THE INTERNAL ORIGINS Of ISOLATION 



The Internal Origins of Isolation 



Isolation, as we have been discussing it, arises from the external 

 genetical relations of one species with others. It can also arise from 

 purely internal causes in a variety of ways. Of these the simplest is 

 provided by the mere habit of inbreeding itself. Isolation is complete, 

 or virtually so, while artificial crossing is readily achieved, in many 

 inbreeding organisms. In wheat or oats, for example, all the species, 

 even the diploids, can be readily crossed. Yet, when grown together, 

 the species show no natural crossing. Here, however, in contrast to 

 the Antirrhinum species, the lack of crossing between plants of 

 different species is only a special case of the general failure of the 

 individuals to outcross even with others of their own species. They 

 are habitual inbreeders, and their isolation is a concomitant of their 

 inbreeding. 



Where crossing of any kind is absent, restrictions on crossing 

 between groups, as opposed to crossing within them, are obviously 

 unnecessary. Indeed, since inbreeding freezes the flow of variability, 

 the production by recombination of groups of genes producing 

 bars to crossing will be impossible: so likewise will be the divergence 

 of genie combinations from which hybrid incapacity arises. It is 

 not surprising, therefore, that there is an absence of all genetic bars 

 to crossing other than the normal inbreeding mechanism. Nor is 

 it surprising that the species hybrids, when artificially produced, 

 are both fully vigorous and fully fertile; apart, of course, from the 

 effects of the numerical hybridity which follows the crossing of 

 species having different chromosome numbers. 



These principles are seen in their simple form in wheat, oats and 

 tobacco. They are illustrated even more critically in the species of 

 Oenothera, which happen by their own special device to be highly 

 heterozygous. They are habitual inbreeders, and they have an 

 unlimited capacity for crossing within the genus. 



Isolation and inbreeding represent restriction on crossing of 

 different degrees and with different consequences. They are both 

 achieved, however, by means within the limits of the sexual 

 mechanism. Apomixis, on the other hand, resolves the instability of 

 a mating continuum by the abolition of the normal sexual cycle. 

 Fertilization is done away with, and meiosis either does not intervene, 



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