526 Cytogenetics and Evolution 



other intermediate types more or less resembling one or the other 

 species might be scored 1 or 3. In this manner, each of the char- 

 acters of a certain plant is given a value. All the values are 

 added together and the sum is the index value of the plant. The 

 index value is a rough estimate of the position of the given plant 

 with respect to the theoretical concepts of the two species for all 

 the characters scored. If a natural population is under observa- 

 tion, the index values of all the plants are plotted in a histogram. 

 The histograms for the two species do not overlap and are found 

 at opposite extremes of a curve, but the histograms of various 

 hybrid populations may fall between the others, depending upon 

 the structure of the various populations. 



Anderson made an interesting study between Tradescantia vir- 

 giniana and T. canaliculata, the two most common Tradescantias 

 of eastern North America. The former usually grows in shade 

 or semi-shade; the latter is usually found in full sun, often on or 

 near rocks or in dry sands. They are thus separated by an eco- 

 logical barrier. Hybridization was studied in Jefferson County, 

 Missouri, in a region little disturbed by man in which hybridiza- 

 tion, therefore, was taking place under natural conditions. Popu- 

 lations of each species were studied as well as six populations 

 of hybrids. Index values were assigned and plotted, as in Fig. 

 149. The distributions of these values show definitely that these 

 were hybrid populations. 



Introgressive Hybridization. If there is free intercrossing 

 among the plants of the two species, among the hybrids, and be- 

 tween any hybrid plant and any member of either species, and if 

 the two species are in equal numbers at the start of the hybridi- 

 zation, theoretically the hybrid swarm should be distributed, 

 when scored by the index frequency method, according to a 

 normal frequency curve. However, if one species is far more 

 abundant than the other, the Fi and subsequent hybrids will 

 have far more opportunity to backcross to that species than to 

 the less abundant species, and the hybrid swarm will present a 

 curve which is strongly skewed in the direction away from the 

 more abundant species. After a few such repeated backcrosses, 

 most of the individuals of the hybrid swarm will appear rather 

 as extreme variants of the pure species than as hybrids. This 

 "absorbing" of one species by the other or, to look at it from the 

 other angle, this "infection" of the second species by the first 



