FIXING TRANSGRESSIVE VIGOR IN NICOTIANA RUSTICA 173 



linked polygenes which give heterotic effects as helerozygoles. Heterosis is 

 explained genetically as due to the accumulated effect of the favorable domi- 

 nants and/or coadapted heterozygous combinations. It is an adaptive evolu- 

 tionary phenomenon (Dobzhansky, 1950). 



Selfing 



In naturally selfed populations there are accumulated, for the most part, 

 favorable genes that are either dominant, recessive, or lacking in strong 

 allelic interactions. Dominance is of little evolutionary significance, and 

 hence a preponderance of favorable dominant genes is not to be expected. 

 Furthermore, there would ordinarily be no adaptive significance to favorable 

 heterozygous combinations. One possible exception is suggested by Brieger's 

 (1950) demonstration that "if survival values for both homozygotes should 

 be below 0.5 (compared to the heterozygote value of 1.0) in selfed populations, 

 a final equilibrium is reached with all three genotypes remaining in the 

 population." Such a condition might have adaptive value in maintaining 

 variability in selfed organisms. Hybrid vigor in self-pollinated plants, in 

 view of the above considerations, is a chance manifestation, an "evolutionary 

 accident" causing luxuriant growth (Dobzhansky, 1950), and not an adap- 

 tive product of mutation and selection. 



However, from published data on crosses within selfed species of culti- 

 vated plants, it appears that hybrid vigor is of frequent rather than chance 

 occurrence. Reported results with flax (Carnahan, 1947), wheat (Harrington, 

 1944), barley (Immer, 1941), tomatoes (Larson and Currance, 1944), egg- 

 plants (Odland and Noll, 1948), and soybeans (Weiss, Weber, and Kalton, 

 1947) all demonstrated that hybrid vigor is characteristic of Fi's. If these 

 data constitute a representative sample, then, although hybrid vigor is an 

 evolutionary accident in naturally selfed species, it is not a genetical accident. 



The result may be interpreted genetically as follows: Selfed species are 

 purged of deleterious genes by selection. Different varieties within the 

 species have accumulated different alleles all of which control "non-defec- 

 tive," slightly different physiological reactions. The combination of divergent 

 alleles in heterozygous condition may, more frequently than not, act as East 

 has suggested in a complementary manner to produce a more efficient physio- 

 logical condition. This is expressed phenotypically by the hybrid manifesting 

 more vigorous growth than midway between the homozygotes. Subsequent 

 selection and inbreeding, however, would permit an accumulation of the most 

 favorable alleles or gene complexes in the homozygous condition. 



As a simplified schematic example, let us assume that two varieties. Pi 

 and P2, differ by three alleles or linked polygene complexes: X' is dominant 

 and favorable for vigorous growth, F^ is a favorable recessive, and at the Z 

 locus the product of the heterozygous condition is above the mean of the 



