NATURE AND ORIGIN OF HETEROSIS 223 



as a result of an evolutionary history controlled by natural or by artificial 

 selection. The normal heterotic state can be disru])ted by sudden inbreeding, 

 which is evidently a disturbance of the reproductive biology to which the 

 species is adjusted. The heterotic state can also be restored by intercrossing 

 the inbred lines. This is true heterosis, or euheterosis. Euheterosis is a form 

 of evolutionary adaptation characteristic of sexually reproducing and cross- 

 fertilizing species. 



Numerous instances are known, however, when hybrids between si)ecies, 

 neither of which can be regarded as inbred, are larger, faster growing, or 

 otherwise exceeding the parental forms in some quality. Similar luxuriance is 

 observed in some hybrids between normally self-fertilizing species, races, or 

 strains. This kind of luxuriance of hybrids cannot be ascribed to sheltering of 

 deleterious recessive mutants, because the latter are sheltered in the parental 

 populations. It is also unlikely to arise from overdominance since, at least in 

 wild species, natural selection would be expected to have induced such bal- 

 anced heterosis in the parental species or strains. 



Luxuriance is, from the evolutionary standpoint, an accidental condition 

 brought about by complementary action of genes found in the parental form 

 crossed. Two sets of facts are important in this connection. First, in cases of 

 luxuriance there is usually no indication whatever that the luxuriant hybrids 

 would prove adaptively superior in competition with the parental forms in 

 the natural habitats of the latter. Second, luxuriance appears to be more 

 frequently encountered in domesticated than in wild species. 



It stands to reason that increase in body size, or in growth rate, is by no 

 means always an adaptively superior change. To equate size with vigor, fit- 

 ness, or adaptive value would be a height of anthropomorphic naivete. The 

 rate of growth and the size attained by an organism in its normal environ- 

 ments are evidently controlled by natural selection. Excessive as well as de- 

 ficient sizes are adaptively about equally disadvantageous. The checks upon 

 excessively rapid growth and excessive size are, however, very often relaxed 

 under domestication. In man-controlled environments those qualities often 

 become desirable from the standpoint of the breeder if not from that of the 

 organism. Luxuriance is, really, pseudoheterosis. 



