222 TH. DOBZHANSKY 



Now, it has been shown by observation both on natural and on experi- 

 mental populations of some Drosophila species, that the heterozygotes for 

 the naturally occurring inversions possess considerable adaptive advantages 

 over the homozygotes. For example, taking the adaptive value of the 

 heterozygotes for ST and CH inversions in Drosophila pseudoobscura to be 

 unity, the adaptive values of the ST/ST and CH/CH homozygotes are 

 about 0.8 and 0.4 respectively. Further, it has been shown that the heterosis 

 in the ST/CH heterozygotes occurs only if the constituent chromosomes are 

 derived from the same population, or from populations of nearby localities. 

 Chromosomes with the same gene arrangements, ST and CH, derived from 

 remote localities (such as Central and Southern California, or Southern 

 California and Mexico) exhibit little or no heterosis. 



This finding is most compatible with the assumption that the over- 

 dominance in fitness observed in the heterozygotes is the property not of a 

 single gene locus, or of a chromosome structure, but rather of integrated sys- 

 tems of polygenes. Such polygenic systems are coadapted by natural selec- 

 tion to other polygene complexes present in the same populations. The role 

 of the chromosomal inversions in the formation of the heterotic state of bal- 

 anced polymorphism is due to the suppression of crossing over caused by 

 most inversions, at least in Drosophila. Elimination of crossing over prevents 

 the breakup of the adaptively integrated polygene complexes which are 

 carried in the chromosomes involved. 



It should be noted that adaptively integrated polygene complexes can be 

 maintained in crossbreeding populations with the aid of genetic mechanisms 

 other than chromosomal inversions. Any factor which restricts or prevents 

 crossing over in chromosomes, or parts of chromosomes, can accomplish the 

 same biological function. Localization of chiasmata may be such a factor. 

 If, for example, chiasmata are found chiefly or exclusively at some definite 

 points in a chromosome, the genes carried in the sections which intervene 

 between these points are inherited in blocks. Such gene blocks may act 

 exactly as gene complexes bound together by inversions. 



Balanced heterosis differs profoundly from mutational heterosis. The 

 latter is due simply to the sheltering of deleterious recessive mutants by 

 their dominant alleles. Balanced heterosis is a result of overdominance. Mu- 

 tational heterosis is a protective device of a sexual species with a certain 

 population structure against the mutation pressure. Balanced heterosis is an 

 evolutionary contrivance that permits maintenance in a population of a mul- 

 tiplicity of genotypes that may be adaptive in dififerent ecological niches 

 which the population occupies. 



LUXURIANCE 



Mutational and balanced heterosis resemble each other in one important 

 respect — both are normal adaptive states attained in outbred sexual species 



