Abstract.  This paper develops a theoretical framework to explain changes in crop genetic resources.  For this purpose the genetic diversity of crop plants is treated as a social/natural co- construct of human beings in particular historical settings.  To illustrate the potential of such approach, the history of rapeseed (Brassica rapa L. and Brassica napus L.)  breeding in Canada from 1954 to 1991 is examined.  Pedigree and cluster analyses are used to evaluate the genetic diversity of rapeseed cultivars as well as its change through time.  Relationships between the genetic diversity of crop plants and ecological sustainability of agriculture is discussed.