PHYLOGENY 



necessary for plants. Because of the phenomenon of allopolyploidy— to be 

 discussed in detail below— hybridization of related species may result in 

 new species. Hence branches may form, then fuse again, so that a network 

 results. This phenomenon is of great importance for plants, less so for 

 animals. 



Rates of Evolution. The rates of evolution have not always been uni- 

 form. In general, periods of major geological change have been periods 

 of rapid evolution, while periods of geological uniformity have been pe- 

 riods of slow evolution. This might be expected a priori, for the selective 

 forces themselves should be in a state of Hux during geologically unstable 

 periods, while they should be quite stable during geologically uniform 

 periods. Thus in a period of uniformity, organisms generally should tend 

 to become well adapted to their environments. In such a situation, almost 

 any change would be disadvantageous, and would tend to be eliminated 

 by natural selection. But in periods of geological change, only those or- 

 ganisms which also changed could possibly have a selective advantage. 

 Thus there was a rapid burst of evolution in the Silurian and Devonian 

 when plants and animals were colonizing the land. Not only were vast 

 new habitats thus opened up, but this was a time of mountain-building 

 and of great changes in sea level. In the Mississippian, physical conditions 

 were again quite stable, and evolution was slow. Many such alternations 

 of periods of rapid and slow evolution have occurred. It appears that we 

 are at present in the midst of a period of rapid evolution. 



Two special situations favor rapid evolution. The first of these is adap- 

 tation of a group to a new mode of life, as invasion of land by aquatic 

 plants and animals. During the transition, the organisms must be rather 

 ill-adapted, hence strong selection pressure favors rapid change, and only 

 those that can respond will leave descendants. This will be greatly facili- 

 tated if, in the earlier environment, characteristics developed which are 

 'preaduptive to the later habitat. Thus many Devonian fishes developed 

 lungs, which were adaptive to life in stagnant, drying ponds. This pre- 

 adaptation later speeded the adaptation to life on land. Simpson has called 

 this "quantum evolution," as it involves sudden change from one "adaptive 

 orbit" to another. He regards the term as unsatisfactory, because quantum 

 events in physics are very small, while quantum evolution is on a large 

 scale. Perhaps macroaduptive evolution would be a better term. 



Second, when a group has invaded a new major mode of life, many 

 variations of habitat are open to it with a minimum of competition, and 

 hence rapid diversification is favored until these empty ecological niches 

 are largely filled. This is the adaptive radiation which was discussed 

 above. 



Entirely apart from changes in the general rate of evolution, the rates 

 in different lines of descent have not been equal. In some, change has 

 been extremely slow for long periods of time. The brachiopod Lingula, 

 for example, has been substantially unchanged since the Ordovician. In 

 this connection, it should be pointed out that the warm, shallow seas 

 which it inhabits constitute perhaps the most stable environment on 



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