234 I The Process of Evolution 



metrically are not known. Many other interesting aspects of varia- 

 tion in this species of Asclepias also are poorly understood but are 

 being studied by Woodson. These include the apparently centrif- 

 ugal variation of A. tuberosa interior, which has resulted in a 

 concentrically distributed peripheral subspecies in the western and 

 northern parts of the range, and an interesting variation pattern 

 with respect to color of the flower (which can be studied biochem- 

 ically ) . 



Exchange of Genetic Information 



In organisms with relatively continuous distributions, exchange of 

 genetic material among populations will limit the amount of diflFer- 

 entiation that can take place. The "mixing" effect of recombination 

 is most pronounced in adjacent populations, which, because of the 

 relative similarity of their environments, are less subject to diflFer- 

 entiation through selection. The situation is reversed for widely 

 separated populations in a series. A certain amount of genetic in- 

 formation is passed back and forth through the intervening popula- 

 tions, but the ability of this weakened gene flow to swamp out 

 genetic differences is greatly reduced. In most instances these more 

 distant populations have been subjected to quite different selection 

 pressures, enhancing the trend toward differentiation. This is well 

 illustrated by the Rana pipiens and Coenonijmpha tullia cases, in 

 which distant populations are strongly differentiated but pairs of 

 adjacent populations show little differentiation.^ A classic example 

 of isolation by distance is found in populations of a single species of 

 a California desert plant. Around the southern and western edges of 

 the Mojave Desert a small annual, Linanthus parrijae (Polemonia- 

 ceae), shows an interesting pattern of variation. With good rainfall, 

 the plants form an essentially continuous carpet over large areas. In 

 1941 L. parryae was studied intensively between Palmdale and 

 Lucerne Valley, where it had developed a practically continuous 

 population over some 700 square miles. In most areas investigated, 

 the plant samples consisted of white-flowered forms, but in three 

 isolated sections samples with varying numbers of blue-flowered 

 individuals were collected. The composition of the samples taken 

 from the westernmost of these areas between Palmdale and Llano 

 is shown in Fig. 10.11. The central variable area is separated from 



' It is possible that gene flow between distant populations of some series is 

 so slow that the migration effect might be practically indistinguishable from 

 mutation as a source of variabihty. 



