ratio 58.1 109.7 41.2 126.1 26.6 156.0 General herd 694 0 + 0 + 7.15 97.0 FI inbred 189 0.09 0.33 6.75 93.7 F, inbred 64 0.33 0.42 4.26 90.6 (From McPhee, Russel and Zeller, 1931. Mter Wright, 1977.) Harmful changes in reproductive characters. typically occur upon in- breeding; a large part of inbreeding depression is made up of such changes. A survey of inbreeding experiments led to the generalization that increasing the inbreeding coefficient by 10 percent induces a 5 to 10 157