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CLASSIFICATION, PHYIOGENY, AND EVOLUTION: 



Figure 7.10 Orthogenesis, directional trends in evolution as exemplified by horse evolution. {From 

 W. D. Matthew, Ooor*. Rev. Biol., vol. 1, 1926.) 



Adapfive Radiation. The evolutionary explosion of 

 an ancestral group or groups into several nevif lines of 

 development (hence, many new higher taxa) is called 

 adaptive radiation (Figure 7.11). It is one of the 

 strongest cases made for "big step" evolution; how- 

 ever, simple neo-Darwinian speciation mechanisms 

 can explain the phenomenon. Adaptive radiation 

 mechanisms can be hypothesized in four steps. First, 

 a potential ancestral group is restricted to a particular 

 environment. It does not occur elsewhere because 

 other organisms, generally representing a much larger 

 taxon, restrict the ancestor's habitat distribution. 

 However, the ancestor could live in vast expanses of 

 these other situations if it were not blocked by the 

 species already in these areas. Therefore, the future 

 ancestor is preadapted to other localities; it could exist 

 in certain places, even though it does not occupy 

 them. Second, the competitors, the large taxon keep- 

 ing the ancestor out of extensive areas, becomes ex- 

 tinct. Third, the ancestor moves into the available 

 localities, which constitute a diversity of habitats. 



Fourth, different segments of the ancestor become 

 isolated and go through a period of rapid changes 

 because more than the usual number of the chance 

 mutations are of greater benefit in the hereditary 

 complement allowing bare survival. An example of 

 adaptive radiation occurred in the small mammals 

 that lived at the time of dinosaurs and radiated into 

 many mammal types, but only after most of the 

 dinosaurs became extinct. 



Extinction. Extinction is one of the great mysteries 

 of the geological record. Generally, only one or a few 

 species die out at a given time; but in a relatively 

 short period, large taxa can be destroyed or even cre- 

 ated. However, no past geological and climatic con- 

 ditions seem to explain the rare times of widespread 

 extinction that tended to wipe out many past taxa or 

 completely changed the nature of the species in some 

 taxa. Only single-species death appears likely to be 

 something inherent within life. For this reason, one 

 of the generally accepted hypotheses proposes that 

 extinction results from the overaccumulation of harm- 



