Races and the Origin of Species 



247 



having 24, 36, 48, 60, 72, 96, 108, and 144 

 chromosomes. These two examples sug- 

 gest that autopolyploidy has played a role 

 in the speciation of these two genera. Auto- 

 polyploidy, however, is not considered an 

 important mechanism of speciation in forms 

 reproducing primarily by sexual means, 

 since autopolyploids having more than 2n 

 chromosomes form multivalents at meiosis 

 and, therefore, numerous aneuploid gametes. 

 Autopolyploids can succeed, though, if they 

 are propagated asexually, by budding or 

 grafting, as in the case of the triploid apples 

 — Gravenstein and Baldwin. Triploid tulips 

 are also propagated asexually. 



Speciation Involving Two or More Species 



Many new cross-fertilizing species originate 

 not only from a single species or its races, 

 but — in relatively recent times — from hy- 

 bridization between two or more different 

 species, that is, via interspecific hybridiza- 

 tion. Although interspecific hybrids pose no 

 threat to the isolation of the gene pools of 

 their parental species, they may form a suc- 

 cessful, sexually-reproducing population that 

 has its own closed gene pool. Interspecific 

 hybrids, particularly of plants, can be con- 

 verted into stable, intermediate types iso- 

 lated from their parental species by three 

 methods. 



The first method involves amphiploidy 

 (allopolyploidy, see p. 155). If one species 

 has 2n = 4 and another has 2n = 6, the Fi 

 hybrid between them will have five chromo- 

 somes (Figure 18-2). If the hybrid sur- 

 vives, it may be sterile because each chro- 

 mosome has no homolog and, therefore, no 

 partner at meiosis. As a result, meiosis pro- 

 ceeds as if the organism were a haploid and 

 produces mostly aneuploid gametes. If, 

 however, the chromosome number of the 

 Fi hybrid is doubled — either artificially (via 

 colchicine) or spontaneously — the individual 

 or sector will be 2n = 10; each chromosome 



2n 4 



2n 6 



Aneuploid 

 -*■ Meiotic 

 Products 



AMPHIPLOID 



Euploid(n) 

 "*" Meiotic 

 Products 



figure 18-2. Interspecific hybridization lead- 

 ing to new species formation via amphiploidy 

 (allopolyploidy). 



will have a meiotic partner; and euploid 

 gametes of n = 5 will be formed. Upon 

 uniting, such gametes produce 2n = 10 

 progeny, which are fertile and more-or-less 

 phenotypically intermediate to and isolated 

 from both parental species. 



It has been estimated that twenty to 

 twenty-five per cent of the present flowering 

 plant species originated as interspecific hy- 

 brids whose chromosomes doubled in num- 

 ber (therefore being "doubled hybrids" or 

 amphiploids). Moreover, in the past many 

 (or more) species originated in this way, 

 then diverged to form different genera. Nat- 

 urally-occurring amphiploidy was involved 

 in the origin of cotton in the New World 

 and in the appearance of new species of 

 goatsbeard during the present century. 



In the early 1800's, the American marsh 

 grass, S parti na alterni flora (2n = 70), was 



