LECTURES IN BIOLOGICAL SCIENCES 



between races to be sterile, the related species of Trillium differ 

 from T. kamschaticum in respect to more numerous chromo- 

 somal differences of the same kind. Furthermore, although hy- 

 brids between diploid species of Trillium are not known, a 

 wealth of evidence from hybrids between species in other plant 

 genera (Stebbins, 1950, 1958, 1959a) shows that chromosomal 

 differences such as those which exist between the diploid species 

 of Trillium can be responsible for hybrid sterility. Further- 

 more, this sterility can be overcome by doubling the chromosome 

 number. This places together in the same hybrid nucleus dupli- 

 cate sets of chromosomes, which are consequently able to pair 

 normally and produce viable gametes. 



Once reproductive isolating barriers have arisen, they can 

 be strengthened and reinforced by natural selection which favors 

 those individuals of a species having an instinctive tendency to 

 mate with others of their own species. This process has been 

 fully discussed by Dobzhansky (1951, 1958). 



A final way in which species-separating barriers can be 

 formed is as a secondary result of hybridization between pre- 

 existing species. The process of amphiploidy, or the produc- 

 tion of fertile, true-breeding species by doubling the chromo- 

 some number of sterile hybrids, is now well known to geneticists, 

 and numerous examples have been described, both in the gar- 

 den and in nature (Stebbins, 1950, 1959a). In addition, more 

 recent experiments have shown that highly sterile, though 

 slightly fertile interspecific hybrids can produce more fertile 

 offspring in later generations without change in chromosome 

 number. Furthermore, some of the fully fertile strains which 

 can be bred from such offspring may form partly sterile hybrids 

 when crossed with either of the original parental species (Steb- 

 bins, 1959a). This is apparently brought about by the effects 

 of genetic recombination, which in rare instances may build 

 up a new harmonious recombination of those genetic differ- 

 ences which were responsible for the original sterility barrier. 



Half a century ago, many evolutionists believed that the criti- 

 cal experiment which would demonstrate our understanding of 

 evolutionary processes would be the production under con- 

 trolled conditions of a new species. This they defined as a 

 population which would breed true and would be reproduc- 



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