New Light on Evolution from Research on the Genus Crepis

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Abstract
In a survey of 25 years'' research on the genus Crepis, the author outlines evidence on the causal events and mechanisms of evolution. Morphological criteria on which Crepis sections were first arranged in the main phylogenetic sequence were later correlated with chromosome length, asymmetry of chromosome arms, and especially with reduction of chromosome numbers from 6 eventually to 3. Life cycles also became progressively shortened from perennial to brief annual in the youngest spp. The validity of the sections was further upheld by the more vigorous, more highly fertile progeny given by intra- than by intersectional crosses. Various expts. have shown that closely related spp. differ in numerous minor genic changes, and that certain 4- and 3-chromo-some spp. have approx. the same genetic material as closely related 5- and 4-chromosome spp., respectively. Evidence from geogr. distribution points to central Asia as the center of origin of the genus, and a time of origin at least as ancient as the early Miocene. Great antiquity is further supported by fossil seeds from middle Pliocene deposits of western Europe. The first of 3 general conditions of importance in evolution in Crepis is the 20- or 30-million-yr. interval since the origin of the genus, giving adequate time for production of new types. Secondly, major environmental changes which can be correlated with evolutionary processes include drying and cooling during Miocene and Pliocene; mountain building in N. Africa, Europe, Asia, and N. America; and glaciation. The 3d general condition, isolation, is accomplished by migration, as well as by a vital change in the plants themselves, a reduction of chromosome number leading to cross-sterility but having no effect on external appearance. Two other vital changes, gene mutation and consequent differentiation of species under the pressure of natural selection, complete the basis of an evolutionary pattern which provides further support for the neo-Darwinian theory of evolution.