58 INTROGRESSIVE HYBRIDIZATION 



recombinations resembling either parental inbred are easy 

 to achieve, whereas recombinations of one quality to a degree 

 resembling one parent and of another quality to a degree 

 resembling the other parent are difficult, if not impossible. 



However, the question of just how strong the cohesive ef- 

 fects of linkage might be, were it the only barrier between 

 species or races, is an academic one. In most cases that have 

 so far been investigated there were other isolating mecha- 

 nisms, all of them operating in the same general direction. 

 The selective effect of the habitat, discussed in detail in 

 Chapter 2, is almost universal in such crosses. Usually, it 

 will be remembered, it favors hybrids and backcrosses closely 

 resembling the parental species. In addition, there are such 

 barriers as geographic isolation, differences in blooming 

 season, differential pollen-tube growth, inversions of chromo- 

 some segments, chromosome interchanges, polyploidy, and 

 the like. Species are kept apart by barriers of various kinds, 

 both internal and external, working together in various ways. 

 Like linkage, many of these barriers continue to operate in 

 hybrid populations. Though they operate in different ways 

 and at different times in the life cycle, their overall effect is 

 the encouragement of gene recombinations like those of the 

 parental species at the expense of more radical rearrange- 

 ments. 



It has been found that species which are completely inter- 

 fertile in the experimental plot often yield no hybrids unless 

 artificially cross-pollinated. Anderson and Schafer (1931), 

 for instance, found that, though Aquilegia plants were out- 

 crossed within the species, no hybrid seed were produced 

 when several plants of various species were grown side by 

 side. Mather (1947) has begun the exact analysis of such a 

 situation in Antirrhinum. He finds the barrier to reside in 

 the flower-visiting habits of the insects responsible for cross- 

 fertilization. A delicately adjusted barrier of this sort would 

 restrict gene flow to particular times and places, rendering 

 the two species effectively shut off from each other most of 

 the time, yet allowing introgression frequently enough to 



