Impacts of Inbreeding in Natural and Captive Populations of Vertebrates: Implications for Conservation

IMPACTS OF INBREEDING IN NATURAL AND CAPTIVE POPULATIONS OF VERTEBRATES: IMPLICATIONS FOR CONSERVATION ROBERT C. LACY* Many wildlife populations that were once large, widespread, and diverse have been reduced to small, isolated populations in a few remaining natural areas, nature preserves, and zoological parks. Once a population becomes very small and isolated from potential sources of immigrants, random demographic and genetic processes can lead the population rapidly toward extinction. Demographic problems faced by small, fragmented populations include difficulty in finding mates, random skewing of the sex ratio, and simply the misfortune of all animals dying or failing to breed in a given year due to chance, a disease outbreak , a temporary scarcity of food or an abundance of predators, or local weather conditions. These and other random demographic and environmental events can cause a small population to decline still further . Even if the causes of such a decline are temporary, the resulting very small population may be forced to inbreed. It has long been observed that inbreeding causes greater mortality and reduced fecundity in many species, the phenomenon called "inbreeding depression" [1—3]. Inbreeding and the loss of genetic diversity may lower fitness and reduce the potential for the population to adapt. This can lead to further decline, making the demographic problems worse, in turn making the genetic problems worse—a feedback that drives small populations ever faster toward extinction. This process has been termed the "extinction vortex" [4], and the size below which a population is likely to get drawn into the extinction vortex is one useful definition [e.g., 5] of the Minimum Viable Population size (or MVP) [6]. This paper was first presented at a Conference on Genetics and Wildlife Conservation at the Istituto Nazionale di Biologia della Selvaggina, Bologna, Italy, September, 1990. *Department of Conservation Biology, Chicago Zoological Park, Brookfield, Illinois 60513.© 1993 by The University of Chicago. AU rights reserved. 0031-5982/93/3603-0835$01.00 480 Robert C. Lacy ¦ Inbreeding in Natural and Captive Populations Conservation of remnant wildlife populations requires knowledge of the minimum population size below which the combined effects of random genetic changes and demographic variation would likely result in extinction [4-11]. Estimating the MVP for a population will require considerable knowledge of the effects of inbreeding on endangered populations [12]. In particular, we need to know: (1) whether the inbreeding depression that is so prevalent among domesticated animal stocks is equally characteristic of most wild populations; (2) the severity of inbreeding depression (the function relating fitness traits to past and present inbreeding) in natural populations; (3) whether inbreeding depression is consistent and predictable; and (4) whether a depression in fitness traits can be avoided or reversed by combining inbreeding with selection to purge a population of the genes that cause inbreeding depression . Causes ofInbreeding Depression The common explanation for the hazards of inbreeding (inbreeding depression) rests on the presence of deleterious recessive genes, a "genetic load," in virtually all diploid organisms. Natural selection will reduce the frequencies of deleterious alleles in a population (or keep newly arisen harmful mutations from becoming common), but it is very inefficient at eliminating wholly a recessive allele because such alleles are almost always shielded from selection in heterozygotes. One consequence of inbreeding is that it makes it much more likely that an individual is homozygous for a rare gene because it is more likely that two related parents simultaneously possess a rare allele and transmit it to their inbred offspring than that two unrelated individuals independently transmit the same rare allele to a non-inbred offspring. Thus, inbreeding seems to reduce fitness because it reveals harmful genes in homozygotes. If this explanation of inbreeding is correct, then we should be able to predict which populations would be most susceptible to inbreeding depression [12]. When inbreeding occurs, deleterious genes will be expressed in homozygotes and cause the death of individuals bearing them, but those genes are thereby eliminated from the population. Thus, populations with histories of inbreeding should now be largely purged of their genetic load of recessive deleterious genes. On the other hand, populations that have long been large and diverse may harbor many, individually rare, deleterious, recessive genes. Thus, the black...