The Geotectonic Factor 335 



EFFECT ON EVOLUTION 



The dynamics of the crust may have been instrumental in keeping 

 earth chmates within the range of the ecological tolerances of life. 

 The obverse is certainly true. Life has evolved within the climatic 

 limits that have existed. 



The most obvious effect of these geotectonic factors has been to 

 produce a constant changing and shifting of climates and physical 

 conditions with time over the entire earth including the oceans. 

 The geographic extent of particular sets of ecological conditions 

 have moved, contracted, and expanded, or even disappeared, de- 

 pending on the prevalent direction of geotectonic influence. The 

 geographic ranges of organisms adapted to these conditions have 

 moved with the conditions. Some of the most important evolu- 

 tionary processes occur because of these shifting geographic ranges: 



( 1 ) Before the origin of life, pre-lif e stages may have developed 

 in isolation and then, when previously separated pools or seas 

 united, combined to form the next step toward a living organism. 



(2) In both plants and animals the great bulk of present-day 

 species owe their origin to geographic isolation. This isolation has 

 occurred because of a splitting of ranges (almost entirely by cli- 

 matic shifts ) or by the colonization of isolated areas whose separa- 

 tion is maintained by topographic irregularities. 



(3) The greatest number of species not created by geographic 

 isolation are the allopolyploid species arising from plant hybrids. 

 The contact between the parental species is usually due to the 

 overlapping of previously separated species ranges brought together 

 by climatic shifts or land bridge formation. 



(4) The evolution of biotic communities as integrated mixtures 

 of species resulted from the congregation of species formed chiefly 

 in isolation which were then brought together by climatic changes 

 after the species had evolved. 



(5) The evolution of simple biotic communities into a series of 

 successional stages and biomes came about by various processes, 

 the most important of which was the splitting and reunion of com- 

 munities and their segregated parts by oscillations of climatic zones. 



Crustal changes have also produced areas having new types of 

 edaphic and climatic characteristics. These areas have furnished 

 opportunities for the evolution of new types of organisms adapted 

 to live under the novel conditions. 



It is obvious that without the crustal disturbances which con- 

 stantly counter the action of erosion, organic evolution would have 

 been entirely different. If crustal movement ceased now and only 



