Isolating Mechanisms 519 



Barriers or isolating mechanisms that tend to keep species and 

 to a lesser extent subspecies from intercrossing and producing 

 hybrids and perhaps also hybrid swarms, occasionally break 

 down and permit one species to cross with another. Let us 

 examine some of the barriers that appear to separate the various 

 species of a genus into natural populations relatively isolated 

 from one another. 



Geographical Isolation. One of the more obvious isolation 

 mechanisms is geographic isolation. This type of barrier serves 

 to prevent two species from interbreeding by keeping them apart 

 in space. If two animals cannot come close together they cannot 

 mate, and if two plants are not near enough for the pollen to be 

 transferred from one to the other by wind or by insects they can- 

 not produce offspring. If the two animals or plants represent 

 different species, any factor that will prevent them from coming 

 sufficiently close to one another during periods of reproduction 

 to be within what might be called the effective crossbreeding dis- 

 tance will prevent the origin of hybrids. Such geographical bar- 

 riers might be linear distance but might also be some other factor, 

 such as mountains or rivers; and a factor that might constitute 

 a barrier for the species of one genus might not operate as such 

 between the species of another genus. Geographical isolation 

 alone would not bring about the origin of new species, but it can 

 set the stage for other necessary conditions such as the accumu- 

 lation of different mutations in different localities. 



If geographical barriers are the only isolating mechanism, the 

 two isolated and differentiated species will cross and produce 

 fertile hybrids and hybrid swarms once that barrier is removed. 



Ecological Isolation. Of all the isolating mechanisms, the geo- 

 graphical one alone is a purely environmental mechanism. All 

 the other types reflect at least some internal condition in the 

 organisms. One of these barriers that depend upon something 

 intrinsic in the organism is ecological isolation. 



Ecological isolation operates in the same manner as geograph- 

 ical isolation. The two species are isolated in space and, as a 

 result, are not crossed. The differences are that ecological isola- 

 tion operates effectively over a much smaller area and the funda- 

 mental causes of the separation are different. For example, 

 Tradescantia canaliculata grows in full sunlight at the tops of 

 cliffs, and T. suhaspera var. typica in the shade at the bottom. 



