THt BREAKDOWN OP CONTINUITY 



or does not run its normal course. Segregation, even from a hybrid, 

 is thus cither avoided completely, so that reproduction is clonal in 

 all but appearance, or is so severely restricted as to give something 

 near the genetical uniformity of a clone. 



Apomixis, as we have already seen, is shown very commonly by 

 individuals so hybrid, even in regard to chromosome number, that 

 only by side-stepping sexual reproduction can they propagate 

 themselves; but it is not confined to such hybrids. In the aphides 

 it is shown by species which under other circumstances are sexual. 

 The sexual phase is to be seen at those seasons of the year which 

 keep their numbers small, and the parthenogenetic phase in the 

 summer when populations are rapidly increasing in size. As we 

 have earlier observed, the use of parthenogenesis during a stage of 

 rapid expansion of the stock is a reproductive economy. We can 

 now see that it is also something more, for it avoids the deterioration 

 which would result from sexual recombination at a period of 

 prosperity when natural selection is dangerously relaxed. 



Isolation and inbreeding on the one hand, and apomixis on the 

 other, though differing sharply in their mechanisms, agree in 

 avoiding recombination by the abolition of segregation. Isolation 

 and inbreeding do so by avoiding heterozygosity for at least the 

 critical gene combinations; apomixis does so by accepting hetero- 

 zygosity but avoiding sexual reproduction altogether. Polyploidy 

 offers a device which accepts both heterozygosity and sexual 

 reproduction, yet still avoids segregation of the old differences. 



In an allopolyploid, or ampliidiploid [A1A1A2A2), the individual 

 is heterozygous for the differences between the sets of chromosomes 

 of the original parents [A^Ai and A2A2). Yet in so far as the two 

 representatives of each set segregate from one another on the 

 scheme of Primula kewensis {Ai from A^ and A2 from /Ig), rather 

 than from the sets of the opposite parent [A^ from A2) these parental 

 differences arc passed on as wholes through each gamete to each 

 offspring. They never segregate and their parts never recombine, 

 so that a polyploid is, as we saw, potentially a true breeding 

 hybrid. So even when descended from cross-breeders, a polyploid 

 species can be an inbreeder without becoming homozygous 

 and hence without serious inbreeding depression. It offers, in fact, 

 a short cut from the flexibility of outbreeding to the fitness of 



310 



