CYTOGENETICS AND EVOLUTION OF THE GRASS FAMILY 187 



ice age and permitted types with different climatic preferences to come to- 

 gether and hybridize. The products of this hybridization advanced into new 

 habitats made available by the effect of the periods of warm, dry climate 

 which followed the pluvial periods, and for reasons which are not completely 

 clear but which could be interpreted on a genetic basis, these hybrid derivatives 

 were more successful after they had doubled their chromosome number to 

 become tetraploids. The third phase of evolution was associated with human 

 activity, since Dactylis seeds can be transported by domestic animals and the 

 plants spring up readily in cut-over and burned-over forests and brushlands. 

 The tetraploids which spread as a result of this type of habitat alteration 

 came in contact with additional diploids, with which they could sometimes 

 hybridize. As MUntzing (1937) has shown, the triploids resulting from such 

 hybridization are by no means completely sterile and can give rise to many 

 vigorous and fertile tetraploid offspring. 



As an example of a polyploid complex which, in spite of a long and com- 

 plex evolutionary history, still preserves intact most or all of its original 

 genetic types, Dactylis illustrates very well the accompanying factors which 

 have made evolution by polyploidy highly successful in the higher plants. 

 These are principally the origin of new adaptive types through hybridization 

 between diploids adapted to widely different ecological conditions and the 

 availability of new ecological niches which can be occupied by these polyploid 

 hybrid derivatives. 



Many genera of grasses contain polyploid complexes which have reached a 

 more advanced stage of maturity and in which many diploid species have 

 become extinct. An example is Bromus. This genus is predominantly Eurasian 

 and American, with a few species in Africa. Because of its unspecialized 

 spikelets, it is generally regarded as one of the most primitive genera of the 

 Festucoid series. A closely related and hardly distinct genus endemic to the 

 Juan Fernandez Islands off the coast of Chile possesses the unusual and 

 primitive characteristic of three stigmas on the ovary. The evidence from 

 both external morphology and geographic distribution suggests that Bromus 

 is relatively old. On the other hand, the distribution and aggressiveness of 

 many of its species indicate that the genus is still in an active state of evolu- 

 tion. 



Bromus is divided into five clearly marked sections. These are, nevertheless, 

 related closely enough to each other so that intersectional hybrids can be 

 made, though with difficulty, between nearly all of them. Some of the sections 

 are rather young, and others are much more ancient, as is evident from their 

 morphology, geographic distribution, as well as the nature of their'polyploidy. 



The two sections Eubromus and Bromium contain annual species native to 

 Eurasia, concentrated chiefly in the Mediterranean region and the Near East, 

 but which have recently been carried by man and his livestock throughout 

 the world as weeds. Both sections are highly developed polyploid complexes. 



