example of this is found in the genus Rubus, in which a compli- 

 cated polymorphism and great adaptability have led to wide- 

 spread distribution in Australia and other countries. ^°^ Vegeta- 

 tive propagation is of great value to many species not only in 

 maintaining the population under conditions adverse for seed pro- 

 duction, but also in enabling it to invade adjacent areas. 



Limitation of pollination and dispersal in a small area may pro- 

 mote the formation of new ecotypes. This appears to follow from 

 the conclusion of Ford ^° that when a population is subdivided 

 into small isolated, or partly isolated, units rapid evolution is 

 favored because each group can then become adjusted to the 

 particular habitat conditions instead of to the average conditions 

 over a large area. Similar conditions obtain in marginal popula- 

 tions within the area of distribution of a species, for such popu- 

 lations may possess less heterozygosis than those in the central 

 part of the range of distribution. ^° A high degree of heterozygosis 

 often gives the population greater plasticity, and in many cases 

 leads to greater capacity to cope with variations in the environ- 

 ment. ^'^^ 



The number of individuals in a population, accompanied by 

 heightened variability, increases during favorable environmental 

 periods. This greater intensity of variability permits genes to be 

 combined in new ways, and some of the resulting combinations 

 may give increased adaptability during less favorable environ- 

 mental periods or in relationships with other organisms. Disturb- 

 ance of natural conditions by man, or the formation of new 

 habitats by natural causes such as erosion, recession of glaciers, 

 or emergence from water, may provide suitable habitats for hy- 

 brid populations. As suggested by Anderson,^ the habitat needs 

 "hybridizing" before hybrids can survive, e.g., irises or oaks,^°^ 

 for in the undisturbed habitat they are unable to compete with 

 individuals of the parent genotypes. Populations of different 

 species may be sufficiently close to one another geographically 

 (sympatric) so that ecologically successful hybrids are formed, 

 followed by introgressive hybridization, as in the case of Quercus 

 velutina, for example, which has been enriched by characters from 

 Q. borealis in the Great Smoky Mountains.^°^ In the Canary 

 Islands Pinus canariensis forest forms an ecological pathway con- 

 Ecological CKatracteristics of Species &. Populations • 43 



