6 AGRICULTURE HANDBOOK NO. 3 09, U.S. DEPARTMENT OF AGRICULTURE 



In the wild, natural selection oc- 

 curs through the survival of the 

 fittest based upon function in the 

 environment available. Selection 

 b}' the rabbit l)reeder, if done in- 

 telligently, often proves superior in 

 many ways. Outbreeding with 

 careful selection is generally ac- 

 cepted as a satisfactory procedure 

 for commercial purposes and, with 

 due attention to reproductive ca- 

 pacity, accounts for a moderate de- 

 gree of improvement of breeds, 

 particularly when carried out ac- 

 cording to the breed standards. It 

 cannot produce permanently a high 

 degree of uniformity, even in the 

 hands of the most skilled breeder, 

 nor can it lead to establishment 

 of an outstanding strain with 

 recognizably uniform dependable 

 improvement. 



Outcrossing, or hybridization, 

 consists of wide matings between 

 unrelated rabbits. It is usually 

 done between breeds for special 

 purposes. It is the initial step in 

 the establishment of new breeds, 

 because by bringing together a 

 maximum number of unlike genes 

 of the two breeds (or gene pools), 

 a maximum range of variation 

 from which to select is provided. 

 Because of the extreme degree of 

 relationship, such matings in the 

 first one or two generations fre- 

 quently manifest a maximum 

 amount of fertility, vigor, and 

 growth, commonly referred to as 

 hybrid vigor or heterosis. The 

 first generation is thus often a 

 highly desirable commercial ani- 

 mal especially for meat production. 

 Later generations, however, be- 

 cause or their great variation are 

 of little value commercially. Their 

 breeding value lies only in the 

 range of variation which they pro- 

 vide in a selection program extend- 

 ing over a number of generations 

 in which the aim is selection for 

 the most desirable characteristics 



of both breeds. Some breeders 

 have capitalized on the advantages 

 of outcrossing by involving three 

 breeds, each of which contributes 

 especially desirable characteristics. 

 However, such crosses may be ex- 

 pected to involve a longer period 

 of selection to arrive at the ulti- 

 mate objectives. 



Inbreeding in contrast to out- 

 breeding, is the mating of closely 

 related individuals. The closest 

 form is brother-sister or parent- 

 offspring mating. Carried on for 

 20 generations or more it leads to 

 genetic uniformity. Opinion varies 

 with regard to its use. In general, 

 it is in bad repute because it usu- 

 ally is initiated with a stock pre- 

 viously outbred for many genera- 

 tions; such a stock is likely to carry 

 a large pool of undesirable reces- 

 sive genes covered up by tlie proc- 

 ess of outbreeding. Some of these 

 genes may be lethal, thus reducing 

 viability and reproductive fitness 

 as they are brought together by 

 successive generations of inbreed- 

 ing. However, as these recessive 

 genes are observed and ruthlessly 

 discarded, the strain in each gen- 

 eration tends to become more and 

 more uniform. Inbreeding in it- 

 self does not create harmful genes; 

 it only exposes those that are al- 

 ready present. At the same time, 

 careful selection fixes favorable 

 and desirable dominant and reces- 

 sive characters so that uniformity 

 is progressively increased. In the 

 smaller laboratory animals which 

 reproduce more rapidly than rab- 

 bits, a number of successfully iso- 

 genic (as alike as identical twins 

 in man) strains have been success- 

 fully established for research pur- 

 poses. This means lines which are 

 so much alike that skin and other 

 tissues can be successfully grafted 

 or transplanted among them. The 

 procedure followed is simple and 

 straightforward if no complica- 



