Genetic Evolutionary Processes

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Abstract
From a study of phyletics in the genus Crepis, based on such criteria as comparative morphology, geographic distribution, chromosome number and morphology and cytogenetics, the following conclusions regarding the nature of the genetic changes involved in the evolution of this genus are derived: The primary evolutionary process is some mode of chromosome transformation by which 8-chromosome and 6-chromosome species have developed from 10-chromosome ancestors. Next in importance is interspecific hybridization sometimes followed by amphidiploidy. Autopolyploidy has produced a number of distinct spp. in this genus. Operating concurrently with all the foregoing is gene or point mutation.[long dash]A preliminary survey of the subtribe, Crepidinae, suggests the following hypothesis as to the chief processes of genetic change involved in the evolution of this group: The original stock from which the whole subtribe was derived consisted of one or more 10-chromosome species. New spp. with 8 chromosomes came from the original spp. through some such process as translocation of segments between non-homologous chromosomes, leading eventually to the absence of one pair of chromosomes. The genus Crepis arose from the complex of 10-chromosome and 8-chromosome species thus created. The other genera in the Crepidinae originated through interspecific hybridization between 10-chromosome and 8-chromosome spp., or between 8-chromosome spp., followed by amphidiploidy. Concurrently with all these processes, gene mutation has played an important role in differentiation.[long dash]In the evolution of this large group of spp. gene mutation must be viewed as a secondary process operating concurrently with the primary processes. However, gene mutation is an essential process in specific differentiation, since it provides the genetic variations necessary for the origin of ecotypes, geographic races, varieties, subspecies and species having the same type of genom as the spp. from which they developed.

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