Domesticated plants often show more striking variability at and below the species level than their wild relatives. Nevertheless, no unique processes have operated in crop plant evolution. The ultimate source of variation is mutation, which is then exposed to selection, natural or human. The direction of human selection may change through time and can usually be inferred only indirectly from archaeological artefacts. Where a crop is evolving in contact with its wild relatives, disruptive selection may lead to simultaneous divergence of crop and weed races. The crop may then become genetically isolated by change in breeding system, chromosome number or chromosome structure, though these processes are usually arrested short of total isolation or speciation. In crops that are grown on a garden rather than field scale, reproduce asexually, or have a long generation time, gene flow and disruptive selection are less significant, though human selection may be more intense.