150 • Technologies To Maintain Biological Diversity 



Box 6-C.— Genetic Drift and Inbreeding 



Once the decision has been made to propagate a population in a controlled setting, a plan to en- 

 sure retention of desired levels of genetic diversity must be developed. In large populations with ran- 

 dom mating (i.e., without preferential reproduction by certain individuals), frequencies of the various 

 genes at each locus are essentially constant and genetic change is minimal. Likewise, in large random- 

 mating populations, the mating of close relatives is unlikely. If the population contains only a few 

 individuals, however, the sample of genes passed to the next generation may differ considerably from 

 that found in the parent generation. Rare genes may be lost and others may, by chance, be dispropor- 

 tionately replicated. This process is called genetic drift and is cumulative; once a gene is lost, it can 

 only be re-created through mutation — an exceedingly rare event. Genetic drift can lead to a captive 

 population that differs considerably from the original source population and is much less genetically 

 diverse. 



If population size is limited, the likelihood of mating by close relatives is also greater. Because 

 related individuals tend to carry similar genes, mating of relatives can increase genetic uniformity 

 in the offspring. This increased uniformity is known as inbreeding; in most species, rapid increases 

 in inbreeding lead to decreases in fertility and postnatal survival within the population. Indeed, if 

 the level of inbreeding is too high, the overall viability of the population can be seriously compromised. 



Both inbreeding and genetic drift can be characterized in terms of decreases in genetic diversity 

 and corresponding increases in genetic uniformity. The losses in diversity are directly related to pop- 

 ulation size. If several rare genes are present in the original population, the rate of loss of these genes 

 can be considerably greater than the average rate of loss of genetic diversity in the population (6). 

 Likewise, if gene interactions are important sources of diversity (49), specific gene combinations will 

 be lost much more rapidly than will individual genes. Thus, larger populations will be required to^ 

 ensure retention of rare genes. ™ 



In this assessment, genetic diversity is discussed primarily as effects of independent genes. This 

 approach appears acceptable in terms of the probable fitness and viability of the population and of 

 likely future responses to selection (35). However, results and conclusions regarding required popula- 

 tion sizes will be conservative for preservation of rare genes and gene combinations. 



1 



Genetic drift accumulates generation by gen- 

 eration, not year by year, and animal species 

 differ considerably in this regard (box 6-C). For 

 example, 200 years covers perhaps 100 gener- 

 ations of chickens but only 7 to 8 generations 

 of elephants. Yet in most cases, breeding pop- 

 ulations should not experience inbreeding rates 

 in excess of 1 percent per generation; to meet 

 this constraint, at least 50 breeding individuals 

 per generation are required. 



Manipulation of the breeding structure of a 

 population can have a significant impact on its 

 genetic characteristics. For example, an appro- 

 priate level of subdivision of a population can 

 retard the overall rate of genetic loss in the pop- 

 ulation as a whole: Subdivision increases the 

 rate of loss within each subgroup, but the spe- 

 cific genes that are lost through random drift 



vary among subpopulations. And the number 

 of genes that can be maintained within the sub- 

 divided population will exceed the number that 

 could be maintained in a random-mating pop- 

 ulation of comparable size. In practice, the min- 

 imum size of the subpopulation will depend on 

 the tolerance of the species to the inbreeding 

 rates. Still, some degree of subdivision should 

 be practiced, and the movement of individuals 

 among subpopulations should be fewer than 

 one per generation unless effects of inbreed- 

 ing become evident. Subdivision also protects 

 the population against disease outbreaks or 

 other disasters that might annihilate any one 

 subpopulation. 



The loss of genes from a captive population 

 can also be retarded by controlling mating. In 

 contrast to selection, which presupposes a 



