Races and the Origin of Species 



249 



for hybrids between similar species, there is 

 a second way interspecific hybrids can be- 

 come stabilized as a new species, provided 

 that the two hybridizing species are very 

 similar chromosomally. If the two species 

 have the same haploid number, their Fi 

 hybrid may have all chromosomes synapsed 

 in pairs at meiosis. Segregation, independ- 

 ent segregation, and crossing over may yield 

 progeny of the hybrid whose recombinations 

 can become stabilized in nature yet are iso- 

 lated from either parental species. Con- 

 sider certain species in the larkspur genus, 

 Delphinium: D. gypsophilum is morpholog- 

 ically intermediate between D. recurvatum 

 and D. hesperium; all three species have 

 2n = 16; and the "parent" species, recurva- 

 tum and hesperium, can be crossed to pro- 

 duce an ¥\ hybrid. When the F! hybrid is 

 crossed to gypsophilum, the offspring are 

 more regular and more fertile than those 



produced by backcrossing the F 1 hybrid with 

 either parent species. Similarly, the progeny 

 from crosses between gypsophilum and either 

 of its parent species are not as regular or as 

 fertile as are those from the cross between 

 gypsophilum and the hybrid of the parent 

 species. These results provide good evi- 

 dence that gypsophilum arose as the hybrid 

 between recurvatum and hesperium. Fig- 

 ure 1 8-4 shows the distribution of these 

 species in California. 



The third way that interspecific hybrids 

 can become stabilized as new species is by 

 introgression. In this process a new type 

 arises after the interspecific hybrid back- 

 crosses with one of the parental types. The 

 backcross recombinant types favored by nat- 

 ural selection may contain some genetic com- 

 ponents from both species, may be true- 

 breeding and, eventually, may become a new 

 species. 



SUMMARY AND CONCLUSIONS 



A race of a cross-fertilizing species is characterized by the content of its gene pool. 

 Each race is adapted to the territory in which it lives. Different races can be sympatric 

 or allopatric. Races can become species by accumulating genetic differences whose 

 end effect is genetic discontinuity — that is, the formation of isolated gene pools. Sep- 

 aration of two gene pools is usually accomplished by a combination of different repro- 

 ductive barriers each of which is incomplete by itself and has a polygenic and/or a 

 polychromosomal basis not necessarily correlated with morphological differences. 



It is generally recognized that most cross-fertilizing species arose from the further 

 differentiation of races. Occasionally a new species can arise via autopolyploidy, and 

 it is possible that a new species can also arise by the gradual change of one species as 

 a whole into another species. 



Two (or more) species can give rise to a new one after interspecific hybridization. 

 An interspecific hybrid can form a new species via amphiploidy, by selection of re- 

 combinants among its progeny, or by selection of individuals produced after intro- 

 gression. 



