CHAPTER 



6 



Special Techniques 



for the Study of Introgression 



For the most part this chapter will deal with the special 

 techniques that have been developed for apprehending intro- 

 gression in the field. It should be emphasized at the outset, 

 however, that, although these are powerful techniques and 

 although they allow us to make reliable estimates of the 

 probability of hybridization from field data alone, they will 

 be more fruitful if combined with the more traditional 

 techniques such as transplant experiments, progeny tests, 

 cytological examination of species and hybrids, and the ex- 

 perimental repetition of the suspected cross. Where it is 

 feasible to carry on this kind of experimentation it is par- 

 ticularly important to study artificial hackcrosses of the hy- 

 brid to each parent. Until these have been made, one does 

 not have even a rough estimate of how much undetected 

 hybridization there might be in supposedly unmongrelized 

 populations of the parental species. Of all the kinds of ex- 

 perimental evidence which might be gathered on such a 

 problem, the production of artificial backcrosses is of out- 

 standing importance. The mere demonstration that such 

 and such a species hybrid can actually take place under 

 natural conditions is no longer of any general significance. 

 That these crosses can sometimes take place is now proved 

 beyond a reasonable doubt. Wliat we do not yet know is the 

 role (or rather the roles) such hybridizations play in evolu- 

 tion. If we are going to measure the effect (or lack of effect) 

 of hybridization in natural populations, then one of the most 

 useful kinds of e\'idence we can obtain experimentally is an 

 exact understanding of what is to be expected when the hy- 

 brid crosses back to either parent. 



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