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CHAPTER 18 



i aces of a single species. Sibling species are 

 found in mosquitoes and Other insects as 

 well as in Drosophila; they are also found 

 in plants — among the tarweeds of the aster 



family and in the blue wild rye. 



The study of D. pseudoobscura and D. 



persimilis illustrates two other principles re- 

 lating to species formation. First, any par- 

 ticular reproductive barrier usually has a 

 multigenic and or a multichromosomal basis; 

 second, any two species are separated not 

 by one but by a number of reproductive bar- 

 riers. Although each of the barriers involved 

 is incomplete, together they result in com- 

 plete reproductive isolation — there being no 

 stream of genes between the two gene pools 

 in nature. The known differences between 

 these two particular sibling species include: 



1 . Pseudoobscura lives in drier and warm- 

 er habitats than persimilis 



2. Females accept the mating advances 

 of males of their own species more 

 often than they do male advance of 

 the other 



3. Pseudoobscura usually mates in the 

 evening, persimilis in the morning 



4. Interspecific hybrids are relatively in- 

 viable and when viable, they are mostly 

 sterile. 



The nature and origin of the reproductive 

 isolation mechanisms involved in forming 

 new species from races shows that valid 

 species originate not by a single or simple 

 mutation, but as the result of many different, 

 independently occurring genetic changes. 

 Moreover, as already noted, speciation is 

 accomplished not merely by an accumulation 

 of mutants which distinguish races, but also 

 by those which contribute to reproductive 

 isolation. Usually populations are physically 

 separated while reproductive barriers are 

 being built up; otherwise, hybridization 

 would break down these barriers. Experi- 

 mental evidence also supports our expecta- 



tion that natural selection acts to further the 

 accumulation of the genetic factors promot- 

 ing reproductive isolation between races. 



The preceding discussion illustrates how 

 one species can give rise to two or more 

 species via races which serve as incipient 

 species.- It was stated earlier that a species 

 has an isolated gene pool, that is, a gene 

 pool closed to individuals of some other al- 

 ternative condition (species). A species is 

 expected to undergo numerous changes in 

 its gene pool during the course of many 

 generations. At the end of this time, is it 

 the same or a new species? Here is an 

 example of one type of species formation 

 which would not be recognized by the cri- 

 terion above because the alternative state 

 would no longer exist. Suppose some mem- 

 bers of the original population had been 

 (miraculously) preserved, then we might 

 find that they were reproductively isolated 

 from the members of the new population. 

 In such an event we could admit the forma- 

 tion of a new species whose origin is de- 

 pendent upon the "extinction" of the parent 

 species. This type of speciation will be- 

 come a valid subject of study once man 

 learns how to preserve sample genotypes 

 indefinitely. 



One species can give rise to another via 

 allopolyploidy — an increase in the number 

 of genomes present in a normally cross- 

 fertilizing species. Mechanisms for the pro- 

 duction of autopolyploid cells, tissues, and 

 organisms have already been described on 

 pp. 151-153. In the genus Chrysanthemum, 

 species occur with 2n chromosome numbers 

 of 18, 36, 54, 72, and 90. Thus, it appears 

 that nine is the basic n number. In the 

 genus Solanum (the nightshades, including 

 the potato ) the basic n number seems to be 

 twelve, since species of this genus are known 



'-' See Th. Dobzhansky, L. Ehrman, O. Pavlovsky, 

 and B. Spassky (1964). 



