CHAPTER 15 



MEANS AND METHODS OF 



EVOLUTIONARY CHANGE 



The Problem 



In preceding chapters we have noted evolution manifested in many guises. 



We have seen numerous examples of relatively unspecialized animals 

 which have given rise to descendants specialized for some particular mode 

 of life. We have seen that frequently several lines of descendants have arisen 

 from one ancestral group, and have termed this phenomenon adaptive 

 radiation. 



We have noted independent occurrence of similar evolutionary trends, 

 and have termed this parallel evolution when the independent evolution oc- 

 curred in two related groups of animals, convergent evolution when it in- 

 volved two relatively unrelated groups. 



We have observed the complexity of human evolution and suggested that 

 isolated human populations developed differing characteristics while iso- 

 lated, and that these characteristics were variously combined and recom- 

 bined when subsequently the populations came into contact through migra- 

 tions and conquests. 



We have emphasized that the past and present geographic distribution of 

 animals suggests that a form originates in a certain region (its "center of 

 dispersal") and as descendants spread out from this center they undergo 

 modification, so that eventually they differ from one another and from the 

 parent form. 



We have noted ways in which animals finding themselves on oceanic is- 

 lands have been able to exploit environmental niches they would never 

 have entered on continents. 



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