









•i*'';^*"^^"; 







■t '^^ 



r'4/:- '\MS 







*^:4^';i[isiy^^^M;^ "' ''^ 



Figure 1-19. Vigorous propagation of black gramagrass 

 {Bouteloua eriapoda), by stolons is characteristic of this im- 

 portant range grass in southern New Mexico. (U. S. Forest 

 Service.) 



in a population, the greater will be the genetic diversity and 

 adaptability of the population, and hence the survival under un- 

 favorable conditions. This greater adaptability in growth-form, 

 earliness of growth and maturity, size of seed, and resistance to 

 disease are important in determining the extent of the geographic 

 distribution of a species; the decumbent, rhizomatous growth- 

 form o{ Panicum virgatum, for example, is more common in sandy 

 soils in northern and western Nebraska, while the erect, bunch 

 type is more common in southeastern Nebraska. ^-^ 



The Mendelian population, consisting of cross-pollinating in- 

 dividuals, is a more plastic system of adaptability than an asexual 

 array of organisms. Species equipped with both sexual and 

 asexual kinds of reproduction have distinct advantages over 

 those that have only one, because the former are able to main- 

 tain the same genotype almost indefinitely in propagating bio- 

 types, and at the same time they benefit from the formation of 

 new genotypes in sexual reproduction (Figure 1-19). A striking 



42 • Species ai«d Popvmlatioits 



