The Differentiation of Populations | 241 



among birds (e.g., gulls, ducks, grackles, grosbeaks, honeyeaters, 

 birds of paradise) and is usually interpreted as occurring in zones 

 of secondary contact. Whatever its interpretation, it is evident that 

 differentiation of bird populations may be a much more complicated 

 process than one would assume from the neat arrays of "species" and 

 "subspecies" found in bird checklists. 



Similarly, it has been shown rather clearly that in many groups of 

 plants considerable genetic interchange is possible after the so-called 

 specific level of differentiation has been reached. This may be en- 

 tirely at the diploid level, or it may involve polyploidy and apomixis. 

 Examples of the latter have been discussed in Chap. 9. Most in- 

 stances of such hybridization fall into the category of what has been 

 called introgressive hybridization. The chances of an Fi hybrid 

 offspring crossing with another such hybrid in the early stages of 

 hybridization are much less than the probability of crossing with 

 one or the other of its parents. The hybrid derivatives are almost 

 always intermediate with respect to their ecological requirements, 

 just as they are intermediate with respect to morphological traits. 

 Backcrosses to one parent or another will thus be more likely to 

 find an appropriate ecological niche than the Fi or Fo individuals. 

 The result is that genetic submergence of the two hybridizing enti- 

 ties is unlikely to occur. Rather, portions of the germ plasm of one 

 species will infiltrate the genotype of the other. Variability of the 

 parental types will be increased in the direction of the hybridizing 

 entity, and the species or subspecies may be able to increase its 

 range and to move into habitats previously unoccupied. 



An interesting example of this in the sunflower genus (Helian- 

 thus) in California has been studied by Heiser. Helianthus annuus 

 is a common weedy species found in most of the United States; 

 there is considerable evidence that it was introduced into California 

 by Indians in relatively recent times. Helianthus holanderi occurs 

 in California in two races. One is almost completely restricted to 

 serpentine soils, an ecological situation in which relatively few and 

 specialized plants are found. The other subspecies is a weedy form. 

 Heiser has shown that the weedy race of H. bohnderi probably 

 originated from the introgression of genetic material of the wide- 

 spread weedy H. annuus into the serpentine race of H. holanderi. 

 The details of how this came about need not concern us here, but by 

 making the appropriate crosses the derived subspecies can be syn- 

 thesized in the laboratory. It is interesting that the introgression 

 has been reciprocal; in addition to creating a larger, weedier form 

 of H. holanderi, it has resulted in the formation of a smaller form of 

 H. annuus, apparently with extended ecological amplitude. 



