The Mechanism of Evolution in the Genus Clarkia

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Abstract
The breeding habit, morphological,cytogenetical, and ecological relationships are considered with respect to the mechanism of evolution in Clarkia (Onagraceae). Spp. of Clarkia as well as populations within a sp. are characterized by the same kind of traits that may distinguish individuals within a population. These include morphological, physiological, and chromosomal differences as well as differences in fertility. A complete spectrum exists between colonies with a sp. extending from complete interfertility to inability to cross. Much of the observed differentiation may be due to chance but adaptive changes have doubtless played a more important role. Probably the most important single factor contributing to speciation in this group has been the differential accumulation of chromosomal rearrangements. The pattern of differentiation suggests that speciation may frequently occur rapidly and that the success of the genus may lie in the production of a multiplicity of locally adapted, genetically isolated, and often ephemeral segregates.