THE TRACES OF ANCESTRY 



The Traces of Ancestry 



All forms of discontinuity give us a clue to the relationships of 

 organisms through descent, in other words phylogeny. In so doing 

 they both utiHze and confirm the conclusions we have reached on 

 the origins of discontinuity. The chromosomes remain to mark 

 many of the steps by which species, genera, and even families, have 

 diverged. The changes that we have to use for the larger groups 

 are no longer the inversions and interchanges that serve to trace 

 relationships within a small and inter-fertile group. We have to 

 use changes of number arising from polyploidy, reduplication, 

 and the fragmentation and fusion of chromosomes. 



Polyploidy is in a special position. It can take place effectively 

 only in one direction : the diploid must nearly always be the parent 

 of the polyploid. Moreover, even as simple doubling, it determines 

 a genetic change at the same time as it establishes a discontinuity 

 by creating a new form which will not cross with the old. Finally, 

 a single colonizing polyploid individual can unload a degree of 

 variation that would be far beyond the reach of a solitary diploid. 

 For this reason when sudden changes are needed and a sudden 

 opportunity for colonization created, as after the retreat of the ice, 

 the new polyploid steps in and quickly acquires a large range from 

 which its diploid parent is excluded. In Paeonia the Mediterranean 

 seems to have created a barrier that has favoured the appearance 

 of tetraploids in Europe such as are found only once in Asia and 

 not at all in America. An external discontinuity has called forth an 

 internal one (Table 27). 



Changes of single numbers also require a word of explanation. 

 Fragmentation can be effective in adding to the number of chromo- 

 somes only when the centromere itself is split to give two new 

 chromosomes with terminal centromeres. The result has been found 

 as a fragmentation heterozygote in Campanula pcrsicifoUa. Each 

 fragment pairs with the arm of the old chromosome with which 

 it is homologous, so that a chain of three chromosomes is formed 

 at meiosis {A— AB— B). Homozygotcs with 8 {AB— AB) and 

 with 9 {A— A, B — B) pairs have been derived by segregation 

 from the heterozygote (Darlington and La Cour, unpub.). 



Changes ascribed to fusion, and doubtless preceded by breakage, 



EhmailsofGauiiiS 321 X 



