138 • Technologies To Maintain Biological Diversity 



dangered in their natural habitats. These 

 programs can be considered holding actions 

 designed to reinforce rather than replace wild 

 populations. If a natural population is deci- 

 mated or lost, captive maintenance programs 

 provide a reservoir of individuals to allow re- 

 introduction. 



The genetic diversity of the original popula- 

 tion must not be lost or seriously reduced dur- 

 ing captive maintenance if animals are ex- 

 pected to be able to readapt to life in the wild. 

 Likewise, genetic changes that may be induced 

 during captive maintenance must be mini- 

 mized. Reciprocal transfers of individuals be- 

 tween wild and captive populations can help 

 reduce genetic pressures. Such exchanges, 

 however, involve the capture of wild animals, 

 and they risk the accidental death of some of 

 them. Therefore, risks should be evaluated care- 

 fully before beginning a program of genetic ex- 

 changes between wild and captive populations. 



For domestic species, all populations are by 

 definition maintained offsite. Most of these ani- 

 mals have existed in association with humans 

 for centuries, and their current genetic diver- 

 sity is a reflection of this long interaction. Their 

 genes have been manipulated through genera- 

 tions of selective breeding to meet the diverse 

 needs of humans, and this manipulation has 

 led to a wealth of specialized breeds (boxes 6- 

 A and 6-B). Some wild progenitors of domes- 

 tic species still differ so much from domestic 

 populations that they exist as a reservoir of 

 genetic diversity, but these natural populations 

 are unlikely to contribute much to current com- 

 mercial stocks through traditional breeding 

 methods. 



The aim of programs to maintain genetic 

 diversity in livestock differs somewhat from 

 that for wild animals. In domestic populations, 

 the challenge is to maximize current utility and 

 preserve sufficient diversity to ensure live- 

 stock's continued adaptability to changing— 

 and often unforeseen— human needs. In fact, 

 efforts to raise current rates of food produc- 

 tion may constitute the greatest threat to future 

 flexibility by concentrating unduly on short- 

 term production goals with attendant losses in 



Box 6-A. — Breeds 



In its most restrictive form, the design^ 

 "breed" is reserved for subpopulations that 

 have both distinctive morphological charac- 

 teristics and a pedigree-recording system to 

 document the ancestry of individuals within 

 the breed. In practice, however, a breed is 

 often taken to be any differentiated, identifia- 

 ble domestic subpopulation. 



Breeds that are widespread may often be fur- 

 ther subdivided into national or regional pop- 

 ulations that retain the major characteristics 

 of the parent breed but have become partially 

 differentiated in response to local conditions 

 and selection pressures. Thus, identifiable re- 

 gional populations exist for Holstein-Friesian 

 dairy cattle in North America and most of the 

 European countries, for Shorthorn beef cattle 

 in nations previously colonized by the British, 

 and for Merino sheep worldwide. Some de- 

 gree of interbreeding often occurs among such 

 populations, but in most cases they persist and 

 are referred to as "strains" or "stocks" withinj 

 the breed. i 



For more industrialized species such as 

 poultry, a few ancestral breeds such as the Leg- 

 horn chicken have been used in concert with 

 crossbreeding and selection to develop a wide 

 array of partially differentiated stocks main- 

 tained by commercial breeding firms. These 

 stocks are open to genetic manipulation at the 

 discretion of the firms, but they represent 

 another source of differentiated genetic ma- 

 terial. 



In this assessment, the term breed will be 

 used to describe any differentiated domestic 

 subpopulation and will include both regional 

 strains of major international breeds and iden- 

 tifiable commercial stocks. 



I 



genetic diversity that may be important to fu- 

 ture generations. 



Breeding Pregrams v. Leng-Term 

 Cryogenic Storage 



Using captive breeding programs to retain 

 a considerable proportion of the genetic diver- 

 sity of endangered species or rare breeds for 



