516 - Heredity and Evolution 



ring in just one chromosome produced a new 

 stock that was sufficiently different from the 

 old to be considered as a separate species. 

 Selective Breeding: Artificial Selection. 



Except for species with a long history of self- 

 fertilization, animal and plant poptdations 

 tend to be extremely mixed; that is, com- 

 posed of individuals that are heterozygous for 

 many different pairs of genes. In consequence 

 of this heterozygosity, most species display a 

 high degree of genetic variability, and such 

 species are very plastic from the viewpoint 

 of experimental breeding. 



Artificial selection has produced many 

 "breeds" of animals and plants, according to 

 man's need or fancy. Attempts to improve 

 the quality of domesticated animals and 

 plants were started long before the laws of 

 heredity were understood, and many valu- 

 able breeds were created by hit-or-miss meth- 

 ods. However, modern genetics has greatly 

 accelerated the processes of selective breed- 

 ing, because modern breeders select their 

 crosses logically and efficiently, according to 

 the laws of segregation, assortment, linkage, 

 etc. Accordingly, it is now possible to "pick 

 apart" the hereditary qualities of a given 

 species and to assemble its desirable qualities 

 into a new stock that can be depended upon 

 to breed true in subsequent generations. 



Animal breeding is more difficult because 

 none of man's domestic animals can be self- 

 fertilized. However, pure lines of true-breed- 

 ing animal stocks can eventually be obtained 

 by persistent and continued inbreeding. 

 Strictly speaking, inbreeding involves broth- 

 er-sister matings in every generation; but in 

 practice first cousin crosses are more fre- 

 quently employed. The final goal of the in- 

 breeder is to establish complete homozygosity 

 with reference to all gene pairs that have any 

 effect upon the qualities of the organism that 

 are being "bred for." In the case of self- 

 fertilizing species this goal may be reached in 

 six generations of inbreeding; but brother- 

 sister inbreeding requires about three times 

 as many generations, and cousin crosses even 

 more. 



To take a crude example, suppose that the 

 problem is beef production, and the cattle- 

 man owns a large herd with a long history of 

 random mating. In this case, the muscle 

 weight of the herd would show tremendous 

 variation between the heaviest and lightest 

 animals. If the two heaviest specimens of the 

 herd are selected as parents, the offspring 

 will still vary widely in weight, but the aver- 

 age weight of the selected group will be 

 greater than that of the herd as a whole. 

 Similarly if the two champions of the selected 

 group are crossed, the average weight of the 

 F> selectees will be still greater. Finally, how- 

 ever, after about 20 generations of such selec- 

 tive inbreeding, a maximum weight will be 

 reached. Continued inbreeding will maintain 

 the heavily meated stock, but further im- 

 provement will not occur. The stock has be- 

 come homozygous for all genes concerned 

 with muscle weight and, therefore, no fur- 

 ther improvement can be expected, except 

 on the rare chance of a favorable mutation 

 or aberration. The breeder must now resort 

 to outcrossing with the champion of some 

 other famous pedigreed herd, in hopes that 

 a still more favorable combination of "meat 

 genes" may be obtained. If so, the process of 

 selection may proceed again until a peak of 

 homozygosity is gained in the new and better 

 stock. 



In practice, the problem is not so simple 

 as is indicated by the foregoing description. 

 The breeder must simultaneously select for 

 other qualities in addition to "meat weight." 

 Of what avail is a meat champion if the new 

 stock is susceptible to disease, cold, or other 

 unfavorable environmental factors; or if the 

 stock displays a reduced capacity to multiply? 

 In fact, the very process of inbreeding may 

 lead to a decrease in the general vigor of the 

 inbred line as compared to the mixed popu- 

 lation, unless the breeder pays close attention 

 to the factors of health and resistance in mak- 

 ing his parental selections. The tendencv ol 

 an inbred stock to lose vigor is not due to in- 

 breeding as such, but to an accidental accu- 

 mulation of deleterious recessive genes as the 



