Genetic Evolutionary Processes in Crepis

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Abstract
(I.) Theoretically the basic processes of genetic change are gene mutations and gross structural changes in the chromosomes. (II.) Evidence:[long dash](1) Geographically and morphologically Crepis is a natural group with 1 center of origin, a wide distribution and great variation[long dash]from primitive to advanced spp. (2) Genetic evidence shows that closely related spp. differ in numerous genes. (3) Cytological evidence reveals progressive reduction in number, total length and symmetry of the chromosomes. Parallel trends exist between (a) morphological similarity and chromosome similarity; (b) reduction in chromosome number and reduction in size and life cycle of the plant; (c) increase in asymmetry of the chromosomes and progressive speciation. (4) Cytogenetic evidence:[long dash](a) Meiotic behavior in hybrids indicates more or less genic homology in the chromosomes of numerous species investigated. (b) Structural hybridity in certain spp. throws light on phyletic relations. Thus C. kotschyana, a 4-paired sp., must have originated from a 5-paired ancestor, and 3-paired C. juliginosa, from a 4-paired ancestor, the latter certainly by means of reciprocal translocations between non-homologous chromosomes, (c) The majority of chromosome aberrations caused by x-rays and the aging of seeds are just such translocations. (d) Intraspecific sterility originates through such translocations as well as through gene mutations. (III.) The primary genetic processes causing evolution in Crepis:[long dash](1) Gene mutations lead to (a) morphological and physiological differentiation; (b) accumulation of intra- and inter-specific sterility; (c) probably progressive reduction in chromosome size. (2) Changes in chromosome structure induce (a) inter-sterility which may lead to speciation when accompanied or followed by gene mutations; (b) karyotype evolution. Gene mutations and gross structural changes in the chromosomes proceed independently but apparently fortuitously in Crepis. The secondary genetic processes causing evolution in Crepis are interspecific hybridization, polyploidy and apomixis.

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