THE DYNAMICS OF EVOLUTIONARY CHANGE 



tively isolated by hybrid inviability or sterility from all other 

 pre-existing populations. Amphiploids, or doubled hybrids, ful- 

 fill this qualification in every respect, and the partially isolated 

 segregates with unchanged chromosome number which have 

 been produced in Elymas, Nicotiana, and Delphinium come 

 close to it. While the former constitute a special type, rare or 

 lacking in the animal kingdom, segregates of the latter type 

 might be expected in all types of organisms. Furthermore, since 

 partial sterility is found in many hybrids between different 

 races of the same animal species, the origin of new species from 

 such hybrids would require the addition of only a few muta- 

 tions to the extreme segregants which gene recombination can 

 produce in the progeny of such hybrids. The origin of species, 

 one of the most crucial steps in evolution, is well on the way 

 to becoming understood. 



The Major Trends of Evolution 



The seventh and final basic postulate about evolutionary 

 processes is as follows: The origin of genera and other higher 

 categories, as well as the longtime trends which have given rise 

 to increasingly complex and highly organized forms of life, re- 

 sults from the continuation into geologic spans of time of the 

 processes responsible for evolution on the racial and species 

 level. The only new element which must be considered is the 

 increasingly evident extinction of populations intermediate be- 

 tween the successful lines. 



The first type of evidence in favor of this postulate is the 

 fact that some of the same kinds of differences which in one 

 group of organisms may form the distinction between genera, or 

 even families, can in a related group exist as differences be- 

 tween species of the same genus or even between races of the 

 same species. This has been illustrated elsewhere for the grass 

 family (Stebbins, 1956). 



Even more convincing evidence has been obtained from care- 

 ful studies of fossil lineages, particularly in mammals. This 

 evidence is carefully reviewed by Simpson (1953, 1960). He has 

 shown that the earliest representatives of lines which led eventu- 

 ally to very different kinds of animals were so much alike that 

 they would unquestionably be placed in the same group if their 



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