Abstract
Crop improvement through plant breeding, like crop evolution in general, occurs through selection operating on genetic variability. Selection by plant breeders or by farmers can be intense and has resulted in major crop improvements. However, continued success in plant breeding can only be realized in so far as new variability is available for selection. Genetic diversity is therefore essential for crop improvement. On the other hand, there is a perception and concern that genetic diversity is limited, both within production systems and in breeding programmes, and that there is a need for continuing and concerted efforts to broaden the genetic base of crops. In a wide sense, broadening the genetic base of crops can be considered at 3 levels: increasing the extent of useful diversity available to breeders, that is broadening the genetic base of breeders' material through genetic enhancement, or pre-breeding; increasing the range of useful diversity available to farmers as planting material; and increasing the diversity of crops and varieties grown in the field. In this book, papers on all 3 of these areas have been brought together. It seeks to present and explore the main concepts related to broadening the genetic base of crops, and discuss the extent to which some recent trends (including the greater use of participatory plant breeding and on-farm genetic resources conservation) can contribute to these base-broadening goals. This chapter introduces the topic, provides and overview of the main concepts and surveys some examples.