Races and the Origin of Species 



259 



By what mechanisms can interspecific hybrids 

 form successful, sexually reproducing popu- 

 lations that have their own closed gene pools? 

 There are several methods by which inter- 

 specific hybrids, of plants particularly, may 

 be converted into stable intermediate types 

 that are isolated from their parental species. 

 The first method involves amphiploidy (Fig- 

 ure 29-2). 



Suppose one species has 2n = 4 and an- 

 other 2n = 6. The Fi hybrid between them 

 will have five chromosomes. Even if the 

 hybrid survives, it may be sterile because the 

 chromosomes are all nonhomologous and 

 none has a partner at meiosis. As a result, 

 meiosis would proceed as if the organism 

 were a haploid; the gametes usually pro- 

 duced would not contain a single set of the 

 five chromosomes, but would be aneuploid. 

 If, however, the chromosomes in such an Fi 

 hybrid are somehow doubled, artificially by 

 the drug colchicine, or by some spontaneous 

 agent, then the chromosome number in that 

 individual or sector would be 2n = 10, or 

 n = 5, and since each chromosome would 

 now be paired, the chromosomes would 

 behave normally in meiosis and produce 

 euploid gametes. The progeny would be 

 fertile, phenotypically more or less inter- 

 mediate to, and isolated from, both parental 

 species. 



It has been estimated that 20-25% of 

 the present species of flowering plants have 

 originated as interspecific hybrids whose 

 chromosomes have become doubled in num- 

 ber (being therefore "doubled hybrids" or 

 amphiploids). Moreover, as many or more 

 species have originated in this way in the 

 past, and then later diverged to form diff'er- 

 ent genera. Amphiploidy has been involved 

 in the origin of cotton in the New World. 

 In the present century, new species of goats- 

 beard have arisen in nature via amphiploidy. 

 Amphiploidy also can be produced artificially. 

 Thus, it has been possible for man to cross 

 radish (2n = 18) with cabbage (2n = 18) 



(Figure 29-3). This produces an Fi hybrid 

 with 18 unpaired chromosomes at meiosis. 

 If, however, the hybrid's chromosomes be- 

 come doubled, an amphiploid is produced 

 with 2n = 36 chromosomes (containing 9 

 pairs each from radish and cabbage). This 

 amphiploid is fertile, genetically isolated 

 from both radish and cabbage — and con- 

 stitutes a new species. 



The breeding success of the amphiploid is 

 greater, the more diff'erent are the chromo- 

 somes of the two species which originally 

 provided haploid genomes to the Fi hybrid. 

 For if eachchromosome in Fi is diff'erent, then, 

 after becoming doubled, each will have just 

 one partner at meiosis and segregation will 

 be normal. It is not surprising, therefore, 

 that when the two species hybridizing are 

 very similar chromosomally, an amphiploid 

 of them produces trivalents and quadrivalents 

 at meiosis, which lead to abnormal segrega- 

 tion and sterility. 



While amphiploidy is not successful for 

 hybrids between very similar species, there 

 is a second way such interspecific hybrids may 

 become stabilized as a new species. If the 

 two hybridizing species are very similar chro- 

 mosomally, they could have the same hap- 

 loid number, and the Fi hybrid could have all 

 these chromosomes synapse in pairs at 

 meiosis. Segregation, independent segrega- 

 tion, and crossing over could yield progeny 

 of the hybrid whose recombinations may be- 

 come stabilized in nature, yet isolated from 

 either parental species. Consider certain 

 species in the larkspur genus. Delphinium. 

 D. gypsophilum is morphologically inter- 

 mediate between D. recurvatum and D. 

 hesperium. All three species have 2n = 16. 

 It is possible to cross the "parent" species, 

 recurvatum and hesperium, and obtain an Fi 

 hybrid. If this Fi hybrid is crossed to 

 gypsophilum, off'spring are obtained which are 

 more regular and more fertile than those 

 which would be produced by backcrossing 

 this Fi hybrid to either parent species. 



