CHAPTER 14 



THE BREAKDOWN OF CONTINUITY 



The Mating Coiitimmm Restrictive Practices The Internal Origins of Isolation 

 Floating and Fixed Discontinuity Restriction and Flexibility 

 The Traces of Aticestry 



Selection, as we have seen, distinguishes only between phcnotypic 

 differences. It is, therefore, effective in changing the genetic 

 constitution of a population, or group of individuals, only in so far 

 as the genetic differences of the group are, or can become, expressed 

 in the phenotypcs, whether of cells, individuals or populations. 

 Now the release of potential variability into the free state depends 

 on segregation from heterozygotes. Hence, as we have seen, the 

 potential variability of an inbreeding species is excluded from 

 providing the material for selective change. In such species, change 

 under selection will be confmed to the sorting out of such differences 

 as appear immediately in the phenotype: it will soon be over and 

 will be small in effect. 



Only groups with outbreeding systems can respond by extensive 

 change to the continued action of selection, whether natural or 

 artificial. And this is as true of the production of change in the 

 genetic system itself, a property of the species, as it is of change in 

 the phenotype of the individual, since the one as much as the other 

 depends on change in the genotype. It is, therefore, to the properties 

 of outbreeding that we must turn in order to see how changes in 

 genetic systems come about, and in particular how groups initially 

 unitary in their breeding come to break up. 



The Mating Continuum 



Under an outbreeding system an individual receives its genes 

 from two parents which are always, or at least nearly always, 

 genetically different. The same will have been true of the two 

 parents in their turn. Each individual can therefore be regarded as 

 representing the fusion of a number of lines of descent. In the same 



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