ABSTRACT
Crop genetic variation (VG ) measures the number of alleles, differences between them, and their arrangement in plants and populations. Farmers and the biophysical environment select plants within populations and farmers choose between populations. Together, phenotypic selection and choice determine the extent of population change between generations, and evolution over generations. With in situ conservation on farm, there is evolution in response to local selection pressures, often maintaining a high level of VG . In contrast, ex situ conservation in gene banks conserves the VG present at a given time and place. Sometimes, farmers carry out selection or choice intentionally to change or conserve VG . Yet much of farmer practice is for production and consumption goals, affecting crop evolution unintentionally if at all. The need to understand farmer selection and conservation is increasing with the loss of genetic resources, spread of transgenic varieties with limited VG , development of a global intellectual property rights in crop genetic resources, global climate change, and efforts to make formal plant breeding relevant to traditional farmers.
