Ch. 6— Maintaining Animal Diversity Offsite • 139 



lox 6-B.— Replacement and Genetic Diversity 



Traditionally, breed replacement has proceeded on an evolutionary time scale, with gradual changes'* 

 in breed composition and provision for maintenance of a wealth of local populations. Recently, how- 

 ever, the pace of breed replacement has accelerated, with the emergence of multinational breeding 

 companies in poultry and swine, the widespread use of artificial insemination and intensified sire 

 selection in dairy cattle, and enhanced opportunities for dissemination of germplasm throughout the 

 world. Also, greater standardization of production, marketing, and recording procedures for poultry, 

 swine, and dairy cattle in industrial countries has increasingly promoted replacement of local breeds. 



In domestic species, the greatest threat to genetic diversity involves extensive and sometimes 

 indiscriminate crossing of indigenous stocks in developing countries with breeds from North Amer- 

 ica and Western Europe (3). This crossing stems from needs to increase world food production and 

 from a belief that this goal is best met using stocks with the highest possible genetic merit for individ- 

 ual traits (such as milk or egg production). But breeds developed in temperate-zone industrial coun- 

 tries are often not suited to the more restrictive nutritional, management, and disease conditions of 

 developing countries and may be less efficient than indigenous stocks in using available resources. 

 Only recently has the need for comprehensive evaluation of the total performance of imported breeds 

 begun to be recognized in developing countries. Unfortunately, serious dilution of original breeds 

 may have already occurred. Thus, it is not the process of breed replacement per se that is a problem 

 but the rate of replacement and the danger that useful breeds may be discarded before they can be 

 fairly evaluated. 



Regional strains of estabhshed breeds are especially vulnerable to loss through intercrossing with 

 more popular strains. Extensive use of Holstein bulls from North America in European Friesian pop- 

 ulations threatens serious dilution of the genetic material of these strains. The percentage of Holstein 

 genes in young Friesian bulls entering European artificial insemination programs in 1982 ranged 

 from 8 percent in Ireland to 91 percent in Switzerland and averaged 54 percent for 10 European 

 countries (4). 2 



Genetic diversity can sometimes be reduced in commercial stocks even if population numbers * 

 remain large. These losses can occur when selection is intense and control of breeding stock is con- 

 centrated in a few large breeding farms (as in the commercial poultry industries) or when artificial 

 insemination allows extensive use of a few selected sires throughout the population (as in the dairy 

 industry). In both cases, the result is increased genetic uniformity within the stock despite the large 

 numbers. Several studies (3,25) have concluded that important losses may be occurring in commer- 

 cial poultry breeds. Comparable losses have apparently not yet happened in dairy cattle populations. 

 No imminent losses of genetic diversity within major commercial breeds are foreseen for swine, sheep, 

 goats, or beef cattle. Several populations of chickens are currently being maintained without selection 

 and at sufficient population sizes to substantially retard losses in genetic diversity (42). And some 

 artificial insemination organizations retain semen from bulls that have been removed from service. 

 However, these programs do not represent industry or public policy, and parallel programs do not 

 exist for other domestic species. 



The loss of endangered species and rare breeds is of particular concern in light of likely future 

 advances in molecular biology and genetics. The ability to extract desirable genes from different spe- 

 cies or less productive types and insert them into domestic animals could have important implica- 

 tions for designing superior animals for specific environmental conditions. Unfortunately, knowl- 

 edge of the genetic material of most wild and domestic animals is rudimentary. For instance, it is 

 unclear if adaptive factors such as heat tolerance and disease resistance are controlled by many or 

 a few genes, and no basis yet exists to assess potential single-gene contributions of local breeds and 

 endangered species. 



