Genetic System, Variation, and Adaptation in Eschscholzia californica
Evolution, 16 (3):
pp. 278-299 (1962)Abstract
The variability of Eschscholzia californica in California is partially described and explained. Variation in stamen number, black spot on filament, width of torus rim, color of petals, dormancy of seeds, and longevity are analyzed. Each of these characteristics varies independently and more or less gradually. Parallel variation occurs when several characters respond concurrently to changes in a single environmental factor or to simultaneously varying factors. The steepness of clines depends on the character and the region in which it is examined. The result of such variation is that populations of the species make up a graded patchwork of distinctive populations, each nicely adapted to the particular local conditions for existence. A study of the breeding system reveals that, whether annual or perennial, the plants are obligate outbreeders dependent on sexual processes for reproduction. It is suggested that retention of outbreeding by annuals is related to the large size of their colonies, which in turn may be related to the openness of the communities in which they are found. This breeding system is considered responsible for the clinal variation and the extensive recombination that has taken place. Nevertheless, outbreeding does not preclude the existence of distinctive populations in close proximity. It is suggested that genetic death due to hybridization can be tolerated by populations as long as a sufficient number of adapted individuals is produced to occupy the Lebensraum, and this selection, together with effective isolation by distances of the order of hundreds of feet can account for the close proximity of differentially adapted (genotypically differentiated) populations. The facts and conclusions of this study closely parallel those from studies on other outbreeding higher plants, as, for instance, Agrostis tenuis (Bradshaw, 1959, 1960).