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CLASSIFICATION, PHYIOGENY, AND EVOLUTION: 



single species. Any species, and especially, any line 

 of development, can display the gamut of tempo dur- 

 ing its evolution. 



DISTRIBUTION PHENOMENA 



Evolution of species and larger taxa is to a great 

 extent a reflection of distribution. Biogeography was 

 already mentioned as the primary basis for the origin 

 of a theory of evolution. Actually, two aspects of dis- 

 tribution, barriers and highways, are most influential 

 upon evolution. A barrier is any condition, physical 

 or biological, that prevents an organism from expand- 

 ing its range. A highway, or bridge, is most easily 

 considered a somewhat unfavorable environmental 

 situation but one that still allows the passage of an 

 organism pnward to a favorable habitat. Therefore, 

 barriers cause isolation, and highways, in part, pre- 

 vent isolation. 



Highways are of three kinds, corridors, filter bridges, 

 and sweepstake routes. A corridor is a geographically 

 broad, fairly long-existing connection between two 

 favorable environments; a corridor may be a favor- 

 able habitat. A corridor now exists between Europe 

 and Asia. One often speaks of an influential corridor 

 in the past tense. "It was a broad, favorable area 

 that explains the present isolation of two groups of 

 the same species, or species that evolved from the 

 once broadly ranging species." A filter bridge is nar- 

 rower and less permanent than a corridor and con- 

 tains conditions that allow only certain species to pass 

 between the two similar environments; corridors, on 

 the other hand, enable many species to pass rather 

 freely. A filter bridge now exists between the Amer- 

 icas. Sweepstakes routes are not real highways in the 

 physical sense; rather, they are paths along which 

 some individuals of one or a few species pass by 

 chance alone. The path is only in one direction, and 

 the passage is not possible for the species as a whole. 

 Barriers and highways are best contrasted by examin- 

 ing island and continental organisms. 



Coniinenial Organisms. Distributional phenomena 

 are apparent in the evolutionary history of species in 

 general. When a new species finally is formed from a 

 pre-existing population of another species, of neces- 

 sity the new taxon is restricted in its distribution, it is 

 geographically limited to its center of origin. The next 

 phase of development, if the organism does expand at 

 all prior to extinction, is movement from the center of 



origin via highways. Eventually, all points in the ex- 

 panding "circle" of the species range will encounter 

 barriers. For a while, at least, the species does not 

 expand its distribution. Later, a highway — perhaps 

 only a sweepstakes route — will allow the passage of 

 individuals into a new area. If sufficient numbers 

 reach the new locality and the invaded area is favor- 

 able, the species will increase in numbers and again 

 start a cycle of range expansion. In this case, the new 

 locality is termed a center of dispersal. 



Some time later, geological changes occur and 

 cause environmental changes. If the geological cycle 

 is not too rapid but still exterminates the species, the 

 following cycle of events is likely: Barriers such as 

 arms of the ocean may split the range of the species 

 without much influence on the separated populations. 

 However, the cycle of extinction itself is more dra- 

 matic. The species first becomes less and less 

 numerous in various parts of its range. Then it no 

 longer occurs in many places, and the distribution is 

 island-like with populations remaining only in geo- 

 graphically separated areas, perhaps including true 

 islands. Finally, only a single population of the or- 

 ganisms exists; then, even this is no longer present 

 and the group becomes another fossil species. 



The evolutionary history of a species might fall far 

 short of the above history of the more detailed types. 

 Also, the actual history might be modified by certain 

 peculiar circumstances. For example, the horse has 

 its center of origin in North America and a center of 

 dispersal in eastern Asia. During the last glacial age, 

 the horse became extinct in North America but sur- 

 vived in Asia. 



Island Organisms. From an evolutionary point of 

 view, there is only one important kind of island, the 

 biological island. Biological islands are of two general 

 types, geographical and ecological. A geographic 

 island is an island in the usual sense; however, only 

 those having organisms isolated from their relatives 

 on adjacent land are true biological islands. An 

 ecological island is an isolated habitat such as an 

 alpine environment that is remote from like mountain 

 communities. Therefore, biological island life is not 

 found on all geographic islands and is not limited to 

 such areas. On the other hand, biological islands of 

 any kind possess common characteristics, so that one 

 can describe conditions of suitable geographic islands 

 that will apply to ecological ones as well. 



Although geographic island organisms typically 



